For your added reading pleasure, and as an aid to understanding the oft-complicated “lingo” that usually accompanies the science of geology, this author has included an exhaustive list of references cited in the text of this geologic guide and their meanings, organized alphabetically for convenience. 

aa:  Hawaiian word used to describe a basaltic lava flow with a surface broken into angular, jagged fragments related to its slightly higher than average viscosity.

ablation:  All processes by which snow and ice are lost from a glacier; also, the amount lost.

abrasion:  Erosion of rock by the impact of sediment particles carried by water, wind, or glacial ice.

absolute age:  Specific time when an event occurred or a particular number of years that have elapsed since an event occurred.

Accreted terrane:  See exotic terrane.

accumulation:  All processes that add snow or ice to a glacier or to floating ice or snow cover, including snowfall, avalanching, and snow transport by wind.

active volcano:  A volcano that is currently erupting or, based on its recent history is expected to erupt; the distinction between active and dormant is not sharply defined.

agglutinate: A volcanic deposit formed by the accumulation of flattened and welded rock fragments somewhat resembling cowpats, typically derived from showers of molten particles ejected in magma fountains.  The liquid fragments may accumulate to form a stream of lava.

aggradation:  The process of building up a surface by deposition.

aggrading stream:  A stream that is actively building up its channel or flood plain by being supplied with more load than it is capable of transporting.

aggregate:  A mass or body of rock particles, mineral grains, or both.

alumina:  Aluminum oxide, Al2O3.

alluvial fan:  Fan-shaped mass of alluvium that forms where stream flow abruptly exits a confined channel onto an unconfined valley floor; commonly forms at the base of a mountain range.

alluvial stream:  Stream that generally transports, deposits, and/or flows in a channel comprised of loose sediment (alluvium).

alluvium:  Generally fine-grained sediment deposited by streams, often comprising floodplains.

andesite:  Aphanitic igneous rock solidified from an intermediate-composition magma.

aphanitic:  An igneous rock texture resulting from rapid crystallization of magma to form very small crystals that are generally invisible to the unaided eye.

aphyric: Of the texture of fine-grained igneous rocks, showing two generations of the same mineral but without phenocrysts.

arête:  A rocky sharp-edged ridge or spur, commonly present above the snowline in rugged mountains sculptured by glaciers, and resulting from the continued backward growth of the walls of adjoining cirques.

arkose:  Sandstone containing at least 25 percent feldspar in addition to abundant quartz.

ash:  Fine particles of pulverized rock blown from a volcano, measuring less than about 0.1 inch in diameter.  The ash may be either solid or molten when first erupted.  By far the most common variety is vitric ash, glassy particles formed by gas bubbles bursting through liquid silicic magma.  Lithic ash is formed of older rock blown apart during a phreatic (nonmagmatic) or phreatomagmatic eruption.  Ash may also be composed of crystals with only traces of glass adhering to them.  Many volcanic ash deposits contain mixtures of all three kinds in various proportions.

ash cloud:  A turbulent cloud of ash particles rising above a volcanic vent that is generated by a pyroclastic eruption; it can also refer to an eruption column or ash plume.

ashfall:  A rain of ash from an eruption cloud.

ash flow:  A ground-hugging cloud of hot volcanic ash and gases that typically travels great distances at high speeds from an erupting vent.  Large-volume ash flow deposits, emplaced while still hot, commonly solidify to form ash-flow tuffs (ignimbrites).

asthenosphere:  A weak layer of Earth’s mantle below the lithosphere, generally considered a hot, plastic material that may contain small amounts of magma.  Its nature allows it to flow convectively, the process referred to as mantle convection, a major driving force behind plate tectonics.

atom:  The smallest unit of matter that engages in chemical reactions and cannot be chemically broken down into simpler components.

barchan:  Crescent-shaped sand dune that forms on desert surfaces where wind-transported sand is not abundant.  Barchans resemble parabolic dunes except that the ends of the crescent on a barchan dune point in the downwind direction.

basalt:  Aphanitic igneous rock solidified from a mafic-composition magma.

basin:  Depression caused by subsidence of the crust and typically accumulating thick sediment and sedimentary rocks.  May also refer to an eroded depression; also see drainage basin.

