0.0 (0.0) Refer to Map 1B.1. Intersection of Franklin Avenue and U.S. Hwy 97 (3rd Street) in Bend, OR. Drive south, remaining on Hwy 97 (3rd Street) through town.
0.3 (0.3) Underpass beneath the Burlington Northern Santa Fe Railroad. Road cuts on either side of the road expose typical basalts from Newberry Volcano’s lower flanks (Sherrod et al., 2004). These basaltic lava flows have been offset by NW-SE trending faults along the Tumalo Fault Zone which extend through the Bend area (Jensen, 2006) and are highlighted in Field Trip 1C.
1.6 (1.3) The highway passes over the Central Oregon Canal which diverts water from the Deschutes River for irrigation.
2.9 (1.3) Hwy 97 (3rd Street) crosses a major northwest trending fault trace associated with the Tumalo Fault Zone. The fault block to the northeast has been dropped downward. Look carefully, another fault scarp of the Tumalo Fault Zone is clearly visible on Map 1B.1 north of Pilot Butte.
3.0 (0.1) Junction of U.S. Hwy 97 (3rd Street) and the Bend Parkway. Turn left (south) and remain on Hwy 97.
4.6 (0.9) Refer to Map 1B.2. Southbound off ramp to Baker Rd and Knott Rd, remain on Hwy 97. Almost immediately, the highway crosses the Arnold Canal and another northwest trending fault trace here, with the northeast block dropped downward.
5.1 (0.5) Northbound off ramp to Baker Rd and Knott Rd lies on the opposite side of the highway here. Field Trip 1C diverges from the route here.
5.9 (0.8) Good view of Lava Butte at 12:00; notice Lava Butte’s relatively small size and conical shape; geologist’s refer to this type of volcano as a cinder (or scoria) cone); U.S. Hwy 97 passes around the eastern side of this cinder cone. This volcano and its associated lava flows form part of a northwest trending linament of cones and flows on Newberry Volcano’s lower northwest flank (Figure 1A.1). They erupted along the Northwest Rift Zone (Jensen, 2006), a system of youthful normal faults that cut across Green Mountain northwest of Lava Butte and continues southeast to the East Lake Fissure above East Lake in Newberry Volcano’s caldera rim.
9.4 (3.5) Intermittent road cuts on both sides of the highway for about the next half mile reveal a thickening and then thinning blanket of dark tephra erupted from Lava Butte as the highway traverses across the NE trending axis of the cinder cone’s ash plume (Figure 1A.2). Charcoal obtained from the base of the deposit indicates the tephra erupted about 7,000 years ago (Jensen, 2006). A well-preserved section of Mazama ash occurs beneath the Lava Butte tephra. Several cylindrical columns of reworked Lava Butte tephra were excavated here into the top of the Mazama ash during highway expansion in 1988 and Jensen (2006) indicates that these features represent the molds of trees rooted in the underlying Mazama ash. The tephra buried the trees deeply enough to kill them, and as they subsequently rotted, volcanic cinders from the surrounding tephra filled the void to form the tree molds.
9.8 (0.4) FS Rd 9710 lies on the opposite side of the highway here. Field Trip 1B merges with the route at this point.
10.0 (0.2) The basaltic aa lava flow margin from Lava Butte lies on the right side of the highway (Figure 1A.2); Lava Butte is in the background. The highway continues around the eastern margin of the flow for about the next mile.
10.4 (0.4) The Northwest Rift Zone of Newberry Volcano is expressed by a shallow fault scarp to the left side of the road here.
10.5 (0.1) Gas-Line lava flows occur to the left and Lava Butte flows are to the right (Figure 1A.2). The Gas-Line flows are also dated at about 7,000 years, but underlie the Lava Butte flows. Jensen (2006) suggests eruptive activity along this portion of the Northwest Rift Zone began with vents near the Gas-Line flows and finally coalesced at a vent now under Lava Butte.
10.9 (0.4) Junction of U.S. Highway 97 and FS Rd 9702 to Lava Lands Visitor Center and Lava Butte (these sites are described in Field Trip 1A). Continue south on Hwy 97.
