Road Log

0.0 (0.0)       Refer to Map 3A.1.  Intersection of U.S. Hwy 97 and 1st Street (Reed Rd) in La Pine, OR.  Drive north and remain on Hwy 97 through the outskirts of town.

La Pine, OR sits approximately on the central axis of the La Pine Basin and is the geographic location after which the basin is named.  As Newberry Volcano grew during the Pleistocene, lava flows on its western flank gradually accumulated higher and expanded further westward.  This process forced the Deschutes River to the west and formed a natural barrier to downstream transport of sediment, creating the basin which now contains as much as 1300 feet of lacustrine and fluvial deposits at its center. sediments of the La Pine Basin covered by.  The uppermost La Pine basin sediments are chiefly reworked material derived from a dark lapilli tuff blanketing the western slopes of Newberry Volcano that are probably related to at least one caldera-forming event, capped by a veneer of Mazama Ash (from the more famous Crater Lake caldera southwest of here).

5.2 (5.2)       Crossing Paulina Prairie, the floodplain of Paulina Creek.  There is a nice view to the right at 2:00 of Newberry Volcano and the highest point on its caldera rim, Paulina Peak.  Newberry Volcano was named after Dr. John Strong Newberry, the first geologist to explore this area in 1855 while accompanying the Pacific Railroad surveys. Note Newberry Volcano’s broad, shield-like profile covered with dimples.  Newberry Volcano is known to geologists as a shield volcano, generally composed of stacked basaltic lava flows (although its anatomy and recent volcanic history, which we will soon examine, suggests a much greater complexity).  Its back is covered with smaller basaltic cinder cones and rhyolite domes, giving the dimpled appearance.

5.8 (0.6)       Refer to Map 3A.2.   Junction of U.S. Hwy 97 and FS Rd 21 (Newberry – Paulina Lake Rd).  Turn right (east) onto FS Rd 21.  Continuing north on Hwy 97 would take you to Bend, OR completing the first leg of Field Trip 2A.  FS Rd 21 traverses sediments of the La Pine Basin covered by a veneer of Mazama Ash for about the next three miles.  The uppermost La Pine basin sediments are chiefly reworked material derived from a dark lapilli tuff blanketing the western slopes of Newberry Volcano thought to be related to caldera-forming eruptions.

7.2 (1.4)       Refer to Map 3A.1.  Entering the Deschutes National Forest.  Newberry Rd to the right; stay on FS Rd 21.

8.4 (1.2)       Begin crossing Paulina Prairie; Paulina Creek has incised deeply into sediments of the La Pine Basin here.

8.5 (0.1)       Junction of FS Rd 21 and FS Rd 21-050, entrance road to the Ogden Group Camp.  Remain on FS Rd 21.  The Ogden Group Camp is located on a gravelly stream terrace 10 to 15 feet above Paulina Creek that is not covered by Mazama Ash.  These terraces are common along Paulina Creek; the lack of Mazama Ash suggests that they formed subsequent to the eruption of Mt. Mazama about 7700 years ago that formed Crater Lake.  Jensen (2006) indicates that their formation was probably associated with an outburst flood caused by rapid downcutting of Paulina Creek’s outlet from Paulina Lake.  Shoreline terraces around Paulina Lake mark a former higher water level in Paulina Lake prior to the downcutting event.  Flood waters roaring down Paulina Creek likely stripped the Mazama Ash from the stream terraces.

8.7 (0.2)       Junction of FS Rd 21 and FS Rd 2120 to McKay Crossing Campground.   A five-mile round-trip drive on FS Rd 2120 takes you to the campground on Paulina Creek and more flood features.  On the north side of the stream near the campground is another gravelly terrace stripped of its Mazama Ash veneer; although due to a deflection in the stream channel, an isolated remnant of the same terrace on the south side of the stream at the campground still contains its ash.  The campground also provides easy access to a small waterfall about 500 feet downstream.  Much of the waterfall is now abandoned, suggesting it may have formed (or was extensively enlarged) during the post-Mazama flood event that stripped most of Paulina Creek’s stream terraces of Mazama Ash.

9.4 (0.7)       An excellent road cut here exposes a compositionally-zoned ash-flow tuff (Jensen, 2006).  The lower, thicker unit here is the basaltic andesite lapilli tuff (known as the Black Lapilli Tuff) of MacLeod et al. (1995).  At this location, the tuff consists dominantly of small black bombs ranging from a few centimeters to about 25 centimeters in diameter composed of silicic aphyric andesite and/or porphyritic dacite Bomb compositions are identical to pumice clasts in the overlying andesitic tuff unit, an indication that these volcanic materials were produced by the same eruption.

12.4 (3.0)     Intersection of FS Rd 21 and FS Rd 9736.  Continue on FS Rd 21; heading left (north) on FS Rd 9736 would take you to McKay Crossing on Paulina Creek and heading right (south) would take you past the McKay Buttes, three rhyolite domes erupted in succession about 600,000 years ago (MacLeod, 1981).

13.0 (0.6)     Refer to Map 3A.3.  From approximately this location, road cuts along FS Rd 21 show a substantial amount of Black Lapilli Tuff.

15.1 (2.1)     The extensive scree slope to the left of the road here is a typical exposure of  the Black Lapilli Tuff.  It is one of the most widespread ash-flow tuff units exposed on Newberry Volcano’s flanks, discussed in more detail momentarily.

15.7 (0.6)     Entrance to 10-Mile Snow Park on the left.  Turn in here, and drive to the upper end of the parking area to examine good exposures of the Black Lapilli Tuff (MacLeod et al., 1995; Jensen, 2006).

On Newberry’s western flank, this tephra unit is widespread over an area of about 30 square miles.  It is deeply eroded, but exceeds 200 feet in thickness in some locations, consisting of dark pumiceous lapilli and less common blocks and bombs in an ashy, lithic-rich matrix.  Clasts of this tuff are a common constituent in reworked, gravelly, colluvial and alluvial deposits on all flanks of the volcano.  Local outcrops of underlying basaltic lava flows protrude through it, and it is in turn overlain by other ash-flow tuffs on the upper west slopes of the volcano.  The overall distribution, large volume, and poor sorting and stratification of this unit suggests that it was emplaced as a hot pyroclastic flow or several flows erupted during the first of two closely spaced episodes of caldera collapse of Newberry Volcano about 80,000 years ago (MacLeod et al., 1981; Jensen, 2006).

From the back of the parking area, FS Rd 9736-550 provides access (about one and a half miles round-trip) to Footbridge Falls on Paulina Creek and excellent features associated with the post-Mazama flood mentioned earlier (Jensen, 2006).  The road is rough, but serviceable for passenger cars.  Hike or drive FS Rd 9736-550 about 0.3 miles, turn left onto FS Rd 9736-500, and hike or drive about 0.5 miles to the trailhead for Footbridge Falls.  A short trail leads to Paulina Creek.  Two parallel channels here merge about 500 feet downstream.  The northern channel is dry with an abandoned waterfall displaying nice potholes bored into the basalt by gravel carried in the flood water, while Paulina Creek rushes over a small portion of the southern channel to form Footbridge Falls.  The channels are separated by a long, rocky peninsula swept clean of Mazama Ash, soil, and regolith when inundated by water during the Holocene flood event (Chitwood and Jensen, 2000).

15.9 (0.2)     Return to FS Rd 21 and turn left (east).

16.1 (0.2)     The cinder cones ahead at 12:00 are located on a ring fracture related to caldera formation and are the likely source of the basaltic lava flows underlying the Black Lapilli Tuff in this area.  The cinder cones are buried on their upslope flanks by andesitic ash-flow tuffs (also known as the Andesite Tuff) of MacLeod et al. (1995).  Figure 3A.1 displays a more detailed geologic map of Newberry Volcano’s upper flanks and caldera.  The Andesite Tuff occurs as a wedge-shaped unit high on the western slope, widening toward the caldera rim.  This unit is younger than the Black Lapilli Tuff.

Figure 3A.1.  A geologic map of Newberry Volcano’s upper flanks and caldera (modified from MacLeod et al., 1995).

16.4 (0.3)     Here and for about the next quarter mile, exposures on the left reveal pyroclastic material from the cinder cones described at mile 16.1.

16.8 (0.4)     Road cuts on the left for the next two-tenths of a mile expose the Andesite Tuff (Figure 3A.1).  These ash-flow tuffs blanket an area of about 5 square miles on the upper west flank of Newberry Volcano (MacLeod et al., 1995).  They are considered near-vent deposits and grade upslope toward the caldera rim into pyroclastic material having strong agglutinate characteristics (the material was hot and taffy-like when extruded).  Similar ash-flow tuffs occur on the upper east flank of the volcano as well, likely erupted from a ring fracture during a caldera collapse of Newberry Volcano about 80,000 years ago (MacLeod et al., 1981; Jensen, 2006).

17.1 (0.3)     The road to the right (south) here leads to a cinder pit on the northwest side of Mixture Butte (observed from a distance at mileage 16.1).  A quick walk to the pit reveals typical blocks, bombs, agglutinate spatter, lapilli, and ash primarily composed of basalt and basaltic andesite.  Rhyolitic zenoliths occur within the mafic cinders and as separate fragmental material and pumice lapilli.  These may be associated with the rhyodacite dome that crops out on the slope to the southeast and/or its associated pumice ring now buried by younger colluvium and pyroclastic material.  Remnants of silicic domes, tuff- rings, and flows are exposed intermittently on Newberry’s upper flanks and are likely more extensive at depth, associated rhyolitic material brought to the surface by the eruption of younger mafic cinder cones (Figure 3A.1).

17.6 (0.5)     Entrance station for Newberry National Volcanic Monument.  Entrance to the monument requires purchase of a pass, either here or at Lavalands Visitor Center off U.S. Hwy 97 near Lava Butte.

18.7 (1.1)     The entrance road to Paulina Creek Falls Picnic Area is to the left.  Turn in here, and drive to the parking area and trailhead for the short trail to Paulina Falls.

Two short hiking trails exit from the parking area.  The lower trail leads to a very scenic viewpoint at the base of the falls (Figure 3A.2), be sure not to miss this short hike.  The upper trail takes you to a the picnic area and an awesome viewpoint looking down onto the falls (Figure 3A.3), and can be followed about half a mile further to the outlet for Paulina Creek at Paulina Lake, a short hike well worth your time.  The outlet is dammed and its outflow is regulated; crossing the dam, one can walk down stream on the opposite bank to another great viewpoint.

Figure 3A.2.  Paulina Creek Falls, formed in alternating layers of weakly and strongly indurated Andesite Tuff.

Figure 3A.3.  Doubly plunging Paulina Creek Falls as seen during high discharge on Paulina Creek in early July.

When you reach the viewpoint at the base of the falls take a few moments to consider how they formed.  Some background information on the location’s geology will help.  The rock that forms the resistant cliffs over which Paulina Creek Falls tumble are thin to thick layers of agglutinated andesitic pyroclastic deposits associated with the Andesite Tuff (Figure 3A.1).  These rocks continue both northward and southward along the caldera wall for about one and a half miles.  They grade westward and down slope into ash-flow tuffs like those described at mile 16.8 and are interpreted to represent near-vent pyroclastic deposits erupted from a ring fracture system bordering the western rim of Newberry Volcano’s caldera (MacLeod et al., 1981 and 1995; Jensen, 2006).  The rocks exposed at the base of the cliffs are poorly sorted and stratified volcanic ash, lapilli, and blocks containing abundant lithic fragments.  The contact between these lower rocks and the agglutinated pyroclastics above is gradational, and these units together comprise the Andesite Tuff.    MacLeod et al. (1981) suggest that volcanism associated with the ring fracture first deposited cool materials related to  phreatic eruptions, but as eruptions progressed, temperatures of the ejecta increased to the point that it became agglutinated as it was deposited.

As for a general description of the formation of Paulina Creek Falls, consider the following.  Where stream gradients are relatively steep, channels tend to incise into the material underlying the bed, forming a fairly straight pattern in a generally narrow, V-shaped valley.  Where stream gradients are gentle, channels tend to wonder laterally, cutting into their bank on one side while simultaneously depositing sediment on their opposite bank, forming a meandering pattern on a floodplain in a generally wide, flat valley.  Over time, the stream’s valley would broaden and its channel gradient would decrease at the downstream end, while maintaining a narrow, V-shaped valley and steeper gradient upstream.  The stream’s channel would lengthen in the upstream direction by a process called headward erosion because the maximum erosive ability would remain concentrated higher in the watershed.

The earth’s crust is rarely uniform, especially over the distance of a gradually deepening and lengthening stream valley.  As the channel incises its bed, the stream would encounter multiple rock and/or sediment types with variable resistance to erosion by running water.  Eventually, at some location along the stream’s channel, flowing water may begin to erode into a relatively resistant material.  Where the channel first succeeds in eroding through the resistant bed material and into a weaker substance below, erosion would continue more slowly upstream, but more rapidly downstream.  This process creates a short steep segment of stream channel, what a geologist calls a “nick point” in the stream and a waterfall begins to develop (Figure 3A.4).  Over time, the downstream segment of the stream overlying the weaker material would erode more deeply (especially at the base of the waterfall) and begin to undercut the resistant material above, causing it to break loose, only to be carried away in the flowing water.  Waterfalls are self-perpetuating, once the process has begun, they may back-waste in the upstream direction by differential erosion at the nick point for considerable distances.

Figure 3A.4.  An illustration of waterfall formation and headward erosion on a high gradient stream channel.