Basin and Range:  A geologic province of the western United States in which the earth’s crust stretched, breaking into normal faults that form a pattern of alternating mountain ranges and valleys that trend generally to the north.  Ongoing crustal extension in northern California and Oregon abuts against the Cascade Range.

batholith:  Large body of intrusive igneous rocks formed by the crystallization of a magma chamber beneath the surface.

bedded:  Arranged or deposited in layers or beds, especially used to describe sedimentary rocks.  The term may also be applied to stratified material of other origin, e.g. volcanic ash.

bedding:  The arrangement of a sedimentary rock in layers; stratification.  Also, the general character or pattern of the beds and their contacts within a rock mass, as cross-bedding and graded bedding.  The term may be applied to a layered arrangement in igneous or metamorphic rock.

bedload:  Large grains that roll, slide, and bounce along the bottom of a stream.

bioturbation:  The churning and stirring of a sediment by organisms.

block-and-ash-flow:  A variety of pyroclastic flow consisting of lava blocks in a matrix of fine ash that avalanche downslope during an eruption.  They are commonly triggered by the collapse of a lava dome while it is still hot or by the disintegration of lava flows that are erupted on steep, icy slopes.

block faulting:  A type of normal faulting in which the crust is divided into structural or fault blocks of different elevations and orientations.  It is the process by which block mountains are formed.

blocky lava:  A lava flow with a surface characterized by a jumble of large angular blocks (particularly coarse aa lava).

bomb:   A lump of plastic or molten lava thrown out during an explosive eruption.  Bombs range in size from 2.5 inches to many feet in diameter.  Their plastic condition when first ejected causes bombs to be commonly modified in shape during their flight through the air and/or by their impact on the ground.  As the outer crust cools and solidifies, continued expansion of the bomb’s interior by gas pressure sometimes causes crinkling and cracking, which may form a surface resembling the crust of freshly baked bread.

braided:  Describes a coarse, alluvial stream notable for abundant bars that divide the stream flow into threads that separate and rejoin around the bars; so named for the pattern of water flow around the bars that resembles braided rope or hair.  These streams are usually associated with rapid changes in transport energy, often caused by fluctuating discharge, and/or rapid change in channel slope.

breadcrust bomb:  A volcanic bomb with a checkered and cracked exterior resulting from expansion of the interior after solidification of the crust.

breccia:  Rock deposits composed of many distinct fragments, typically sharp and/or angular, embedded in a matrix of fine material.  Breccias are sometimes formed when shattered lava fragments are carried in a block-and-ash flow or by the avalanching of rock from a growing lava dome; however, most breccias are sedimentary, formed from mass wasting as slides and flows.

butte:  A conspicuous isolated flat-topped hill with steep slopes or precipitous cliffs, often capped with a resistant layer of rock and bordered by talus, and representing an erosion remnant carved from flat-lying rocks; the summit is smaller in extent than that of a mesa.  An isolated hill having steep sides and a craggy, rounded, or pointed summit; e.g. a volcanic cone.

caldera:  A large basin-shaped volcanic depression at least a mile in diameter.  Calderas are usually formed during violent and voluminous explosive eruptions that drain magma from the underlying magma chamber, causing the roof to collapse and the volcanic edifice above it to subside.

calc-silicate rock:  An igneous or metamorphic rock consisting mainly of calcium-bearing silicates.

chattermark:  A small, curved scar made by vibratory chipping of a bedrock surface by rock fragments carried in the base of a glacier.  Each mark is roughly transverse to the direction of ice movement, and usually convex toward the direction from which the ice moved.