11.9 (1.0) The highway passes over Lava River Cave near here. Not to worry though, the lava tube’s roof is at least 50 feet thick.
12.0 (0.1) Road cuts for about the next 0.3 miles expose the basalts of the Lava River Cave Flow, believed to be the equivalent of the middle to late Pleistocene Basalt of Bend. The vent for this extensive flow occurred upslope from here, but is now buried by younger lavas.
13.4 (1.3) Refer to Map 1B.3. The highway passes over younger basaltic lava flows here, erupted from a chain of cinder cones to the left (east), the largest of which occurs at the site of the Camp Abbot cinder pit (visited just ahead).
14.1 (0.7) Refer to Map 1B.4. Sunriver Junction highway interchange. Take Exit 153 which provides access to FS Rd 9720 and a continuation of Field Trip 1B on the left (east), or FS Rd 40 and a divergence to Field Trip 1A to the right (west).
14.4 (0.3) Junction of Hwy 97 off-ramp and FS Rd 40 (right)/FS 9720(left). Turn right (east) onto FS Rd 9720 toward Lava Cast Forest to remain on Field Trip 1B.
15.2 (0.8) Camp Abbot Cinder Pit on the right side of the road. This cinder pit is located on the north flank of the largest cinder cone in a 2-mile-long chain of ten cinder cones. Lava extruded from these cones forms the Basalt of Sunriver (Jensen, 2006). The cinder pit offers excellent exposures of the volcanic materials and stratigraphy making up typical cinder cones, and it contains beautiful scoria fragments in an array of brilliant blue, green, gold, and red colors. This is an active pit, so the best time to explore is on weekends. Exposures are constantly changing, but in the fall of 2008 (the last time this author visited the site), a basaltic feeder dike was clearly visible in its rear floor. In several places, the pit walls expose outwardly sloping layers of cinders deposited by individual eruptive pulses during cone building.
15.6 (0.4) The road makes a 90° right-hand turn here and leaves the basalts erupted from the Camp Abbot chain of cinder cones, passing onto older lava flows from Newberry Volcano, but continuing to follow the cinder cones and lava flow margin for another mile. Sherrod et al. (2004) named these lavas the Basalt of Sunriver.
21.7 (6.1) Refer to Map 1B.5. The road crosses onto a younger lava flow from Klone Butte and remains on the flow for about three miles.
Klone Butte is associated with eight pre-Mazama spatter cones with very fresh appearances, all of which are sources of the Klone Butte lava flows which underlie the volcanics of the Northwest Rift Zone we will be examining in this area. Fissure vents of the Northwest Rift Zone lie beyond (just down slope of) Klone Butte and these spatter cones; and eruptions of lava (such as the Lava Cast Forest Flow) from these vents are post-Mazama in age, occurring about 7,000 years ago.
22.9 (1.2) “Y” junction of FS Rd 9720 and FS Rd 9720-950. Turn onto FS Rd 9720-950, the right fork (in this case the obviously more travelled road despite its numerical designation). This road passes between two lava flows erupted along the Northwest Rift Zone; the Forest Road Flow is to the left (north) and the Lava Cast Forest Flow is to the right (south) (Figure 1B.1).
23.5 (0.6) Begin a one-way loop around the parking area for the Lava Cast Forest Geological Area.
Park at the Lava Cast Forest Trailhead and walk the paved Lava Cast Forest Trail (see the Lava Cast Forest Trail, under Optional Hiking Trails at the end of this road log for a complete description of this hike). This trail traverses a small portion of the Lava Cast Forest Flow which exhibits spectacular examples of tree molds preserved at the lava flow surface.
The Lava Cast Forest Flow is a young, basaltic, pahoehoe to aa lava flow associated with the Northwest Rift Zone of Newberry Volcano (Figure 1A.1); one of several flows erupted about 7,000 years ago along a 7-km-long section of the rift from the East Lake Fissure on Newberry’s caldera rim to the fissure vent producing the Forest Road Flow just northwest and down slope from here. The much larger Lava Butte Flow explored on Field Trip 1A was erupted at about the same time. Other lava flows in the immediate area that will be visited on this trip which are thought to have been generated by the same eruptive period include the Forest Road Flow downslope from this location, and the Lava Cascades Flow and Hidden Flow just upslope (Figure 1B.1).