Here at Paulina Falls, we have a perfect example of just such a process.  The lower pyroclastic unit is less indurated and thus weaker and more prone to stream incision than the upper agglutinated unit.  Paulina Falls has formed where Paulina Creek has downcut through this less resistant material.  As you look upstream at the falls (Figure 3A.2), notice the relatively “fresh” section of cliff face between the falls and the large pile of coarse debris below it that plugs much of the channel.  In May of 1983, a large section of the cliff fell away, in part due to water undercutting the weaker rock layer at the base of the falls, and in part due to the forces of frost action and gravity working on this lower part of the cliff face behind the falls.  In this way, the falls migrate in a headward direction as the stream continues to downcut.  Imagine at some point in the distant future (as humans measure it), the falls will eventually migrate upstream to the outlet at Paulina Lake, releasing another deluge of water down Paulina Creek, substantially altering its channel and valley characteristics, similar to the Holocene flood described earlier.

      Retrace your steps to the parking area.

18.9 (0.2)     Return to FS Rd 21 and turn left (east).

19.2 (0.3)     Entrance road to Paulina Lake Lodge to the left, followed closely by the parking area for Paulina Lake Day Use Area.  Turn into the parking area for the day use area and make a quick stop at the Visitor Information Center, a short walk across FS Rd 21 to your right.  From the parking area, you can also walk to the trailhead for the Peter Skene Ogden Trail which follows Paulina Creek (to Paulina Falls if you like), the Paulina Lakeshore Trail which traverses the shoreline of Paulina Lake, and the Crater Rim Trail that circumnavigates the entire perimeter of Newberry Volcano’s caldera rim.

Before returning to your vehicle, walk to Paulina Creek’s outlet from Paulina Lake (to your left).  The outlet has been artificially raised by the construction of a small dam.  Chitwood and Jensen (2000) suggest that the Holocene flood on Paulina Creek resulted from 5 to 8 feet of rapid incision of the stream channel here when it encountered a weakly indurated layer in the Andesitic Tuff that makes up the caldera rim in this area.

Paulina Lake covers an area of about 1500 acres to an average depth of 163 feet (its maximum depth is 250 feet).  The water of Paulina Lake contains an unusually high concentration of dissolved ions, significantly higher than even East Lake, the other major lake within the caldera.  Both lakes receive a substantial influx of water from mineral-rich hot springs.  These hot springs are mostly located on the floor of the lakes, although Warm Springs, a hot springs on the northeast shore of the Paulina Lake basin, can be reached by hiking the Paulina Lakeshore Trail.

19.5 (0.3)     Intersection of FS Rd 21, FS Rd 21-530 to Paulina Lake Campground on the left, and FS Rd 21-500 to Paulina Peak on the right.  All of the Forest Service campgrounds within the caldera are quite nice, this one being no exception.  Turn right (south) onto FS Rd 21-500 for an 8-mile round-trip drive to the summit of Paulina Peak, the highest point on Newberry Volcano’s caldera rim.  The road was improved in 1998 as a single lane road with turnouts and an improved gravel surface, but as with all new construction, traffic has increased in density and speed; drive carefully.

If you have limited time, this side trip to the summit of Paulina Peak should not be missed.  If you intend to skip this side-trip, you can pick up the main route description at mile 27.1.

20.1 (0.6)     Parking area on the right for the Paulina Peak Trail to the summit of Paulina Peak.  This trail can be combined with the Crater Rim Trail to traverse the entire perimeter of the Newberry Volcano’s caldera rim.  The parking area was built on the drilling platform for Occidental Geothermal, Inc.’s exploratory geothermal drill site NC 72-03 (Arestad et al., 1988).  The test well was drilled to a depth of 4501 feet, encountering a bottom hole temperature of 311 ˚F.  The well passed through 1800 feet of interlayered cinders, lithic tuffs, pumice, mudflows, and basaltic lava flows; 500 feet of silicic lava flows, and about 2200 feet of basaltic andesite lava flows.  The basal unit represents the early shield-building phase of Newberry Volcano’s complex history, while the overlying units probably represent post-caldera infilling.

20.6 (0.5)     The road leaves the Andesite Tuff described earlier and crosses onto talus from the silicic domes that comprise Paulina Peak (Figure 3A.1).

21.1 (0.5)     Good view to the right of the High Cascade volcanoes (from Diamond Peak in the south to Mt. Hood in the north).  The rhyolitic domes of McKay Butte and its immediate partners are visible on the central slope of Newberry Volcano at 3:00 (Figure 3.2).  These domes are dated at about 600,000 years old (MacLeod et al., 1981).  The relatively flat area below and in the foreground is underlain by the andesitic tuff encountered earlier, while the cinder cones just beyond at the break in slope formed from basaltic eruptions associated with a ring fracture system described earlier.

21.6 (0.5)     Road cuts on the left expose rhyolite of the Paulina Peak domes (Figure 3A.1).

22.1 (0.5)     Good views to the south.  In the distance, the Fort Rock– Christmas Valley pluvial lake basin and the Fort Rock tuff ring, the remnants of a maar volcano, lie to the southeast, and to the southwest, Walker Mountain and the Walker Rim fault zone.  In the near distance, numerous basaltic cinder cones dimple Newberry Volcano’s southern flanks; while in the foreground, the long, narrow ribbon of the 7000-year-old Surveyor Lava Flow is clearly visible.

22.3 (0.2)     The Crater Rim Trail crosses the road at this location.  If you are interested in hiking this trail, park at the summit parking area and take the trail to the left (west) out of the parking area which first climbs to the top of Paulina Peak, then branches to the right to become the Paulina Peak Trail described at mile 20.1 or branches to the left to become the Crater Rim Trail which descends past this point.

22.5 (0.3)     A road cut here exposes flow-banded rhyolite of the Paulina Peak domes (3A.1).

23.1 (0.6)     Paulina Peak Tephra exposed in a road cut on the right, the product of an ancient silicic pyroclastic eruption associated with construction of the Paulina Peak domes (Figure 3A.1).

23.3 (0.2)     Paulina Peak summit parking area.  Park here and explore.  First walk to the summit just to the left of the parking area, the views are marvelous!  You are standing on the highest point of the rim surrounding Newberry Volcano’s 17-square-mile caldera.  MacLeod et al. (1981 and 1995) provide an extensive description of the rock units that comprise Newberry Volcano, including its upper flanks, caldera rim, and caldera fill (Figure 3A.1).  From this position, one can easily observe Newberry Volcano’s southern flank, and caldera.  Paulina Peak is a rhyolite dome about a mile in diameter and its associated silicic lava flows that extend about 3 miles down slope to the southwest.  These features are perched on and make up a substantial portion of the southwestern rim and uppermost flank of the caldera and volcano.

Take some time to examine the floor and rim walls of the caldera.  The floor, inner slopes, and rim of Newberry’s caldera are comprised of post-caldera formation volcanic fill, much of it of a more silicic composition than the volcano’s flanks (MacLeod et al., 1981 and 1995).  Several prominent geological features formed as a consequence of volcanic eruptions within the caldera or along the caldera rim, some with well-expressed topographical form (Figure 3A.1).  The two lake basins, Paulina Lake and East Lake, are probably remnants of a once larger lake that occupied much of the caldera (similar to Crater Lake within Mt. Mazama’s caldera today).  Central Pumice Cone, formed of pumiceous ash and lapilli, Little Crater, a basaltic tuff cone, and several smaller pumice cones, tuff rings, and associated tephra deposits now fill much of the central caldera and separate the lakes.  Superimposed on these features are several major obsidian flows, including the 7,100 year old Interlake Flow with a vent source at the base of the north-central caldera wall, the 3,500 year old East Lake Obsidian Flows with a vent source on the southeastern caldera floor, and the 1,250 year old Big Obsidian Flow with a vent source near the base of the south-central caldera wall.  Figure 3A.5 and Figure 3A.6 display prominent views of the caldera to the north and northeast respectively, from Paulina Peak’s summit area.

Figure 3A.5.  Paulina Lake, bounded on the north and west by the caldera rim, and on the east by the Central Pumice Cone, Little Crater, and the western arm of the Interlake Flow.

Figure 3A.6.  The eastern portion of Newberry Volcano’s caldera as viewed from the summit of Paulina Peak.  The caldera rim forms the ridge in the background.  East Lake can be seen in the upper left, behind and just to the right of the Central Pumice Cone.  The Big Obsidian Flow occupies much of the middle ground, with Little Crater pinched between it and the Central Pumice Cone.

The southern caldera wall near Paulina Peak is composed of rhyodacite lava flows associated with the growth of the Paulina Peak silicic dome (Figure 3A.1).  Based on potassium-argon dating, dome growth occurred between 580,000 and 240,000 years ago.  Further east on the south wall near the Big Obsidian Flow and on much of the north wall, volcanic rock is comprised of a complex sequence of platy rhyolite and more massive basaltic andesite flows, as well as pyroclastic deposits (scoria and cinder).  The northern and eastern caldera walls contain sequences of rhyolite and basaltic andesite lava flows, palagonite tuff, and other pyroclastic deposits; while near-vent, welded, rhyolitic pumice deposits occur on the eastern wall.  Two Pleistocene rhyolitic obsidian flows cap the caldera rim above the Big Obsidian Flow and above East Lake, now mostly buried by a thick accumulation of Holocene silicic tephras.  Isolated vents and a long fissure system erupting basalt and basaltic andesite occupy portions of the north wall near Paulina Lake (at Red Slide) and East Lake (at East Lake Fissure and Sheeps Rump) and eastern wall (at the East Rim Fissure).  The East Rim Fissure produced the Resort Flow about 11,000 years ago, a long narrow tongue of basaltic andesite that flowed from near the caldera rim nearly to East Lake.  The East Lake Fissure is the only caldera rim volcanic feature that is post-Mazama eruption in age.  It is the southeasternmost vent of the Northwest Rift Zone (Figure 3A.7), a series of basaltic vents and lava flows erupted about 7,000 years ago along an en echelon fracture zone that trends northwestward along the northwest flank of Newberry Volcano (and first explored on Field Trip 1A and 1B in FIELD GUIDE TO THE BEND AREA).

Figure 3A.7.  A schematic diagram showing vents and lava flows of the Northwest Rift Zone, the East Rim Fissure and Resort Flow are its only expressions within Newberry Volcano’s caldera.

Beyond Newberry Volcano, several other noteworthy features can be observed.  To the north, one can see the Gray Butte-Smith Rock rhyolite dome complex in the distance (visited on Field Trip 1F in FIELD GUIDE TO THE BEND AREA), Pilot Butte cinder cone in the near distance (visited on Field Trip 1C in FIELD GUIDE TO THE BEND AREA), and Lava Butte cinder cone and the lava flows of the Northwest Rift Zone in the foreground (visited on Field Trip 1A and 1B in FIELD GUIDE TO THE BEND AREA).  To the east, the High Lava Plains are marred by the en echelon normal faults of the west to northwest trending Brothers Fault Zone and a similar trending belt of age-transgressive silicic domes and basaltic lava flows.  The 5-million-year-old complex of rhyolitic domes known as Glass Buttes can be seen on the skyline in the distance, while the youngest members of the belt, the domes and associated lava flows of China Hat and East Butte lie in the foreground (visited later on this field trip).  The youthful basaltic Devils Garden Lava Flow can be seen in the middle distance (visited on Field Trip 3B).  To the south and southeast, one can see the Fort Rock-Christmas Valley pluvial lake basin, dotted with maar volcanoes such as Fort Rock’s tuff ring, its southern edge the normal-faulted blocks of the northernmost extension of the Great Basin (visited on Field Trip 3B).  Walker Rim and Walker Mountain, sliced by normal faults of the Walker Rim Fault Zone, some of which extend to the lower flanks of Newberry Volcano, lie to the southwest .  On the western skyline lie many prominent stratovolcanoes of the High Cascades volcanic arc (visited on Field Trips 2 in FIELD GUIDE TO THE CASCADE LAKES AND WILLAMETTE PASS AREAS and 4 in FIELD GUIDE TO THE MCKENZIE PASS AND SANTIAM PASS AREAS); on a clear day, from north to south, Mt. Adams, Mt. Hood, Mt. Jefferson, Three Fingered Jack, Mt. Washington, the Three Sisters, Broken Top, Mt. Bachelor, Diamond Peak, Mt. Thielsen, Mt. Scott and the rim of Crater Lake’s caldera (Mt. Mazama), Mt. McGloughlin, and Mt. Shasta can be observed.  In the nearer distance lies the valley of the Deschutes River, its upper tributaries having suffered the effects of multiple glaciations (visited on Field Trips 2A and 2B in FIELD GUIDE TO THE CASCADE LAKES AND WILLAMETTE PASS AREAS), and its primary drainage channel blocked and rerouted several times during the growth of Newberry Volcano (visited on Field Trip 1A in FIELD GUIDE TO THE BEND AREA).

Return to your vehicle and proceed back to FS Rd 21.   

27.1 (3.8)     Intersection of FS Rd 21-500 and FS Rd 21.  Turn right (east) onto FS Rd 21.  Road cuts here expose a layer of Mazama Ash overlain by a thin (2 inches thick) layer of fine white ash from eruptions related to the Big Obsidian Flow (Jensen, 2006);  although not exposed, sand and gravel deposits of higher lake levels within the caldera are known to lie beneath the Mazama Ash.

28.1 (1.0)     Intersection of FS Rd 21, FS Rd 21-563 to Chief Paulina Horse Camp on the right, and FS Rd 21-565 to Newberry Group Camp on the left.  Lake terraces associated with a higher stand of Paulina Lake are found just north of the group camp (Jensen, 2006).  These terraces are not covered by Mazama Ash, but are covered by the white ash related to the Big Obsidian Eruptive Period.  This indicates that the high lake level must have occurred between approximately 7700 years ago and 1300 years ago, ages obtained for the Mazama Ash and ash of the Big Obsidian Eruptive Period, respectively.