Figure 1B.1. A simplified geological map showing the relationships between the vents, cinder cones, lava flows, and tephra deposits along a segment of the Northwest Rift Zone in the Lava Cast Forest-Mokst Butte area (modified from Jensen, 2006).
This section of the Northwest Rift Zone provides a great opportunity to investigate the relationships between source vents and eruptive cycles along a system of interconnected, en echelon fissures because of the overlapping nature of the cinder cones, lava flows, and tephra plumes that were produced. Detailed mapping of these features reported by Jensen (2006) indicates that volcanic eruptions from different vents in this area were probably concurrent. The Lava Cascade Flow laps onto the southern margin of the Lava Cast Forest Flow; however, the tephra from the Lava Cast Forest vent partially buries the Lava Cascade Flow and its tephra plume, suggesting that eruptions producing the Lava Cast Forest and Lava Cascade volcanics were virtually simultaneous, but the Lava Cascade Flow pouring from a vent somewhat higher on the slope, simply took longer to reach the same topographic position as the Lava Cast Forest Flow. Lava Cascade Tephra partially buries the Hidden Flow, suggesting that eruptions from the Hidden Flow fissure occurred earlier than from the Lava Cascade or Lava Cast Forest vents. The timing of eruption that generated the Forest Road Flow is more problematic as this flow and its small tephra sheet do not overlap other volcanics erupted upslope within the fissure zone, although its similar composition and pahoehoe surface texture suggests that it formed during the same eruptive period.
The Lava Cast Forest Flow has been radiocarbon dated between 6,400 and 5,800 years B.P. (Jensen, 2006). The 600-year range of ages is probably due to the age of the wood recovered from the tree molds. Large diameter, old trees would be the most likely to be preserved as molds, and thus, the wood preserved in the molds could range from the tree’s core to its bark, easily a several hundred year age-range. The tree molds preserved in the Lava Cast Forest Flow were formed when the fairly fluid, basaltic lava entered a forested area, completely surrounding the trunks of living trees. As the flow engulfs the base of the tree, a solid lava crust forms around the trunk. The nearby Forest Road Flow and Lava Cascade Flow have radiocarbon dates similar to the Lava Cast Forest Flow, and thus, all three flows were likely erupted concurrently in association with other volcanic activity along the Northwest Rift Zone.
23.7 (0.2) End of the one-way Lava Cast Forest parking area loop road.
24.3 (0.6) Return to the junction of FS Rd 9720-950 and FS Rd 9720. Turn right (northeast) onto FS Rd 9720 and for the next half mile, drive across the two lobes of the Forest Road Flow (Figure 1B.1). When this flow erupted and began advancing down slope, it encountered an older cinder cone and was forced to separate into two arms around the northern and southern flanks of the cone.
24.8 (0.5) Descend off the northern lobe of the Forest Road Flow. In this area, between the young flows from the Northwest Rift Zone, the road is still on the older lava flows from Klone Butte. Good views of Mokst Butte open up to the left (north).
25.2 (0.4) FS Rd 9720 begins following the southern margin of the Mokst Butte Flow (Figure 1B.1) to your left (north). As you drive, notice the steep front of this lava flow and its distinctly blocky surface. This is an aa flow, compare it to the flatter, smoother surface and more gently sloped flow margins of the pahoehoe Forest Road Flow and Lava Cast Forest Flow you recently encountered.
25.8 (0.6) The road curves to the north. Find a convenient place to pull to the edge of the road here. Take the opportunity to explore the nature of the Mokst Butte Flow more closely. Carefully climb the flow margin; from this vantage point, you can see the Mokst Butte cinder cone, the source of this flow, nicely portrayed to the northwest (Figure 1B.2).
Figure 1B.2. The north flank of Mokst Butte and its associated basaltic aa lava flow.