28.2 (0.1)     The road cuts here and for the next quarter of a mile expose the Paulina Lake Ashflow.  This pyroclastic deposit extends from the Big Obsidian Flow onto the caldera floor at the southern edge of Paulina Lake (MacLeod et al., 1981 and 1995).  The ash flow mostly consists of fine white ash with lesser amounts of pumice fragments.  The lack of collapse of the pumice and welding of the ash suggests that the flow was relatively cool when emplaced. The orientation of primary flow features, ridges and furrows at the surface of the deposit, indicate that the source vent for the ash flow is located under the southern part of the Big Obsidian Flow.  Jensen (2006) reports that multiple charcoal stumps and logs were found in the road cuts and road bed when FS Rd 21 was reconstructed in 1993.  Stumps projected up into the ash flow, but were sheared off at an unknown height above the base of the deposit which was not exposed during excavation.  The logs had an average NE to NW orientation along this quarter-mile stretch of the road, indicating a general northly flow direction to the ash as it was deposited.

28.7 (0.5)     Refer to Map 3A.4.  Junction of FS Rd 21 and FS Rd 21-570 to Little Crater Campground. Turn left (north) onto FS Rd 21-570.

The parking area for the Little Crater Trailhead lies just ahead near the boat launch.  Little Crater is a palagonitic tuff cone (Figure 3A.1) formed by explosive eruption of basaltic magma rising upward through groundwater saturated sediment.  In this area, bounding normal faults related to the formation of the caldera have formed a dam which maintains ground water at shallow depths (Jensen, 2006).  Little Crater’s tuff is primarily composed of mafic igneous rock fragments in an ashy matrix, but occasional granitic rock fragments attest to a shallow, cooled body of silicic igneous rock near the surface. A short hike takes you completely around the rim of Little Crater and provides access to outcrops of tuff and interesting views of the nearby Big Obsidian Flow (see Little Crater Trail under the Optional Hiking Trails section at the end of this road log for a complete description of this pleasant hike).

After completing the hike, return to your vehicle and proceed northward on FS Rd 21-570 toward Little Crater Campground.

The entrance to Little Crater Campground is about four-tenths of a mile beyond the boat launch site.  Shoreline exposures near there reveal palagonite tuff from Little Crater.  The campground is built on several wave-cut lake terraces on the lower flank of Little Crater marking former high stands of Paulina Lake.  The presence of these terraces, and others, like those at Newberry Group Camp, indicates that the floor of the caldera has undergone 10 to 15 feet of differential uplift since about 7000 years ago (Jensen, 2006).  As you drive through the campground, look for evidence of these bench-like features.

29.7 (1.0)     North end of the campground and the trailhead for the Paulina Lakeshore Trail (see Paulina Lakeshore Trail under the Optional Hiking Trails section at the end of this road log for a complete description of this short hike).  It is well worth your time to hike this trail about two miles out and back to Warm Springs.  More evidence of raised shorelines can be observed along the trail including gravel bars formed of obsidian from the Interlake Lava Flow.  The trail also traverses the toe of the western arm of the Interlake Obsidian Flow (Figure 3A.1).  The trail passes the Warm Springs hot springs area at the northeast corner of the lake.  Shoreline bottomwaters gradual warm from about 50 ˚F at the campground to about 61 ˚F nearest the hot springs, and offshore, there is a large area of above normal temperatures and rising gas bubbles.

Turn around and retrace your course to FS Rd 21.

30.7 (1.0)     Junction of FS Rd 21-570 and FS Rd 21.  Turn left (east) onto FS Rd 21.

31.1 (0.4)     The entrance road to the Big Obsidian Flow trailhead is on the right (southeast) side of the road.  Turn in here and park in the trailhead parking area.  Follow the signs for the Big Obsidian Flow interpretive trail.  This short hike of about one mile round-trip lets you explore the amazing compositional variations and flow features of the Big Obsidian Flow (see Big Obsidian Flow Trail under the Optional Hiking Trails section at the end of this road log for a complete description of this hike).

Before hiking onto the Big Obsidian Flow, take a few moments to consider what is known of its features and formation.  The Big Obsidian Flow, Paulina Lake Ashflow, and an associated widespread pumice air-fall deposit, the Newberry Pumice, resulted from the most recent episode of volcanism within Newberry Volcano’s caldera, beginning and ending between about 1,450 and 1,250 years ago.  Known as the Big Obsidian Eruptive Period, these rhyolitic volcanic eruptions occurred in three stages: 1) initial gas-rich, explosive eruptions that produced an immense cloud of pyroclastic debris that blanketed the southern portion of the caldera and a large part of Newberry Volcano’s eastern flank with the Newberry Pumice, an air-fall pumice deposit (Figure 3A.8); 2) an intermediate stage of  less volatile eruptions that produced the Paulina Lake Ashflow,  localized pyroclastic surge deposits that cover the caldera floor between the Big Obsidian Flow margin and Paulina Lake; and 3) a final stage involving the eruption of silicic, degassed magma from the vent that generated the Big Obsidian Flow (Figure 3A.1).  The Newberry Pumice covers an area of about 270 miles2 to an average depth of 1.5 feet (it is more than 6 feet thick near the vent source, but thins to less than a foot about 40 miles downwind.  The distinctly elongated tephra plume indicates a strong wind was blowing at the time of eruption.  The Paulina Lake Ashflow covers about 2.5 miles2 to an average depth of 20 feet and the Big Obsidian Flow an area of about 1.1 square miles to a depth of 150 feet.  The Big Obsidian Flow ranks fifth by area and seventh by volume when compared to other similar silicic lava flows in California, Oregon, and Washington, and is among the largest in the world.

Figure 3A.8.  Map showing the distribution of the Newberry Pumice, an air-fall pumice deposit associated with the initial eruption of gas-rich magma during the Big Obsidian Eruptive Period (modified from Jensen, 2006).

Figure 3A.9 shows the topography and extent of the Big Obsidian Flow.  The surface of the flow consists of about 10 percent glassy obsidian and about 90 percent pumice (frothy glass) in an amazing variety of forms.  The flow surface is incredibly irregular, consisting of seemingly random heaped-up mounds of rough, angular blocks of obsidian and pumice.  However, when viewed from above (Figure 3A.4), the mounds form distinctly arcuate ridges.  These ridges are the result of deformation of the rapidly cooling and solidifying rock of the flow surface by the hot, plastic, still-flowing rock of the lava’s interior.  They form perpendicular to flow, and thus indicate the rhyolitic lava’s flow direction (generally away from the vent and downslope away from the caldera rim).  Research by Fink (1983), Fink and Manley (1987), Manley and Fink (1987), and Metz and Bailey (1993) indicates that obsidian flows like this one develop distinctive vertical and lateral zonation (Figure 3A.10).  The flow surface forms as a fine, vesicular, light-colored pumiceous breccia that becomes broken up by continued movement within the flow.  As the flow spreads downslope, its leading edge continually buries previously formed pumiceous breccia as a basal layer beneath the flow.  This surficial carapace grades inward about 10 to 15 feet into a zone of dense, glassy obsidian surrounding a coarsely vesicular pumice, which in turn grades inward to a finely crystalline rhyolite core.  Zonation can be distorted at the flow margins while the flow is still in motion by the rise of buoyant diapirs of low density vesicular pumice toward the flow exterior, and by compression of the flow that causes complex faulting, folding, and ramping.  These distortions are quite evident at the margin of the Big Obsidian Flow along the interpretive trail where one can observe alternate bands of lighter, finely vesicular pumice and darker, coarsely vesicular pumice and obsidian.

Figure 3A.9.  A map of the Big Obsidian Flow (modified from Jensen, 2006).

Figure 3A.10.  Diagram of the internal vertical and lateral zonation within an obsidian flow (modified from Metz and Bailey, 1993).

Molten obsidian, a highly siliceous magma, is an extremely viscous material that must be quite thick in order to flow.  The highly viscous nature of this magma substantially reduces ion mobility, preventing chemical combination and the formation of mineral crystals.  These conditions result in the formation of a natural glass that can contain considerable gas bubbles, forming obsidian and a range of pumice types.  These compositional variations are exhibited in the large obsidian and pumiceous blocks that comprise the flow’s surface (Figure 3A.9), best seen along the Big Obsidian Flow interpretive trail.  These blocks are generally concentrated into ridges on the flow surface, as seen from an elevated position such as that afforded by Paulina Peak earlier (Figure 3A.5).   Some of the pumiceous blocks exhibit finer taffy-like flow structures of alternating density, while blocks of glassy obsidian may contain inclusions of fine-grained pumiceous rhyolite.

Figure 3A.11.  The surface of the Big Obsidian Flow is comprised of large blocks of pumice and obsidian.

In addition to interesting compositional variations and flow structures, the Big Obsidian Flow is pitted by many small craters with a fascinating origin intimately tied to the viscous nature of the siliceous magma from which it formed (Figure 3A.9).  These craters are generally circular in shape, 40 to 200 feet across and 15 to 45 feet deep, and typically rimmed by blocky obsidian and pumice from the flow interior.  Spherical, smooth- and glassy-walled, bubble-like cavities are found on the floor of several of these craters (Figure 3A.12).  Jensen (1993) suggests that these features formed when giant gas bubbles burst outward from near the surface of the cooling, solidifying lava flow.  The bubbles formed as exsolving gases gradually rose from the still-molten interior of the flow to coalesce in the viscous, plastic carapace of the flow’s exterior.  When the pressure exerted by the steadily accumulating gases exceeded the strength of the overlying surface layer, the bubbles vented explosively to form the craters observed today.  Castro et al. (2000) have shown that the formation of these giant gas bubbles is probably controlled in part by flow structures within the obsidian, rather than simply by coalescence of gases alone.

After enjoying the hike on the Big Obsidian Flow interpretive trail, return to your vehicle and proceed back to FS Rd 21.

Figure 3A.12.  A giant (exploded) gas bubble in the Big Obsidian Flow observed from without (A) and within (B); notice the smooth, curved nature of the bubble’s ceiling formed of platy pumiceous obsidian.

31.3 (0.2)     Junction of Big Obsidian Trailhead parking entrance road and FS Rd 21.  Turn right (east) onto FS Rd 21.  The Paulina Lake Ashflow has thinned to only a couple of feet thick here and completely disappears in the next few tenths of a mile.

31.9 (0.6)     Good view to the left at 9:00 of the Central Pumice Cone (Figure 3A.1).  Construction of this small volcano occurred about 7,200 years ago, one of many eruptions subsequent to caldera formation that have been gradually in-filling a once larger, single lake basin that now exists as the modern Paulina and East Lakes.  Central Pumice Cone contains a small obsidian flow (Crater Obsidian Flow) on the floor of its crater.  Remnants of obsidian perched on the inner slopes of the crater indicates that siliceous lava initially filled the crater to a much greater depth, but was drained away through a breach in the southeast side of the cone to form the Game Hut Obsidian Flow (now buried by Mazama Ash).

32.1 (0.2)     FS Rd 21 follows the edge of the Game Hut Obsidian Flow (buried by Mazama Ash) for about the next quarter mile.

32.6 (0.5)     Pullout on the left.  Park here and take in the view of East Lake and the East Lake Obsidian Flows on its southern shore (Figure 3A.1).  The pullout is built on, more or less, the axis of a buried obsidian flow, the South Obsidian Flow, which was erupted about 12,000 years ago.  This flow extends from a silicic dome near the south wall of the caldera behind you (MacLeod et al., 1981 and 1995).  Prior to eruption of Little Crater, the basins of Paulina Lake and East Lake were probably still joined, and geomorphic evidence suggests that a stream still connected East Lake to Paulina Lake until formation of the South Obsidian Flow and then the Central Pumice Cone finally choked off and plugged its valley.

In 1993, this section of the road was reconstructed.  For about the next quarter mile, temporary exposures during excavation revealed an interesting geologic story (Jensen, 2006).  In ascending order, the following stratigraphic sequence was exposed: 1) the top of the South Obsidian Flow; 2) a soil zone containing abundant basaltic tephra, probably erupted from the East Lake Fissure; 3) about 2.5 feet of Mazama Ash; 4) a dark, organic-rich soil zone containing multiple tree and log molds with a general orientation to the south; 5) about 14 feet of pumiceous tephra deposits of the East Lake Tephra; and 6) the overlying modern soil and colluvium.  The East Lake Tephra is widespread in the eastern part of Newberry’s caldera (Figure 3A.1), generated by the Interlake Obsidian Flow Eruptive Period.  The tephra deposits are comprised of massive to crudely bedded pumiceous ash and accidental blocks overlain by mud-armored pumice, accretionary lapilli, pumice, and ash beds.  The upper bedded layers contain volcanic bombs associated with sag features that indicate the sediment was wet and cohesive.  A depression on the southwest floor of East Lake is believed to be the vent source for the eruption of the East Lake Tephra, and its location correlates well with the orientation of the tree and log molds discovered during expansion of FS Rd 21.

33.0 (0.4)     Junction of FS Rd 21 and FS Rd 21-660 to East Lake Campground.  Turn left (north) onto FS Rd 21-660 and park in the East Lake Boat Launch parking area.  East Lake covers an area of about 1000 acres to an average depth of 67 feet (its maximum depth is 180 feet).  Testing of larger fish by the state of Oregon indicates high concentrations of mercury.  Fish bioconcentrate mercury as larger fish feed on invertebrates and smaller fish that in turn feed on plants that naturally uptake mercury from the hot springs-enriched waters of the lake.  Mercury levels in the lake water itself are well below state and federal standards.

Walk along the shoreline of East Lake to your right (northeast) several hundred feet from the boat launch to wave-cut shoreline outcrops (Figure 3A.13) that expose the basaltic, palagonite tuff of the East Lake Tuff Cone (Figure 3A.1).  The tuff is well bedded, many beds containing mud-armored lapilli and accretionary lapilli in an ashy matrix, as well as volcanic bombs and associated sags.  These features suggest the tuff was deposited above water, although it was wet and cohesive, indicating deposition near the lake.  About half way along your walk, shoreline outcrops display a distinct contact between two palagonite tuffs; the western (nearer the boat launch) unit is younger and plastered onto the eastern unit.  The eastern tuff contains abundant, large accidental blocks that are more thoroughly palagonized than the tuff within which they occur (MacLeod et al., 1981 and 1995).  Deposits of the East Lake Tuff Cone are buried by about 2 feet of Mazama Ash, indicating construction of the cone proceeded the 7,000-year-old eruption of Mt. Mazama.

Figure 3A.11.  An outcrop of bedded palagonite tuff on the northern flank of the East Lake Tuff Cone exposed along East Lake’s southern shore.  Note that the dip of the beds is toward the shore (and away from the vent area).

From your position on the south shore of East Lake, several significant geologic features are visible (Figure 3A.1); westward lies the Central Pumice Cone, northwestward, the Interlake Obsidian Flow extends down into the lake, northward, the East Lake Fissure is exposed on the caldera wall, and northeastward lies the pre-Mazama eruption basaltic andesite cinder cone of the Sheeps Rump and somewhat older obsidian flow that form cliffs along the lake shore.  Near the headland formed of palagonite tuff from the East Lake Tuff Cone, a shoreline hot spring emerges.  The water from this hot spring issues at temperatures approaching 176 ˚F, but is quickly diluted by the lake.  A large area of warmer than usual water and rising gas bubbles indicates that hot spring activity occurs in the near shore area as well.

Return to your vehicle at the boat launch parking area and proceed with the field trip.

33.2 (0.2)     Back to the junction of FS Rd 21-660 and FS Rd 21.  Turn left (east) onto FS Rd 21.

33.8 (0.6)     Junction of FS Rd 21 and the entrance road to Hot Springs Campground.  This campground is often closed except during the peak summer tourist season, but the half mile loop road through the campground provides access to the East Lake Obsidian Flow (Figure 3A.1) produced during the East Lake Obsidian Flow Eruptive Period.

34.0 (0.2)     The andesitic Resort Flow can be observed on the hillside to the right (Figure 3A.1).  MacLeod et al. (1981) indicates that this lava flow extends from a vent associated with a ring fracture system on the southeastern wall of the caldera.  The flow and vent area contain rock with a wide range of composition and phenocryst content.

34.2 (0.2)     Junction of FS Rd 21 and FS Rd 21-700.  Continue straight on FS Rd 21-700 which goes to Cinder Hill Campground and East Lake Resort.  This campground is an excellent place to camp while enjoying the many geological wonders of Newberry Volcano, campsites are spacious and the beach access is a real bonus.  It is the last developed campground to be passed on this field Trip.

34.8 (0.6)     Take the spur road to the left (west) here and park in the lot for the Cinder Hill Campground Day Use Area just ahead.  First walk down to the shoreline.  A pleasant summer swim anyone?  Aside from a potentially fine beach stroll, there are also excellent views directly across East Lake to the Central Pumice Cone, and the East Lake Fissure (Figure 3A.1).  Figure 3A.14 presents pleasant sunrise view of the East Lake Fissure, the northeasternmost extension of the Northwest Rift Zone discussed in Field Trip 1A and 1B in THE FIELD GUIDE TO THE BEND AREA.  If you walk far enough northward along the shore, you come upon a small lake cut off from East Lake by a narrow strip of beach sands and gravels (the beach you are walking on); this feature is a fine examples of a spit formed by longshore drift of sediment.  Figure 3A.15 shows the spit from the northern end of the campground looking back in the direction from which you came, while Figure 3A.16 offers a fine sunset view of the Central Pumice Cone with Paulina Peak peering over its left shoulder.

Figure 3A.14.  A fine sunrise view of the East Lake Fissure from the shoreline of East Lake near the Day Use Area in Cinder Hill Campground; the fissure lies on the slope just above the tiny white boat on the lake (left of center).

Figure 3A.15.  The beach and spit on the east side of East Lake near Cinder Hill Campground.

Figure 3A.16.  The northern end of Cinder Hill Campground offers a great view across East Lake to the Central Pumice Cone (with Paulina Peak in the background); note the near-perfect symmetry of Central Pumice Cone’s flanks silhouetted in this photograph.

If you intend to stay longer (the campground is inviting), you may wish to take in a round-trip hike of about six miles that takes you to a young basaltic cinder cone, Cinder Hill, perched on the caldera rim (Figure 3A.1) and an excellent view of East Lake and Newberry’s caldera (described as the Cinder Hill Trail under the Optional Hiking Trails section at the end of this road log).  The trail departs from a small trailhead parking area further up the road about three-tenths of a mile (continue straight rather than turning at mile 34.8).  The trailhead for the Cinder Hill hike is on the right (east) side of the road next to the parking area.

Once you have camped, taking in this great little hike, or just a swim and a stroll on the beach, return to your vehicle and proceed back to FS Rd 21. 

35.5 (0.7)     Junction of FS Rd 21-700 and FS Rd 21.  Turn left (east) onto FS Rd 21 toward the eastern rim of Newberry Volcano’s caldera.

The pavement ends shortly.  If you would rather not continue this field trip over a lengthy section of unpaved road (descent gravel, but often wash-boarded), this junction should serve as your turn-around point.  Retrace your route on FS Rd 21 to U.S. Hwy 97 and return to La Pine, OR or to Bend, OR.

Over the next tenth of a mile, you’ll pass the dual entrances to the East Lake RV Park on your right.  As you pass the first entrance, look ahead at 12:00 to see basaltic cinders, bombs, and spatter associated with vents of the East Lake Fissure.  The low rise to the right after the second entrance is the surface expression of a silicic intrusion that extends southwest to vents of the East Lake Obsidian Flows.  The andesitic Resort Flow is cut by an eruptive fissure related to the silicic intrusion and vents.

36.2 (0.7)     The East Draw Cinder Pit is on the right.  Exposures at the back of the pit reveal an interesting stratigraphy of pyroclastic deposits associated with silicic volcanic eruptions both outside and within Newberry’s Caldera (Jensen, 2006).  From bottom to top, a composite profile includes: 1) several inches of andesite of the Resort Flow; 2) Mazama Ash composed of about twenty inches in place, overlain by an equal amount of reworked ash; 3) about fifty inches of the East Lake Tephra associated with the Interlake Obsidian Flow Eruptive Period; 4) about eight inches of the East Draw Tephra associated with the East Lake Obsidian Flows Eruptive Period; and 5) a similar thickness of Newberry Pumice associated with the Big Obsidian Flow Eruptive Period.

37.5 (1.3)     The Crater Rim Trail crosses the road at this location.  This is also the approximate eastern boundary of Newberry National Volcanic Monument.  The junction of FS Rd 21 and FS Rd 2127 lies just east of here.  Park your vehicle in a safe place along the right side of the road.  The southern segment of Crater Rim Trail leaves from the right-hand trailhead here and offers spectacular views north into Newberry Volcano’s caldera and to the south toward the Fort Rock – Christmas Valley basin, as well as opportunities to examine the Big Obsidian Flow and Newberry Pumice, products of the Big Obsidian Flow Eruptive Period, up close (see Crater Rim and Lost Lake Loop Trail under the Optional Hiking Trails section at the end of this road log for a complete description of this short hike).

After this invigorating and eye-popping hike, return to your vehicle and continue eastward and down the eastern flank of Newberry Volcano on FS Rd 21.

37.8 (0.3)     A prominent road cut on the left exposes a blanket of Newberry Pumice over eight feet thick (MacLeod et al., 1981).  Pumice lapilli and blocks comprise the majority of the material, although accidental fragments of basalt, rhyolite, and obsidian are common.  There is a minimal presence of ashy matrix. Recall that the tephra plume generated by the early stages of the Big Obsidian Flow Eruptive Period is long and narrow and oriented nearly east-west with the prevailing wind direction (Figure 3A.6).  This location is about two and a half miles east (downwind) of the vent and just north of the tephra plume’s axis.

38.0 (0.2)     Pull off into the trailhead parking area on the right side of the road for a short excursion on The Dome Trail (see The Dome Trail under the Optional Hiking Trails section at the end of this road log for a complete description of this short hike).  The beginning of the trail is from the southeast end of the pullout and is not well marked.

The Dome is a large, post-Mt. Mazama-eruption, basaltic cinder cone, breached on its southeast flank. A lava flow was extruded through the breach and flowed a considerable distance downslope (Figure 3A.17).  The outer flanks and rim of the cone are plastered with a veneer of Newberry Pumice, but the interior slopes of the crater expose layered reddish scoria and cinders.

Figure 3A.17.  A map showing latest Pleistocene and Holocene vents and lava flows on Newberry Volcano’s southeastern flank (modified from Jensen, 2006).

38.7 (0.7)     Excellent view of The Dome and its breached southeastern flank at 4:00.

38.9 (0.2)     A road cut to the left here reveals about five feet of tephra overlying an outcrop of dacite.  The pyroclastic material includes nearly 40 inches of basaltic tephras associated with eruption of The Dome, overlain by about 20 inches of silicic pumice lapilli and ash of the Newberry Pumice.

40.8 (1.9)     Junction of FS Rd 21 and FS Rd 9710.  Make a brief stop here.  There is a good view of Red Hill up this road to the north at about 9:00.  Red Hill is a pre-Mazama eruption basaltic cinder cone.  The vent underlying Red Hill is the source of the Red Hill Lava Flow, widespread on Newberry Volcano’s eastern flank (Figure 3A.17), much of it covered by a thin veneer of various younger pyroclastic units, including Mazama Ash and Newberry Pumice.

      Continue straight (east) on FS Rd 21 over several patches of Red Hill lavas.

42.5 (1.7)     Refer to Map 3A.5.  FS Rd 21 crosses onto a kipuka ridge of older lava flows here.

44.0 (1.5)     The road passes back onto the Red Hill Lava Flow at this location.

44.4 (0.4)     Once again, the road leaves the Red Hill lavas.

45.4 (1.0)     Junction of FS Rd 21 and FS Rd 18 (China Hat Rd).  Turn right (south) onto FS Rd 18.  This junction is the starting point for Field Trip 3B (which ends at Fort Rock at mile 78.6 of this field trip).  The Newberry Pumice is still over 40 inches thick here (Figure 3A.6).

If you wish to combine the first part of Field Trip 3A with Field Trip 3B to the Fort Rock-Christmas Valley area, turn left onto FS Rd 18 here and head north following the route of Field Trip 3B (which is described later in this guide).

FS Rd 18 curves around the western slopes of China Hat, a middle Pleistocene rhyolite dome (Figure 3.2), on the left and brings you shortly to the margin of the narrow southern lobe of the Red Hill Lava Flow on the right (Figure 3A.17), although exposure is poor because the lobe is generally buried by the Newberry Pumice.

47.1 (1.7)     Junction of FS Rd 18 and FS Rd 1850 on the left.  Remain on FS Rd 18.  This junction marks the approximate downslope position reached by the Red Hill Lava Flow (Figure 3A.17).

47.4 (0.3)     FS Rd 18 passes beyond the limits of the Newberry Pumice tephra plume near this point (Figure 3A.8).

47.8 (0.4)     A lobe of the Lava Pass Flow comes in from upslope to the right here (Map 3A.5), but doesn’t reach the road; the small lobe ends just shy of China Hat Campground, which you’ll find to your right shortly.  This campground is typical of eastside fair; no facilities, just a patch of ground for a tent or RV (they are free though).

48.6 (0.8)     Outcrops of the Lower East Flank Tuff begin to appear intermittently along the road here, mostly to the right-hand side.  This unit is a dark colored ash-flow tuff containing abundant gray to black pumice lapilli in an ashy lithic-rich matrix with broad, but scattered distribution on Newberry Volcano’s eastern to southeastern slopes (Jensen, 2006).  Its age is unknown.

49.4 (0.8)     The best outcrop of Lower East Flank Tuff exposed along FS Rd 18 lies to the right side of the road here.

51.0 (1.6)     Refer to Map 3A.6.  Lava Pass; FS Rd 18 begins crossing the Lava Pass Flow at this location.  Pull your vehicle to the edge of the road for a brief stop.  Examine the upslope face of the road cut; note the rubbly flow top compared to the denser lavas of the flow’s interior exposed lower in the cut (Figure 3A.18).

Figure 3A.18.  The rubbly flow top and denser interior lavas of the Lava Pass Flow exposed in a road cut on FS Rd 18 present a stratigraphy common to basaltic lava flows.

This stratigraphy is common to basaltic lava flows where the flow interior is exposed.  Notice the presence of coarse, silicic ash within the interstices of the dark, crumbly lava.  This is Mazama Ash that has sifted down into the porous upper layer of the flow indicating a minimum age for the lava flow of 7,700 years.  This narrow ribbon of lava connects the main portion of the flow with a large lobe of lava that extends several miles further downslope and onto quite gentle terrain (Figure 3A.17).  The Lava Pass Flow issued from Weasel Butte about eight miles upslope, high on Newberry’s southeastern flank.  Lava flowing over steeper slopes formed major gutter systems that helped transport lava a considerable distance.  Downslope, the flows tended to stack up and become inflated as fluid lava poured in from higher terrain; well-preserved inflation structures characterize the low-gradient flow lobe to the southeast (not visible from here).  Relief on the Lava Pass Flow is clearly visible on Map 3A.6.