The formation of Mokst Butte and its associated lava flows and tephra sheet followed a similar history to Lava Butte described in Field Trip 1A. Volcanic activity began along a 1-mile-long fissure on the Northwest Rift Zone (Figure 1A.1), probably about 7,000 years ago, around the same time as other fresh-looking lavas of dated eruptions along this major zone of weakness. During the early stages of the eruption, fire fountains built a series of small spatter cones and lava flows along the fissure. These early-formed fissure vents now occur as a kipuka above surrounding Mokst Butte lava flows on the northwest side of the cone (Figure 1B.1). As the eruption progressed, eruptive activity became concentrated at a vent now underlying Mokst Butte, and the gaseous content of the magma decreased. A cinder cone was formed over the vent as lava flows poured down slope to the northwest.
The cinder cone is asymmetric, with the greatest portion of cinders accumulated on its northeast flank in the direction of prevailing wind. The cinder cone was breached on its weaker southwest side and large blocks were rafted way in one of the younger lava flows that wraps northwestward around the base of the cone. Volcanic activity associated with Mokst Butte also produced a wedge-shaped, bi-lobed tephra plume to the northeast of the cinder cone (Figure 1B.1). One lobe spread to the north and another to the east, indicating a shift in prevailing wind direction during eruptions. The tephra lobes cover the upslope portion of earlier lavas erupted to the north of Mokst Butte, while later lavas extruded from the main breach on the cone’s southwest flank spreading to the northwest cover tephra. As you drive further along the flow margin from here, the tephra plume deposited from the Mokst Butte eruption gradually thickens to a near maximum at the junction of FS Rd 9720 and 9710.
From the top of the Mokst Butte Lava Flow, Klone Butte can be seen to the south (Map 1B.5). This cinder cone is the largest of eight pre-Mazama spatter cones and the main source of extensive basaltic lavas immediately underlying those of the Northwest Rift Zone in this area.
26.4 (0.6) Junction of FS Rd 9720 and 9710. Turn left onto FS Rd 9710. The tephra from Mokst Butte rapidly thickens to a maximum of about three feet, then thins gradually to near zero when the road veers away from the lava flow margin (in about one mile distant).
27.0 (0.6) Junction of FS Rd 9710 and FS Rd 9710-490. FS Rd 9710-490 can be used as hiking access to the kipuka formed of older vents and spatter cones of the Mokst Butte fissure system (Map 1B.2) (see the Mokst Butte Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).
27.8 (0.8) Remain on FS Rd 9710. The road follows the margin of Mokst Butte lavas until roughly this location. Mokst Butte Tephra both underlies and covers the lava flows (Jensen, 2006). The northerly tephra lobe is oldest and underlies early lavas on the east side of Mokst Butte, while the easterly tephra lobe overlies early east-side flows, but is itself overlain by the latest east-side flow. The tephra sheet from Mokst Butte has thinned considerably by this point (it is less than a foot thick here and thins to zero in about two tenths of a mile).
The road now returns to pre-Mazama Klone Butte lavas.
29.8 (2.0) Here, FS Rd 9710 leaves the Klone Butte lava flows for good and briefly follows the Mokst Butte Lava Flow margin again.
30.0 (0.2) Refer to Map 1B.2. The road leaves the margin of the Mokst Butte Lava Flow. Klawhop Butte lies to the immediate right. This large cinder cone is the source of a major 39,000 year old eruption of basaltic lavas that poured northward and downslope almost nine miles to the very outskirts of Bend, OR; the lava flow is crossed on Field Trip 1C.
32.7 (2.7) FS Rd 9710 descends from the western margin of the Klawhop Lava Flow at this location and on to the much older Basalt of Bend.
33.8 (1.1) The road crosses dual Pacific Gas pipelines here and shortly begins traversing gradually thickening tephra deposits from Lava Butte.
34.6 (0.8) Junction of FS Rd 9710 and U.S. Hwy 97. Turn right (north) onto Hwy 97, there is no southbound access to Hwy 97 at this location.
44.4 (9.8) Refer to Map 1B.1. Return northward to the intersection of Franklin Avenue and U.S. Hwy 97 (3rd Street) in Bend, OR. This segment of the trip is over the first 9.8 miles of the outward bound route. This ends Field Trip 1B.
Road Route Maps
Map 1B.1. Color shaded-relief map of the Bend 7.5” Quadrangle containing segments of Field Trip 1A-F and Field Trip 2.