As you continue to drive south, you’ll gradually descend into the Fort Rock basin, the northwestern extension of the hydrologically connected compound Fort Rock-Christmas Valley-Silver Lake basin.  Note the change in vegetation as you descend into the much drier climes of the Oregon High Desert country.

52.0 (1.0)     Another narrow cascade of the Lava Pass Flow reaches nearly to the road here (look carefully through the trees on the right; Map 3A.6).

52.9 (0.9)     FS Rd 2236-800 merges with FS Rd 18 here.  A short distant down this road lies the terminus of the Cinder Cone Flow.  Onlapping relationships between the lava flows in this area indicate that the Lava Pass Flow is slightly younger than the Cinder Cone Flow (Figure 3A.17).

54.1 (1.2)     Intersection of FS Rd 18 and FS Rd 22; FS Rd 18 becomes Cabin Lake Rd here.  Turn right (west) onto FS Rd 22 for a short excursion to South Ice Cave.

55.3 (1.2)     The parking area for South Ice Cave is on the left.  Turn in here and park.  Take time to explore this short, but interesting, lava tube which contains patches of perennial ice.  You will need your own headlamp or propane lantern to visit this cave.

South Ice Cave was discovered during timber cruising operations in 1927.  The cave occurs in two sections separated by a collapsed area now floored by breakdown material and soil that serves as the entrance for both (Figure 3A.19).  The northwest section is about 350 feet long and has one area of perennial floor ice after passing through a narrow crawlway.  The southeastern section is more easily accessed and is 650 feet long.  A large area of perennial wall and floor ice is your reward after first crossing an area of seasonal ice (or water) and through a low passage partially filled with breakdown rubble (Figure 3A.20).  The ice found in this cave forms in the spring when the outside air temperatures are high enough to melt snow, allowing meltwater into the cave, but due to the insolating lavas overhead, the inside air temperatures are cold enough to cause the incoming meltwater to refreeze on the cave walls and floor (Larson, 1982).

Figure 3A.19.  Map (A) and cross-section (B) of South Ice Cave.

Figure 3A.20.  Perennial ice buildup within South Ice Cave (this photograph was taken in early August).

Lava tube formation is unique, but vital to the process of removing lava from the near vent area during relatively fluid, mafic, volcanic eruptions.  Active tube formation has been described in detail by Peterson et al. (1994).  The most significant factor in the formation of lava tubes is the viscosity of the extruded lava.  More fluid lavas are less viscous.  In general, lower viscosity lava occurs with greater heat content (higher temperature), greater gas content, and lower silica content.  Lower temperatures, lower gas content, and higher silica promotes mineral crystallization within the lava, and the lava solidifies too quickly and too thoroughly for tube formation.   Another important factor in lava tube formation is the rate-volume relationship of the flowing lava.  Low to moderate rates of lava production from the vent, sustained for long periods of time, work best to develop extensive tube systems.

During a prolonged eruption, lava initially spills out from a fissure, crater, or lava lake over a broad area as a thin sheet, each cooling and solidifying at relatively short distances from the vent.  As flow continues, lava becomes channelized in the depressions between previously formed sheets and begins to flow progressively greater distances from the vent.  Channelized flow and channel formation is a critical first step in lava tube development, but flow from the vent must be sustained for hours to days for one or more of four significant processes to begin actively forming tubes.

The formation of a lava tube may result from the growth of a surface crust rooted to the banks of a lava stream.  If a lava stream flows at a nearly constant rate, volume, and height within the channel for a considerable period of time, lava will cool and solidify at the surface, adhering in thin layers to the banks.  As flow is maintained, lava will continue to accrete to these outwardly growing margins, gradually forming a crust over the entire surface and an incipient lava tube.  If the surface level of the lava stream remains relatively constant, the roof of the lava tube will thicken and grow in the downstream direction.  If the height of lava in the stream abruptly decreases, the fragile roof of the tube will collapse; and if the height abruptly increases, the roof will be torn loose from the lava stream’s banks and carried away in the flow.  Eventually, the roof becomes strong and extensive enough to suspend itself without the support of the underlying flow, and the lava tube is firmly established.

Lava tube formation may also result from the accretion of stream bank levees when flow in the channel fluctuates significantly in rate, volume, and height.  During successive fluctuations, lava is accreted to the upper and inner edges of levees perched on the channel banks and the levees gradually arch toward each other over the lava stream.  Eventually, the slot between approaching levees grows narrow enough, so that the next surge of lava in the stream squeezes into the gap and solidifies, forming a seal over the channel and an incipient lava tube.  This process of tube formation may occur to a limited extent nearer to the vent, where splashing and spattering of lava is enhanced by the vigorous pulsating of gas-rich lava.

Formation of a lava tube may result from the accumulation of rafted slabs of chilled, semi-solid lava at channel bends and/or constrictions in conjunction with other processes.  In this process, a skin of congealed lava forms on the surface of the lava stream where flow is nonturbulent (flow that is not mixing).  Initially, these skins are thin and flexible, able to deform, bending through curves and/or squeezing through constrictions in the channel.  As the raft thickens and becomes more rigid, it will eventually get stuck at a particularly sharp bend or in a narrow constriction, becoming the roof of an incipient lava tube.  Flow of lava in the stream continues to jam additional rafts behind the new roof, thickening it, and extending it upstream.  Individual slabs gradually lose their distinctiveness as the rafts are contorted and fused together.

Where topographic gradient decreases, lava emerges at the lower end of a tube to produce laterally spreading fields of pahoehoe not confined in well-defined channels.  The surface of the spreading lava cools and solidifies to form a crust.  Molten lava beneath this crust continues to move outward and the overlying crust bulges upward.  In this way, small, distributary lava tubes may form.  Some of these tubes stagnate and congeal; others remain active, conducting lava to the flow perimeter and forming new flow lobes, incorporated into and extending the lava tube system.  Molten lava may split the overlying crust and pour out to form new flow lobes that crust over and branch again.  Younger flows are often stacked upon older flows and sometimes develop their own lava tubes fed by those below.  This process is continually repeated at the expanding flow front, creating an extremely complex system of relatively small, short distributary tubes.

This tendency for lava streams to encase themselves in tubes enables the molten lava to retain heat content and remain fluid because the congealed crust acts as an insulator.  Insulation, heat retention, and fluidity allow copious volumes of lava extruded at low to moderate rates to flow great distances and cover large areas.  In contrast, lava of greater viscosity or erupted too rapidly tends to solidify, reducing its ability to spread.  The molten lava in large, long-lived lava tubes will gradual decline in volume and height as the vent extrusion rate decreases.  The upper portion of the tube system often drains to preserve an empty lava tube, but the lower portion may not drain at all.  The lava stops moving, solidifying to become the floor of the tube.  Occasionally, lava tubes may become reoccupied by flows from subsequent eruptive periods.

Return to your vehicle and proceed back to the intersection of FS Rd 22 and FS Rd 18.

56.5 (1.2)     Back to the intersection of FS Rd 22 and FS Rd 18 (Cabin Lake Rd). Turn right (south) on FS Rd 18.  FS Rd 18 now makes a lengthy traverse of older Pleistocene lava flows from Newberry Volcano.

62.3 (5.8)     Refer to Map 3A.7.  Junction of FS Rd 18 and FS Rd 2435.  Turn right (west) onto FS Rd 2435 to visit Flat Top, the first of several unique volcanic maars, features exclusive to the greater Fort Rock-Christmas Valley-Silver Lake basin that you have now entered.

63.9 (1.6)     Junction of FS Rd 2435 and FS Rd 2440-760; the latter road leads to Flat Top Cinder Pit.  Continue on FS Rd 2435.  The cinder quarry is located on a small, youthful cinder cone erupted on the eroded flank of the Flat Top tuff cone and is unrelated to volcanism that produced the tuff cone (Jensen, 2006).

Shortly, the road emerges from the trees and offers a great view of Flat Top’s formidable edifice (also prominent in the upper left corner of Map 3A.7).  Flat Top is a type of basaltic maar volcano known as a tuff cone.  The tuff cone is the northernmost maar volcano in the Fort Rock-Christmas Valley-Silver Lake basin, erupted on the periphery of pluvial Fort Rock Lake, likely where groundwater saturated sediments occurred close to the lake’s edge.  Figure 3A.21 uses Flat Top to illustrate typical stages in the formation of a tuff cone.  During a volcanic eruption, mafic magma rises toward the surface and encounters groundwater saturated sediment at shallow depths to produce violent steam explosions and abundant pyroclastic material (basaltic tuff) which accumulates as a ring around the vent.  If the water supplied to the vent runs out or is sealed off, magma can rise uninterrupted into the center of the tuff ring, forming a lava lake.  After cessation of volcanism, cooling and solidification of the lava forms a hard, dense, resistant plug of basalt capping the central portion of the tuff ring.  Eventual erosion removes the outer ring of tuff to preserve a flat topped, conical hill called a tuff cone.  In Flat Top’s case, lavas actually breached the northwest side of the tuff ring during eruptions to flow down and across the tuff and onto the surrounding terrain, coating that flank of the tuff cone in a resistant cap of basalts, and better preserving this portion of the tuff cone from erosion.  Flat Top is a relatively old maar, its southeast flank has been significantly eroded, partially exposing the internal stratigraphy of the tuff cone,  and its lower flanks are surrounded and onlapped by younger Newberry lava flows.  Flat Top can be seen from the rims of Fort Rock and Hole-in-the Ground, two other maar volcanoes visited later on this field trip.

Figure 3.3A.21.  Development of Flat Top, a tuff cone, one variety of maar volcano to be observed on this field trip (modified from Jensen, 2006).

64.2 (0.3)     Junction of FS Rd 2435 and FS Rd 2435-870.  FS Rd 2435-870 also leads to the Flat Top Cinder Pit.  Continue on FS Rd 2435.  Intermittent views of Flat Top lie to your right.

64.6 (0.4)     Junction of FS Rd 2435 and FS Rd 3145; ignore FS Rd 3145 and continue forward.  The thick, columnar jointed “lava lake” basalts that cap Flat Top come into view on your right.

64.8 (0.2)     Several pressure ridges in Newberry lavas can be observed to the left of the road here.

65.5 (0.7)     The road passes tuffaceous basal surge deposits on the right that mark the outer perimeter of the Flat Top tuff cone.  Notice that the deposits are dipping outward, indicating that they formed on the outer slope of the tuff ring that was initially deposited around the vent.

65.8 (0.3)     Drive past the cattleguard, then park along the roadside near the two-track on the right (this faint road is FS Rd 2435-650).  Use this road to access Flat Top’s summit and explore the composition of a tuff cone (Figure 3A.22).  Walk upslope; you’ll begin passing tuffaceous deposits in about a quarter mile, these too are dipping outward.  Continue upslope, but head to the right toward more outcrops of tuff.  Between four- and five-tenths of a mile, you should pass more tuffaceous deposits with a distinctive inward dip (Figure 3A.22a).  The dip indicates that these sediments were deposited on the inner slope of the tuff ring.  Work your way to the summit by locating a route up through the broken down jumble of columnar basalt blocks that form the flat lying cap rock, once the lava lake of the tuff cone (Figure 3A.22b).  Once at the summit, take in the views of Fort Rock basin to the south as you walk to the left until your see the road below and negotiate your way down to it.  Follow the road back to your vehicle.  The hike is about one and a quarter miles in length.

Figure 3.3A.22.  Tuffaceous deposits, these dipping toward the tuff ring’s interior (A) and columnar basalts comprising the lava lake cap rock (B) make up the primary constituents of the Flat Top tuff cone.

Turn around at your leisure and return to the junction of FS Rd 2435 with FS Rd 18.

69.5 (3.7)     Junction of FS Rd 2435 and FS Rd 18.  Turn right (south) onto FS Rd 18.

69.6 (0.1)     FS Rd 18 passes Cabin Lake Campground to the west; again don’t expect much, but it is a rare designated campground in this area.

70.1 (0.5)     Junction of FS Rd 18 and FS Rd 1800-970 to the Cabin Lake Guard Station; remain on FS Rd 18.  Jensen (2006) reports that a water well drilled at this site encountered about 90 feet of basaltic lavas and interlayered silty sand before penetrating into uniform sandy deposits.  He believes the sandy sediment at the base of the well may be lacustrine in origin, indicating the pluvial lakes occupying the Fort Rock basin likely reached at least this elevation.  Overlying lavas exhibit no evidence of having encountered water, so any former lake was reduced in size before arrival of Newberry lavas.

70.4 (0.3)     Deschutes National Forest Boundary; FS Rd 18 becomes Lake County Rd 5-11 (Cabin Lake Rd).  Ephemeral Cabin Lake lies to the left.

72.0 (1.6)     Here, the road affords a nice view of Flat Top tuff cone to the northwest.

73.8 (1.8)     Lake County Rd 5-11 crosses a small ridge here which offers an excellent view south into the Fort Rock basin.  Fort Rock tuff ring, another maar volcano, lies to the southwest.

75.2 (1.4)     The road leaves Newberry lava flows at this point and crosses onto the sedimentary fill of the Fort Rock basin; notice the fairly abrupt transition from the crenulated topography of the lavas to the relatively smooth terrain of the sedimentary fill.  Lava flows from Newberry Volcano’s southeastern flank buried any evidence of pluvial lake shorelines in this area (Allison, 1979).

78.6 (3.4)     Refer to Map 3A.8.  Junction of Lake County Rd 5-11 (Cabin Lake Rd) and Lake County Rd 5-11-A (Cow Lake Lane).  Turn left onto Lake County Rd 5-11-A toward Fort Rock State Monument.

79.1 (0.5)     Entrance road to Fort Rock State Monument on the right.  Turn in here and drive to the parking area.

Fort Rock (Figure 3A.23) is a maar volcano erupted within the late Pleistocene Fort Rock – Christmas Valley Lake itself (Peterson and Groh, 1963; Heiken et al., 1981); its tuff ring formed an island in the lake.  The wave-eroded remnant of the tuff ring is over 4200 feet in diameter and 200 feet high.  The southern flank of the ring has been breached by wave erosion and the ring interior now averages about 25 feet above the floor of the basin.  A classic wave-cut terrace and cliff occurs about 65 feet above the basin floor; easily observed at the margins of the breach (Figure 3A.24).  This bench occurs at an elevation of 4450 feet, evidence that this lake level, although not the highest recorded in the basin, was a significant one in the pluvial lake’s history.

Figure 3A.23.  Fort Rock Maar Volcano; this maar erupted within the pluvial lake basin, and was subsequently breached by wave action on the southern flank of its tuff ring.

Figure 3A.24.  A well-developed wave-cut bench on the outer flank of the Fort Rock maar.  This feature likely formed during the highstand of late Pleistocene, pluvial Fort Rock Lake.  Notice the notch formed at the same level on the far side of the breach in the tuff ring.

The Fort Rock maar is typical of the smaller, isolated maars of the basin (Heiken et al., 1981).  It is composed of orange-brown, bedded lapilli-tuff and tuff-breccia.  Tuff beds can be traced from within the crater to the outer flanks, the dip of the beds are parallel to the inner and outer walls of the tuff ring (Figure 3A.25), suggesting that the crater is funnel shaped.  The inner rim on the western side of the tuff ring displays a distinctive angular unconformity where the tuffaceous deposits are truncated and offset by slumping of younger beds into the crater.

Figure 3A.25.  The eastern wavecut cliff of Fort Rock’s tuff ring exhibits bedding within the tuff that dips to the right and left, parallel to the inner and outer walls of the tuff ring, respectively.

After exploring the tuff ring of the Fort Rock Maar, return to your vehicle and drive back to Fort Rock.

80.3 (1.2)     Junction of Lake County Rd 5-11-A (Cow Lake Lane) and Lake County Rd 5-11 (Cabin Lake Rd).  Turn right (south) onto Lake County Rd 5-11.

81.3 (1.0)     Junction of Lake County Rd 5-11 (Cabin Lake Rd) and Lake County Rd 5-10 (Fort  Rock Rd) in Fort Rock.  Turn right (west) onto Lake County Rd 5-10.

81.4 (0.1)     Junction of Lake County Rd 5-10 (Fort Rock Rd) and Lake County Rd 5-13.  Continue straight ahead (west).  A left turn here onto Lake County Rd 5-13 would begin Field Trip 5B in reverse direction (Field Trip 3B ends at this junction).

82.8 (1.4)     Entrance to a county Refuse Disposal Site; there is a superb view of the Fort Rock Maar from here.  Another smaller, isolated maar is visible further to the northwest.  A cave cut by wave action into the southern flank of this smaller maar contained tule reed sandals radiocarbon dated at about 9,000 years in age, providing further evidence of the occupation of this basin by early Native Americans.

84.4 (1.6)     The road climbs onto a low ridge here, formed by a spit extending between the small maar to the north and an uplifted ridge of basalt to the south.  This spit formed at the same major stillstand level of 4450 feet observed elsewhere for pluvial Fort Rock Lake.  In early evening sunlight, shading accentuates a succession of low beach ridges that can be observed on this eastern side of the spit; the shoreline features marking recession of the pluvial lake.

As you top the spit, look to the south at the flat-topped basaltic ridge close to you.  A wave-cut bench carved into its northeast side occurs at 4540 feet, the highstand of the compound late Pleistocene pluvial Fort Rock-Christmas Valley-Silver Lake.  Isolated locations such as this ridge provide evidence for this highest lake level and are rare in the basin (Allison, 1979).

84.8 (0.4)     A normal fault associated with the Walker Fault zone cuts basaltic lava flows here; the fault scarp is easily observed to the left at 9:00.  This fault forms the eastern margin of a shallow graben, the western margin is formed by Bell Rim, ahead and to the south.

86.4 (1.6)     Refer to Map 3A.9.  Bell Rim, a normal fault scarp and the western edge of the graben mentioned previously, cuts basaltic lava flows to the left at 9:00.

87.6 (1.2)     Junction of Lake County Rd 5-10 (Fort Rock Rd) and Oregon Hwy 31.  Turn right (northwest) onto Hwy 31.

88.3 (0.7)     The flat-topped ridge to the right at 3:00 is capped by the Peyerl Tuff.  Horse Ranch Canyon is ahead.  Road cuts on the right side of the road as you climb expose a 4 million year old pyroclastic ash-flow tuff and tuffaceous sediments, overlain by the welded Peyerl Tuff, in turn overlain by a basaltic lava flow.

91.6 (3.3)     Refer to Map 3A.10.  Junction of Oregon Hwy 31 and FS Rd 3145.  Turn right (east) onto FS Rd 3145 toward Hole-in-the-Ground.

92.4 (0.8)     “Y” junction of FS Rd 3145 and FS Rd 3145-300.  Take the left (northeast) fork and remain on FS Rd 3145.

92.7 (0.3)     “Y” junction of FS Rd 3145 and FS Rd 3130.  Turn left (northeast) onto FS Rd 3130.

93.6 (0.9)     Junction of FS Rd 3130 and 3130-400.  Turn right (east) onto FS Rd 3130-400 and climb to the rim of Hole-in-the-Ground.

93.9 (0.3)     Park in a convenient location along Hole-in-the-Ground’s rim, then walk around; Map 3A.8 shows an obvious cratered feature on the landscape and you’re now strolling on its rim.  Hiking the entire perimeter of the rim requires roughly an hour and provides some nice views of the Fort Rock-Christmas Valley basin to the east and Newberry Volcano’s cinder cone-dimpled southern flank to the north.  There are also several nice primitive camping sites back down the road among the ponderosa pine at the base of the rim.

Hole-in-the-Ground is a maar, or a volcanic explosion crater (Figure 3A.26), located on the edge of the Fort Rock -Christmas Valley basin (Peterson and Groh, 1961 and 1963; Lornez, 1971; and Heiken et al., 1981).  It probably formed while the adjacent basin was filled with a pluvial lake, perhaps as recently as about 13,000 years ago, but possibly as long as 50,000 to 100,000 years ago.  Therefore the site of the eruption was close to the shoreline, in groundwater-saturated rock and sediments.  The crater formed in only a few days to weeks by a series of hydrovolcanic explosions triggered by the rise of basaltic magma along a NW-SE trending fissure that came in contact with saturated sediment at depths of about 975 to 1640 feet.  Magma likely rose along the fault exposed in the crater walls.

Figure 3A.26.  A view of Hole-in-the-Ground’s tuff ring looking to the north; the dimpled profile of Newberry Volcano lies on the skyline.

Four major explosive events are recorded in the tuffaceous deposits around the crater, caused by repeated slumping and subsidence of tuffaceous debris into the vent after each explosive eruption that lead to changes in water supply and an accumulation of pressure in the volcanic pipe.  Ejected debris forms a rim about 110 to 210 feet above the original ground surface and the crater lies about 350 to 500 feet below the original surface. Most of this material is fine-grained ash, lapilli, and basaltic rock fragments formed by rapid quenching of magma in water, ejected with a slightly asymmetric distribution somewhat south of east in the prevailing wind direction.  The presence of beds of vesiculated tuff and accretionary lapilli suggest basal surges during eruptions.  Larger material, forming four layers of coarse lag nearer the vent, and composed mostly of blocks of older lava flows ripped from the volcanic pipe, reaches over 25 feet in diameter and was thrown as far as 2.3 miles downwind from the crater.  A geologic cross-section developed by Lorenz (1971) and modified by Heiken et al. (1981) based on geophysical data, drill holes, and surface exposures (Figure 3A.27), indicates a domal intrusion below the crater floor extending upward as ring dikes around the crater margin.  The crater is backfilled with a layer-cake of inward-dipping basalts and tuff.

Figure 3A.27.  Maar volcanoes typically exhibit explosion craters and an accumulation of pyroclastic material preserved as a tuff ring that results from a violent phreatomagmatic eruption related to the forceful injection of magma upward through water-saturated sediment and rock; this diagram is modeled after Hole-in-the-Ground (modified from Lorenz, 1971).

After taking in the view, return to your vehicle and proceed back to the junction of FS Rd 3145 and Oregon Highway 31.

96.2 (2.3)     Junction of FS Rd 3145 and Oregon Highway 31.  Turn right (northwest) onto Hwy 31 toward La Pine, Oregon.

98.7 (2.5)     Refer to Map 3A.11.  The road crosses a northeast trending normal faulthere associated with the Walker Fault Zone (note the prominent faults displayed on the map).

Several road cuts occur for about the next half mile through basaltic lava flow pressure ridges.  These are typical of the observable pressure ridges found in the area associated with older lava flows from the southern flank of Newberry Volcano.

101.8 (3.1)   FS Rd 2451-400 to the left.  This loop road provides access to Big Hole, another maar volcano crater formed by the eruption of magma in contact with groundwater-saturated material.  Big Hole is 6000 feet in diameter and about 425 feet deep, forming a nearly circular crater even larger than Hole-in-the-Ground (Map 3A.11).  Tuff ring deposits of bedded basaltic tuff and tuff-breccia cap the crater rim to depths of 100 feet and extend outward 6000 to 8000 feet.  Tuffaceous deposits are thicker on the northeast side of the rim forming Big Hole Butte, indicating the dominant direction of wind drift of the ejecta.

103.5 (1.7)   OR Hwy 31 crosses another normal fault at this location, displacement is down to the southeast.

103.9 (0.4)   The highway crosses the drainage divide between the Fort Rock-Christmas Valley-Silver Lake basin to the rear and the La Pine basin ahead.  Recall that the Fort Rock-Christmas Valley-Silver Lake basin is the northwesternmost internally drained basin of the Great Basin, while the La Pine basin is drained to the Pacific Ocean via the Deschutes and Columbia Rivers.  These basins may have been intermittently connected by outflow from high stands of pluvial Fort Rock-Christmas Valley-Silver Lake during the Pleistocene (Allison, 1979).

105.3 (1.4)   Although not obvious from the ground at this location, OR Hwy 31 crosses another normal fault associated with the Brothers Fault Zone (Map 3A.11).  This time displacement is down to the northwest.

106.1 (0.8)   Refer to Map 3A.12.  A road cut here exposes a prominent pressure ridge in a Newberry lava flow.

107.7 (1.6)   A road cut here passes through a NE-SW trending fault scarp associated with the Walker Rim Fault Zone.  Although not topographically significant, this is a rare location where the fault displacement (down to the northwest) can be observed in cross-section.

108.3 (0.6)   The road passes through another substantial pressure ridge in a Newberry lava flow.

110.4 (2.1)   Moffit Butte can be observed intermittently to the right for about the next mile.  The butte is named after James A. Moffit, an early homesteader on Long Prairie near La Pine, Oregon in the 1880s.  Moffit Butte is another maar volcano similar to Big Hole and Hole-in-the-Ground in that it formed from a hydrovolcanic eruption involving the rise of magma upward into contact with groundwater saturated sediment (Heiken et al., 1981).  The maar volcano’s main tuff ring is about 400 ft. high and 4600 ft. in diameter; a smaller tuff ring about 1700 ft. in diameter on Moffit Butte’s southern flank is filled with lava that erupted from a dike on its northwest edge.  Moffit Butte and other small maar volcanoes are located along a southwesterly trend that possibly indicates a Pleistocene age drainage course connecting the compound pluvial Fort Rock-Christmas Valley-Silver Lake basin and the La Pine basin (Jensen, 2006).

110.8 (0.4)   Refer to Map 3A.13.  OR Hwy 31 crosses a small draw here as it begins passing around the steep western side of Moffit Butte.  The road abruptly enters the La Pine basin at this point.

113.6 (2.8)   Leaving Deschutes National Forest.

118.4 (4.8)   Refer to Map 3A.14.  Entering Long Prairie; once the bed of a fairly large stream (Allison, 1979).  This location may have been the overflow stream channel for Pleistocene pluvial lakes occupying the Fort Rock-Christmas Valley-Silver Lake basin.  The bones of fossil salmon found in the Fossil Lake subbasin indicate that this pluvial lake system must once have had an outlet to the Pacific Ocean.  Jensen (2006) further suggests that the presence of a species of snail in the pluvial lake basin which today only occurs in the Columbia River watershed is additional evidence for this connection to the Pacific.

120.2 (1.8)   Refer to Map 3A.15.  The road crosses the center of Long Prairie here.

121.4 (1.2)   Intersection of Oregon Hwy 31 and U.S. Hwy 97.  Turn right (north) onto Hwy 97.

122.7 (1.3)   Entering La Pine, Oregon.

123.6 (0.9)   Refer to Map 3A.1.  Intersection of U.S. Hwy 97 and 1st Street (Reed Rd).  This is the end of Field Trip 3A.

Road Route Maps

Map 3A.1.  Color shaded-relief map of the Findley Butte 7.5” Quadrangle containing segments of Field Trip 2A and Field Trip 3A.

Map 3A.2.  Color shaded-relief map of the Anns Butte 7.5” Quadrangle containing segments of Field Trip 1A and 1B,  Field Trip 2A, and Field Trip 3A.