Map 1B.2. Color shaded-relief map of the Lava Butte 7.5” Quadrangle containing segments of Field Trip 1A, 1B, and 1C, as well as Field Trip 2A.
Map 1B.3. Color shaded-relief map of the Benham Falls 7.5” Quadrangle containing segments of Field Trip 1A, 1B, and Field Trip 2A.
Map 1B.4. 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 1B.5. Color shaded-relief map of the Lava Cast Forest 7.5” Quadrangle containing a segment of Field Trip 1B.
Optional Hiking Trails
Lava Cast Forest Trail (Tr 1B.1)
This is a short, easy hike on a paved trail with several sign posts. An interpretive trail guide is normally available at the trailhead corresponding to numbered posts located along the trail. It is designed to be family-friendly and handicap accessible (although the state of the pavement in some areas would make the trail difficult to navigate in a wheelchair without aid). Map 1B.6 shows the trail route, although specific distances indicated below are not displayed. The trail traverses a portion of the Lava Cast Forest Flow, a young lava flow associated with the Northwest Rift Zone of Newberry Volcano erupted about 7,000 years ago, and famous for its well-preserved tree molds.
The tree molds preserved in the Lava Cast Forest Flow were formed when the fairly fluid basaltic lava entered a forested area, surrounding the trunks of living trees, engulfing them and forming a solid lava crust around the trunk. The lava cools in part because of the much lower temperature of the tree trunk and due to steam released from the rapidly heating tree. As a result of this cooling, the inner zone of the mold is often finer grained than the outer zone; look for this feature here. The level of the lava flow may drop before the tree trunk burns through, leaving a vertical, open tree mold or collar, referred to as a lava tree. Many fine tree molds occur here (Figure 1B.3). The tree’s trunk may remain upright, burning in place to form a vertical tree mold called a lava tree (Figure 1B.3a); or it may burn through and topple into the flow, or be pushed over by the flow, the log becoming coated with lava and subsequently burning away to form a horizontal tree mold called a lava log (Figure 1B.3b). As the wood burns within the lava encasing the tree trunk or log, lava on the interior of the mold may cool and solidify enough to preserve bark impressions on the mold’s inner surface (Figure 1B.4a), or remain hot enough to ooze or flow, producing vertically-oriented drippings on the mold’s inner surface (Figure 1B.4b); both features are preserved within tree molds along the trail. Based on the shape of the bark impressions preserved within tree molds and the dominant tree living in the forests at this elevation on Newberry Volcano’s flank today, the trees inundated by the Lava Cast Forest Flow were probably Ponderosa pine.
Figure 1B.3. Many tree molds are found within the Lava Cast Forest Flow; preserved as upright tree trunks, they form lava trees (A), preserved in a toppled-over state, they form lava logs (B).
Figure 1B.4. Some tree molds preserve bark impressions (A) and/or drip marks (B) on their inner surfaces; the inset in (B) is a closeup of the drip marks shown at the arrow’s location (a dime for scale).
Begin at the trailhead sign at the upper (east) end of the parking area. Since this is such a short hike and so easily navigated, no shaded-relief hiking map is provided; however, detailed mileage is described. The first lava tree mold (Figure 1B.3a) is encountered in 0.19 miles at post #6. Three lava tree molds are found to either side of the trail between 0.22 and 0.23 miles. Two large lava tree molds occur on the left side of the trail between 0.25 and 0.26 miles. The first has well-preserved lava armoring high on the trunk. A very nicely preserved double-trunked lava tree mold is encountered at post #7, 0.28 miles into the hike, while a triple-trunked lava tree mold is preserved at post #8 at 0.30 miles. Another lava tree mold at 0.35 miles is followed shortly by a bridge over an inflation fracture (pressure ridge) in the lava flow surface. Pressure folds form in the more rigid surface of the flow as the actively moving lava beneath rafts one sheet of the surface material against another sheet, buckling both downward into the flow. Two more well-preserved lava tree molds occur at 0.38 miles.