Map 3A.3.  Color shaded-relief map of the Paulina Peak 7.5” Quadrangle containing a segment of Field Trip 3A.

Map 3A.4.  Color shaded-relief map of the East Lake 7.5” Quadrangle containing a segment of Field Trip 3A.

Map 3A.5.  Color shaded-relief map of the China Hat 7.5” Quadrangle containing segments of Field Trip 3A and 3B.

Map 3A.6.  Color shaded-relief map of the South Ice Cave 7.5” Quadrangle containing a segment of Field Trip 3A.

Map 3A.7.  Color shaded-relief map of the Cabin Lake 7.5” Quadrangle containing a segment of Field Trip 3A.

Map 3A.8.  Color shaded-relief map of the Fort Rock 7.5” Quadrangle containing segments of Field Trip 3A and 3B.

Map 3A.9.  Color shaded-relief map of the McCarty Butte 7.5” Quadrangle containing a segment of Field Trip 3A.

Map 3A.10.  Color shaded-relief map of the Hole-in-the-Ground 7.5” Quadrangle containing a segment of Field Trip 3A.

Map 3A.11.  Color shaded-relief map of the Big Hole 7.5” Quadrangle containing a segment of Field Trip 3A.

Map 3A.12.  Color shaded-relief map of the Grass Well 7.5” Quadrangle containing a segment of Field Trip 3A.

Map 3A.13.  Color shaded-relief map of the Moffit Butte 7.5” Quadrangle containing a segment of Field Trip 3A.

Map 3A.14.  Color shaded-relief map of the Masten Butte 7.5” Quadrangle containing segments of Field Trip 2A and Field Trip 3A.

Map 3A.15.  Color shaded-relief map of the La Pine 7.5” Quadrangle containing segments of Field Trip 2A and Field Trip 3A.

Optional Hiking Trails

Big Obsidian Flow Trail (Tr 3A.1)

This is a short hike of about three-quarters of a mile round-trip on a trail that provides access to some really amazing geological features.  It traverses a portion of the Big Obsidian Flow (Map 3A.1.1), one of several young, silicic lava flows within Newberry Volcano’s caldera (Figure 3A.1).  Several interpretive signs provide information on the geology observed along the trail.  It is designed to be family friendly, but be aware that it climbs over loose, jagged rock that can easily cut feet (that are not adorned with proper footwear) and exposed hands, arms, and legs (should you fall).

Start by hiking on a paved path past the picnic area at the south end of the parking area for about 600 feet.  Climb a series of metal stair cases up onto the gray, blocky lava flow, studded with glistening slabs of black volcanic glass.  The flow front is particularly steep because of the extremely viscous nature of this silicic lava.  To the left of the stairs, Lost Lake sits in the crater of a tuff ring partially filled by a lobe of the Big Obsidian Flow where it spilled over the tuff-ring’s southern rim (Figure 3A.9 and Figure 3A.1.1).  After about two-tenths of a mile, the trail levels out and breaks into a loop, follow it counterclockwise to your right.  As you walk along, note the many blocks of frothy pumice and glassy obsidian that make up the rhyolitic lava flow’s surface, some exhibiting interesting flow features that exemplify its pasty nature (Figure 3A.1.2).  The “foaminess” of the lava is related to the concentration of water (as tiny bubbles of steam) within, greater gas content causes the lighter colored, grayish pumice.  At about four-tenths of a mile, a short spur trail to the left takes you to a viewing platform.  This platform affords wonderful views to the north and east of Paulina Lake, Central Pumice Cone and Little Crater, and East Lake (Figure 3A.1).  Lost Lake Pumice Ring, containing Lost Lake and partially filled by the Big Obsidian Flow, lies immediately downslope to the northeast.

Figure 3A.1.1.  Lost Lake and the Big Obsidian Lava Flow.  The steep front of the flow is a result of the viscous nature of this silicic lava.  The lake is confined within an old tuff ring, invaded from the south by the obsidian flow.

Figure 3A.1.2.  The surface of the Big Obsidian Flow is comprised of large blocks of pumice and obsidian; some blocks exhibit finer taffy-like flow structures of denser obsidian within a pumiceous matrix (penny for scale) (A), and may contain inclusions of fine-grained rhyolite (dime for scale) (B).

From this location, it is a relatively simple matter to find the nearest of the giant gas bubbles preserved in the lava flow that were described earlier (Figure 3A.9 and Figure 3A.12).  Look toward Paulina Peak.  You should see a tall rock protruding out of the flow with a small bulbous knob sticking out from it.  Just to the left are three scraggly limber pines, the center, more “Christmas tree-like” of the three, is aligned dead-on with the gas bubble.  Keeping the tree and observation platform in view, walk about 150 feet off the platform to the southwest (toward the tree). A note of caution: watch your step on the rough surface of the flow!  The gas bubble is in the bottom of a small depression in the lava flow surface (Figure 3A.12).  Snow often remains well into the summer inside the bubble.  Climb carefully down into it.  You should immediately notice its smooth walls and ceiling, and its semi-circular shape.  Now try to imagine cutting a soap bubble in half, freezing it, and climbing inside.  Return to the viewing platform and the main loop trail; continue counterclockwise around the trail to close the loop.  From here, simply descend the trail back to the parking lot and your vehicle.

Hiking Trail Map

Map 3A.1.1.  Color shaded-relief map of portions of the East Lake, Fuzztail Butte, and Paulina Lake 7.5” Quadrangles containing multiple hiking trails (Tr 3A.1-3A.6) within Newberry Volcano’s caldera.

Cinder Hill Trail (Tr 3A.2)

This trail climbs to a basaltic andesite cinder cone (the Sheep’s Rump) perched on the inner slope of the northeastern rim of Newberry’s caldera (Map 3A.1.2), a total distance out and back of around six and three-quarters miles.  The Sheep’s Rump is a late Pleistocene or early Holocene (pre-Mazama eruption) cinder cone, probably associated with volcanism along the ring fracture system that includes the East Rim Fissure, also on the caldera rim, as well as The Dome, Sand Butte, and other cinder cones on Newberry’s southeastern flank (Figure 3A.1).  The uniqueness of the cinder cone’s position, combined with the stellar views of East Lake and several of the large obsidian flows filling the caldera, makes the hike well worth it.

Begin your ascent to the caldera rim at the trailhead sign immediately to your right across the road from the day use parking area.  After a brief 500 feet, head left on the main trail coming from the south end of the campground.  In about four-tenths of a mile, a secondary access trail from the north end of the Cinder Hill campground merges from the left, but continue straight on the main trail.  In about one tenth of a mile, an outcrop of basaltic andesite from the lava flow associated with the Sheep’s Rump can be seen to the left of the trail (much of the lava flow is covered in a veneer of various younger pyroclastic units, including Mazama Ash and Newberry Pumice).  The trail begins to rise more rapidly toward the caldera rim after at about eight-tenths of a mile.  You have been traversing a lodgepole pine forest, but the trail soon enters a narrow valley formed between a rhyolite flow to the left and the older volcanic rock of the eastern caldera wall to the right.  Increased shading here forms a cooler microclimate and greater soil moisture retention and the vegetation changes dramatically to Douglas Fir, with some western hemlock, subalpine fir, and lodgepole.  At roughly one and nine-tenths of a mile, outcrops of silicic volcanic rock (obsidian and rhyolite) can be observed to the left of the trail associated with post-caldera lava dome formation.

The Cinder Hill Trail joins the east-west Crater Rim Trail at about two and one-tenth miles.  Turn right (east) on the Crater Rim Trail for the remainder of the distance to the Cinder Hill overlook at the Sheep’s Rump.  This portion of the trail is a fairly unremarkable trek through old lodgepole pine forest with little to view.  In roughly one and a quarter miles, you arrive at your destination.  The cinder cone here (Cinder Hill) is perched on the inner edge of the caldera rim.  The cinder cone is breached in the downslope direction; scoria and cinders were ejected from the vent and accumulated on the upslope, windward side, while a small tongue of basaltic andesite lava poured over the edge on the downslope side and into the caldera below.

The views from this vantage point are marvelous.  Stratovolcanoes perched on the Cascade Crest can be seen on the skyline.  Nearer to you, much of Newberry Volcano’s caldera lies at your feet (Figure 3A.2.1); you can almost reach out a grab Paulina Peak, the Big Obsidian Flow and the Central Pumice Cone (Figure 3A.2.2).  Paulina Peak’s rhyolite dome rises prominently from the southeastern caldera.  The peak and caldera rim to the east were sculpted into a steep cliff by glacial erosion.  The Central Pumice Cone rises prominently from the middle of the caldera, separating Paulina and East Lakes.  East Lake is laid out serenely in the left foreground, and all three major obsidian flows within the caldera (Figure 3A.1), the Big Obsidian Flow, the Interlake Flow, and the East Lake Flows, can be observed around its further margin.

When you have finish gazing, retrace your steps from here back to the trailhead at the Cinder Hill Day Use Area.

Figure 3A.2.1.  The Sheep’s Rump on Newberry Volcano’s eastern caldera rim affords spectacular views of Paulina Peak in the background, East Lake in the foreground, and the caldera filling Central Pumice Cone and obsidian flows in the middleground.

Figure 3A.2.2.  Paulina Peak, the Big Obsidian Flow, and the Central Pumice Cone from Cinder Hill.

Hiking Trail Map

Map 3A.2.1.  Color shaded-relief map of portions of the East Lake, Fuzztail Butte, and Paulina Lake 7.5” Quadrangles containing multiple hiking trails (Tr 3A.1-3A.6) within Newberry Volcano’s caldera.

Crater Rim and Lost Lake Loop Trail (Tr 3A.3)

This hike follows the Crater Rim Trail (misnamed, “Caldera” would be more appropriate) along Newberry Volcano’s southwestern caldera rim to a vantage point directly overlooking the Big Obsidian Flow (Map 3A.3.1).  After taking in this magnificent view, the hike returns part way on the Crater Rim Trail, and then descends the Lost Lake Trail along the edge of the Big Obsidian Flow to join with the Pumice Flat Trail at the west end of Pumice Flat.  After a quick side trip to an overlook of the obsidian dome built on the source vent for the Big Obsidian Flow, the hike returns to Pumice Flat and traverses along Pumice Flat Trail to the east. This trail eventually rejoins the Crater Rim Crater and after a short distance, the hike ends where it began.  This hike is roughly eight miles in length, not short by any means, and it does involve some elevation gain, but its climbing sections have fairly gentle grades.  The aerial views of Newberry’s caldera, close up views of the Big Obsidian Flow, and the hike across Pumice Flat, just downwind from the vent and covered in pumice lapilli as large as cantaloupes, make this a wonderful, geologically significant hike.

Begin at the trailhead sign immediately to the right (southwest) side of the FS Rd 21 (Map 3A.3.1).  The trail climbs gradually along the outer perimeter of the caldera rim, providing ample views to the south and southeast of Newberry Volcano’s cinder cone-dimpled flank and on into the ancient pluvial lake, Fort Rock-Christmas Valley basin (Figure 3.1).  Initially, intermittent views of The Dome lie to the left.  The west slope of this young cinder cone is covered in Newberry Pumice ejected during the Big Obsidian eruption.  A thick layer of this pumiceous material lies at your feet, blanketing the caldera rim in this area.  Scraggly lodgepole pine and Douglas-fir cling to this poor, silicic soil.  At roughly six-tenths of a mile, Sand Butte, another Holocene age cinder cone comes into view to your left, accompanied by a line of similar cones marching to the southwest.  These likely formed along a fault-controlled lineament.  In about one mile, you come to a trail junction in a narrow, nearly east-west running valley cut into the caldera’s rim.  This valley formed by erosion along a caldera ring fault (Figure 3A.1).  The Crater Rim Trail continues to the left and the Pumice Flat Trail climbs into the valley to the right.

Remain on the Crater Rim Trail, and after switchbacking up the outer slope of the upper caldera rim, the trail rapidly reaches the ridgeline in about one-tenth of a mile.  Views to the north of the Paulina Mountains and Pumice Flat are generally obscured by trees; however, views to the south begin to open up.  In the background, you can see the Fort Rock-Christmas Valley basin, on a clear day the Flat Top and Fort Rock maar volcanoes are prominently displayed at the basin’s northern margin.  In the middle ground, the silicic dome complex of Willow Butte and Indian Buttes is clearly visible, disrupted by the northeasternmost normal-faults of the Walker Fault Zone (Figure 3.3); while in the foreground, several chains of aligned cinder cones cover the flanks of Newberry Volcano (Figure 3.2).  An easily recognized chain of vents that begins with the East Rim Fissure and passes through The Dome, Sand Butte, and other cinder cones, gently sweeps to the southwest down Newberry’s flank.  Also to the southeast along a line of sight with The Dome is Weasel Butte, the source of the Lava Pass Lava Flow.  As you move up the ridge, it becomes progressively covered with more and larger pumice lapilli and obsidian fragments of the Big Obsidian eruption.

The trail gradually slips from the caldera rim, but corrects itself with a couple of switchbacks at about one and six-tenths of a mile.  The trail continues along the caldera rim, gradually climbing to a highpoint on the ridge and then descending into a saddle.  The rim is fairly narrow here, but gently sloped to either side, offering occasional views of the distant Cascade Crest.  The rim here is covered in a forest of stunted lodgepole and limber pine; many of the older trees are dead, a result of pine beetle infestation.  When the trail dips to the inner side of the ridge at about two miles, more Douglas-fir indicate moister soil conditions on this north-facing slope.  The Crater Rim Trail descends into a saddle in the rim at roughy two and four-tenths of a mile.

The Lost Lake Trail veers to the right at about two and nine-tenths miles from the trailhead, just after ascending from the saddle, but remain on the Crater Rim Trail for now (Map 3A.3.1).  The trail continues along the ridgeline, climbing gently.  In another six-tenths a mile beyond the trail junction, the trail angles sharply to the right and approaches closely to the caldera rim just before straightening out to encounter a bald, grassy hill on the right; this is your destination.  Your view to the south along this exposed, grassy slope as you mount the hill is excellent.  Ascend upward to the north and to the edge of the caldera rim at the highest point on the promontory.  Watch your step on the scree of the steep inner slope of the ridge, but you should be able to find the perfect spot for marvelously unobstructed views into Newberry’s caldera and directly onto the Big Obsidian Flow (Figure 3A.9 and Figure 3A.3.1).  The obsidian dome, built over the Big Obsidian Flow’s vent, lies just to the northeast with East Lake in the background.  Particularly impressive are the flow ridges spreading in ever larger curves outward from the dome and downward along the length of the lava flow.  These flow ridges formed as the cooler, more solid, outer surface of the lava flow was pushed into a series of wave-like crests by downslope movement within the hotter, mushy plastic interior of the Big Obsidian Flow.  Try to mentally trace lines perpendicular to these ridges in order to estimate flow directions outward from the dome and source vent.  Look carefully, clearly the obsidian dome sits toward the outer and upper eastern margin of the lava flow.  Obsidian backed up to fill the western end of the basin formed by the southern caldera rim and the Paulina Mountains, but mainly poured downslope to the north and west toward Paulina Lake.  Pumice Flat lies immediately east (and downwind) of the dome and vent source (Figure 3A.3.1), directly in the path of the tephra plume generated during the early stages of the Big Obsidian Eruptive Period (Figure 3A.8).  Keep this view in mind for future reference.

Figure 3A.3.1.  An impromptu overlook along the Crater Rim Trail offers an incomparable view into Newberry Volcano’s caldera.  Paulina Peak is the promontory on the caldera rim to your left.  Paulina Lake and East Lake lie in the background, separated by intracaldera volcanics, including the Central Pumice Cone and Little Crater.  The Big Obsidian Lava Flow lies directly below your lookout; note the prominent dome formed over the vent area on the upper east side of the flow and the curvilinear flow bands indicating that the lava spilled mainly downslope to the north and northwest.  The white splotch right of the flow is Pumice Flat, thickly blanketed by pumiceous tephra from the Big Obsidian eruption.

To the northwest, Paulina Peak’s steep, rugged inner-rim face can be observed.  Osborn and Bevis (2001) suggest that this erosion is the product of heightened freeze-thaw action associated with one or more minor episodes of glaciation.  Protalus ramparts, arcuate ridges of coarse, bouldery material, can be found at the base of the cliffs below Paulina Peak. These ridges are the product of debris accumulating at the outer margin of perennial snow fields that would have formed in cooler climates along the sunlight-protected northeast facing slope below Paulina Peak.  Paulina Lake and the northwestern caldera rim can be seen in the middle distance, and the High Cascades strato-volcanoes from Mt. Bachelor to Mt. Jefferson are on the skyline.

Retrace your steps about four and three-tenths miles back to the trail junction of the Crater Rim Trail and the Lost Lake Trail.  Once there, turn left and follow the Lost Lake Trail as it descends from the caldera rim to the western edge of Pumice Flat near the eastern margin of the Big Obsidian Flow (Figure 3A.9 and Map 3A.3.1).  In about three quarters of a mile, you will arrive at another trail junction for the Lost Lake Trail and Pumice Flat Trail.  Continue straight ahead on the Lost Lake Trail as it follows along the eastern side of the Big Obsidian Flow, (eventually it would lead back to the caldera floor at the Big Obsidian Flow parking area).  Hike about two-tenths of a mile, gradually climbing a low hill, to where you should see a small promontory a short distance to the left of the trail.  Walk about 100 feet left of the trail to the promontory which occurs at the edge of a low cliff overlooking the Big Obsidian Flow.  This vantage point affords an amazingly intimate view of the obsidian dome overlying the source vent of the Big Obsidian Flow (Figure 3A.9 and Figure 3A.3.2).  Notice the extremely rough nature of the dome’s surface, covered in large obsidian and pumice blocks.  Now begin walking southeast along the crest of the steep slope that separates you from the margin of the lava flow.  You should begin to see flow ridges arcing to the west, down and away from the dome.  Now walk back to the Lost Lake Trail and return to its junction with the Pumice Flat Trail.  This trail heads to the left (east) across Pumice Flat and up through the narrow valley formed along a ring fault to eventually reconnect with the Crater Rim Trail at the trail junction you left back at one mile (Map 3A.3.1).

Figure 3A.3.2.  An intimate view of the obsidian dome overlying the source vent of the Big Obsidian Flow; Paulina Peak is in the background at the right side of the photograph.

Hike left across Pumice Flat (Map 3A.3.1 and Figure 3A.3.3a), and as you walk along the trail for the next quarter of a mile, observe the pumice lapilli at your feet.  Some of these pumice fragments are quite large (as big as cantaloupes, Figure 3A.3.3b).  Consider your position downwind of and in close proximity to the source vent of the Big Obsidian Flow (the obsidian dome you just saw overlies the vent), which is also the source for this coarse pyroclastic material, the proximal composition of the Newberry Pumice (Figure 3A.8).  Walking east across Pumice Flat and up through the ring fracture-generated gap in the caldera rim, you should be able to observe a substantial decrease in the size of the pumice clasts.  After nearly a mile and a half, you return to the trail junction with the Crater Rim Trail that you passed early in the hike.  Head left and follow the Crater Rim Trail back to the trailhead on FS 21 and your vehicle.

Figure 3A.3.3.  Pumice Flat (A) lies buried in coarse pumice lapilli, some of which reaches enormous size (quarter for scale) (B) attesting to its proximal location downwind of the source vent for the Newberry Pumice underlying the lava dome of the Big Obsidian Flow.

Hiking Trail Map

Map 3A.3.1.  Color shaded-relief map of portions of the East Lake, Fuzztail Butte, and Paulina Lake 7.5” Quadrangles containing multiple hiking trails (Tr 3A.1-3A.6) within Newberry Volcano’s caldera.

Little Crater Trail (Tr 3A.4)

This is a short hike of a little more than one and three-quarters miles with a steep grade at the beginning and end, but is otherwise fairly gentle.  The trail forms a loop as it traverses up the western flank of Little Crater’s basaltic palagonite tuff cone and around the rim of the cone’s crater (Map 3A.4.1).  Several interesting rock outcrops display the composition and depositional history of a tuff cone; and Paulina Lake, the Central Pumice Cone, and the Big Obsidian Flow can be seen along the northern and southern parts of the rim walk.

Begin at the trailhead sign at the northeast end of the parking area.  After a short uphill stretch of two-tenths of a mile, nice outcrops of palagonite tuff exhibiting stratified layers that dip outward and inward relative to the crater’s interior can be observed immediately after a switchback in the trail (Figure 3A.4.1).  A short distance further brings you to a trail junction with right and left forks.  Turn left here and traverse clockwise around Little Crater’s rim.  Ignore the trail to Little Crater Campground encountered at about four-tenths of a mile and continue upslope past more outcrops of tuff, steeply inclined downward and inward toward the cone’s central crater.  After another three-tenths of a mile, the trail ascends to the northeast crest of the cone for a great view of Paulina Lake to the northwest.  Notice that along the cone’s rim crest, the outcrops of tuff exhibit either nearly horizontal stratification or stratified layers that dip down and outward from the crater.  Examine closely the composition of the tuff here, large basaltic rock fragments lie embedded in a matrix of finer devitrified ash (Figure 3A.4.2).

Figure 3A.4.1.  Bedded basaltic palagonite tuff of Little Crater tuff cone, steeply inclined toward the cone’s central crater.

Figure 3A.4.2.  The palagonite tuff of Little Crater is comprised of large basaltic rock fragments embedded in a matrix of finer devitrified ash and small pumice lapilli.

At about one mile, nice views of the Central Pumice Cone present themselves to the northeast; and in an additional tenth of a mile, the first of several good views of the Big Obsidian Flow appears.  From your vantage point, it is easy to see from the orientation of its darker flow bands where the silicic lava flow poured downslope toward you, and where it overtopped the southeast rim of the Lost Lake Tuff Ring, partially filling its crater (Figure 3A.4.3).  In roughly half of a mile, you return to the trail junction and closure of the loop.  Turn left and descend back to the parking area for a total distant of just about one and nine-tenths miles.

Figure 3A.4.3.  The Big Obsidian Flow as seen from Little Crater’s southern rim.

Hiking Trail Map

Map 3A.4.1.  Color shaded-relief map of portions of the East Lake, Fuzztail Butte, and Paulina Lake 7.5” Quadrangles containing multiple hiking trails (Tr 3A.1-3A.6) within Newberry Volcano’s caldera.

Paulina Lakeshore Trail (Tr 3A.5)

      This short hike is simply a pleasure to undertake.  It is nearly level and remains near the shore of Paulina Lake for its entire length (Map 3A.5.1).  Geologically speaking, it does offer a few interesting highlights, including rugged sections of shoreline formed by wave erosion of the margin of an older basaltic andesite lava flow and where the young Interlake Obsidian Flow reached the edge of the lake, as well as a final section of smooth beach containing several hot springs known as the Warm Springs (Figure 3A.5.1).

Figure 3A.5.1.  Small hot springs on Paulina Lake’s northeast shoreline can be reached using the Paulina Lakeshore Trail.

The Paulina Lakeshore Trail begins at the back of Little Crater Campground, following the edge of an old intracaldera basaltic andesite lava flow that poured into an ancestral Paulina Lake.  The source vent of this flow is likely under the Central Pumice Cone which buries much of it.  Its rough, bouldery exterior and steep, lobate flow margin indicates a fairly viscous, aa-type lava flow; wave action has accentuated the steepness and irregularity of the flow’s edge.  At about six-tenths of a mile, the trail crosses onto the margin of the youthful Interlake Obsidian Flow and follows it for a quarter of a mile.  Notice the rubbly exterior of this flow and the abundance of obsidian and pumice chunks.  The trail proceeds onto a grassy shoreline area underline by sands and gravels at roughly nine-tenths of a mile.  Several faint trails lead to the beach at just over a mile, near the area called Warm Springs Campground.  These footpaths invariably reach shallow pools excavated into the sediment where hot springs bubble to the surface and mix with lake water to form comfortable baths (Figure 3A.5.1).

From here, it’s a gentle stroll back to the trailhead from whence you came. 

Hiking Trail Map

Map 3A.5.1.  Color shaded-relief map of portions of the East Lake, Fuzztail Butte, and Paulina Lake 7.5” Quadrangles containing multiple hiking trails (Tr 3A.1-3A.6) within Newberry Volcano’s caldera.

The Dome Trail (Tr 3A.6)

The Dome Trail is a short hike of about one and one-third miles round-trip that leads to the northwest part of The Dome (Map 3A.6.1).  The name is actually a misnomer, The Dome is a cinder cone, not a lava dome, its western rim is however glazed white with a coating of silicic tephra (Figure 3A.6.1). Contrasting reddish basaltic cinders are exposed on the inner slopes of the cinder cone’s crater, although its outer northeastern to southwestern flanks are blanketed in Newberry Pumice.  The Dome is likely an early Holocene (pre-Mazama) cinder cone, breached on its eastern flank where a lava flow poured downslope, some distance from the volcano (Figure 3A.17).  Either direction can be hiked from the point where the trail first reaches the rim, but the southern arm of the cinder cone climbs to the highest position on the rim and provides great scenery to the south and east.

Figure 3A.6.1.  The reddish, oxidized basaltic cinders of The Dome’s western rim contrast sharply with the whitish silicic tephra comprising the Newberry Pumice.

When you reach the steep slopes at the outer end of the cone’s southern ridge, find a comfortable spot for a rest and take in the views.  First look to the south, in the distance, you can see Fort Rock maar volcano in the Fort Rock-Christmas Valley basin, in the middle ground are the normal-faulted dome complex of Willow Butte and Indian Buttes, and in the foreground, several groups of aligned cinder cones cover the flanks of Newberry Volcano.  The alignment of East Rim Fissure -The Dome-Sand Butte cinder cone chain, probably fault-controlled, is easily observed from here (Figure 3.6.2).  Weasel Butte and its accompanying Lava Pass Lava Flow, and Red Hill and its accompanying Red Hill Lava Flow (Figure 3A.17) are prominently displayed to the southeast and northeast, respectively.  To the east-southeast, the rhyolitic domes and lava flows of China Hat and East Butte, and Quartz Mountain just beyond, are clearly visible (Figure 3.2).  East Butte at about 870,000 years old and China Hat at about 800,000 years old, form the northwesternmost occurrences of a westward-younging trend in silicic volcanic vents thought to be associated with the Brothers Fault Zone (Walker and Nolf, 1981).

Figure 3A.6.2.  An aligned chain of cinder cones visible from The Dome; an alignment of vents probably beginning with the East Rim Fissure, passing through The Dome and the adjacent Sand Butte cinder cone, and sweeping gently to the southwest down Newberry’s southern flank.

Hiking Trail Map

Map 3A.16 - Crater Rim Trails

Map 3A.6.1.  Color shaded-relief map of portions of the East Lake, Fuzztail Butte, and Paulina Lake 7.5” Quadrangles containing multiple hiking trails (Tr 3A.1-3A.6) within Newberry Volcano’s caldera.