A rest bench along the trail can be found at 0.41 miles. This location is on the edge of the lava flow; the ridge behind and to the right of you is formed of older lava from Klone Butte that was not inundated by the younger Lava Cast Forest Flow. From this vantage point, a large, elongate mound can be seen to southeast that marks at least one source vent for this younger lava flow. As you descend behind the ridge, notice how it deflected the younger Lava Cast Forest Flow to the left. At 0.47 miles, you encounter the first of several horizontal tree molds (lava logs) partially buried within the lava flow’s margin (Figure 3Bb).
Mile 0.51 offers a good view down the lava flow and of the Cascade strato-volcanoes, Mt. Bachelor, Broken Top, and the Three Sisters. At 0.57 miles, another rest bench is found along the trail next to post #11. At this point, the trail leaves the margin of the Lava Cast Forest Flow and climbs back onto the ridge formed of older, Klone Butte lavas. At 0.64 miles, you descend back onto the Lava Cast Forest Flow with the tree covered ridge to your right. Near the base of the ridge at 0.66 miles, the trail passes several more lava log tree molds. Notice the prominent lava drippings in one of the logs (Figure 1B.4b). Shortly, you reach post #12. Examine the interior of the large lava log tree mold here. The inner surface of this mold preserves beautiful bark impressions (Figure 1B.4a). Notice two sizes to the ridges on the bark impressions; some ridges are larger, thicker, more continuous, and enclose the other ridges. These are the main “plates” of bark formed of several smaller, discontinuous plates; probably from Ponderosa pine which grow prominently in the area today. Watch for more lava trees and logs for the next several hundred feet. You reach the end of the Lava Cast Forest Trail at 0.83 miles. Turn right to return to your car.
Mokst Butte Trail (Tr 1B.2)
This excursion is not for everyone as part of it covers old logging roads and on the rest there is no well-defined trail; the total hiking distance is roughly two and one-half miles depending on how far you wish to go. Map 1B.6 shows the trail route. Turn left onto FS Rd 9710-490 and park (the road progressively worsens, so drive as far as you like at your own risk). Hike along FS Rd 9710-490 until its intersection with FS Rd 9710-491 at about 0.17 miles. You have climbed onto earlier lava flows from Mokst Butte covered in a thick veneer of tephra ejected from the main Mokst Butte vent. Continue right on FS Rd 9710-491; ignore spur roads FS Rd 9710-492 and FS Rd 9710-496 to the right and FS Rd 9710-490 which rejoins from the left at about 0.91 miles (the road becomes impassable to vehicles at this point). Remain on FS Rd 9710-491 as it climbs over a ridge and drops into a swale formed between flow lobes of the younger lavas from Mokst Butte. Now within the swale, at about 1.15 miles, take a right-hand branching of FS Rd 9710-491 and walk along the margin of a small lava flow from one of the spatter cones (at the southeast end of the swale) for several hundred feet and then climb onto the rim of the first visible cinder cone to your right. You can explore several interesting spatter cones aligned along the fissure in this area, but be sure you can retrace your path back to FS Rd 9710-491; it is a little less than half a mile to the furthest spatter cone out and back from here. Good examples of individual buffalo-chip-style spatter bombs and agglutinated spatter, plus volcanic bombs, abound here. From the crest of the spatter cone ridge, you can look back to the southeast at Mokst Butte (Figure 1B.5) and southwest to the younger lava flow along the kipuka’s flank. Notice the NW-SE alignment of the cones and the large rafted blocks in the younger lava flow from the breach on Mokst Butte’s southwest flank. There are also nice views of several High Cascades stratovolcanoes with the Mokst Butte lava flow in the foreground.
Retrace your steps along FS Rd 9710-491 and FS Rd 9710-490 to your vehicle.
Figure 1B.5. The view southeast from the rim of a spatter cone formed along the NW-SE fissure that also underlies Mokst Butte; this and several other nearby spatter cones mark an early stage in the Mokst Butte eruptions.
Hiking Trail Map
Map 1B.6. Color shaded-relief map of the north-central portion of the Lava Cast Forest 7.5” Quadrangle; the Lava Cast Forest (Tr 1B.1) and Mokst Butte (Tr 1B.2) hiking trails are shown.