cinder cone:  A generally small volcanic cone built entirely of tephra (cinders), typically by mildly explosive Strombolian eruptions that eject semimolten or plastic lava fragments from central vent and extrude lava flows from vents near its base.  Most cinder cones are monogenetic, built during a single eruptive episode.

cinders:  Vesicular rock fragments ejected during explosive eruptions.

cirque:  A deep steep-walled recess or hollow, horseshoe-shaped or semicircular in plan view, situated high on the side of a mountain and produced by the erosive activity of a mountain glacier.  It often contains a small round lake.

cirque glacier:  A small glacier occupying a cirque or resting against the headwall of a cirque.

cirque lake:  A small, deep, commonly circular glacial lake occupying a cirque; it is fed by runoff from the surrounding slopes and dammed by a lip of bedrock or by a small moraine.

clasts:  Sedimentary particles larger than 2mm in diameter.

clinopyroxene:  Any of a group of pyroxenes crystallizing in the monoclinic system and sometimes containing considerable calcium with or without aluminum and the alkalies.

col:  A high, sharp-edged pass in a mountain range, esp. one formed by the headward erosion of two cirques, as in the French Alps.  A marked, saddle-like depression in the crest of a mountain ridge; the lowest point on a ridge.

colluvium:  Unconsolidated, poorly sorted, unstratified debris moved down a hillslope primarily by gravity and runoff.

composite cone:  Another term for a stratovolcano, a large volcanic edifice constructed of both lava flows and pyroclastic material.  Most of the world’s great continental volcanoes are composite cones, including Fuji, Vesuvius, Etna, Rainier, Shasta, and Mt. St. Helens.

conglomerate:  A clastic sedimentary rock composed primarily of rounded gravel-size particles (clasts greater than 2mm in diameter).  Conglomerates usually form as a result of high energy stream deposition.

continental drift:  Hypothesis of German meteorologist Alfred Wegener that all continents were once joined as a single continent, which he named Pangaea, from where they drifted to their current positions.

convection:  Process of simultaneously transferring heat and matter by movement of fluid or plastically deforming rock because of density contrast; denser and typically colder material sinks while less dense and typically warmer material rises.

convergence:  The merging of fluid currents (often involving ocean currents or air masses), resulting in the sinking of the denser, colder, or more saline fluid; also, the line or area where this occurs.

convergent plate boundary:  Curving zone on Earth’s surface where plates collide nearly head-on into one another, compressing the lithosphere and causing subduction of one plate beneath the other.

core:  The central region of Earth composed primarily of iron metal and consisting of a molten liquid outer part and a solid inner part.

crater:  The bowl- or funnel-shaped hollow at or near the top of a volcano, through which volcanic gas, lava, and/or pyroclastic material are ejected.  The term derives from the Greek word for “wine-mixing bowl”.

craton:  A part of the earth’s crust that has attained stability and has been little deformed for a long time.  The term is restricted to continents, and includes both shield and platform.

creep:  Very slow flow of rock or regolith detected only by dislocation or bending of features at the surface.

crevasse:  A deep fissure or crack in a glacier, caused by stresses resulting from differential movement over an uneven surface.  Crevasses may be as much as 100 m deep.

cross-bedding:  Cross-stratification in which the cross-beds are more than 1 cm in thickness.  A cross-bedded structure preserved in sediment.

dacite:  Aphanitic igneous rock solidified from felsic magma with a composition between andesite and rhyolite.

daughter isotope:  An elemental isotope produced by decay of a radioactive parent isotope.

debris flow:  A flow of regolith and water that behaves like a high-viscosity fluid.

decompression melting:  A process of magma formation where rock partly melts because pressure decreases at a nearly constant high temperature.  Usually happens by movement of mantle rocks toward Earth’s surface.

deglaciation:  The uncovering of an area from beneath glacier ice as a result of melting.

dehydration metamorphism:  High temperature and pressure metamorphism where water-bearing minerals react to form minerals that lack water.  Water is released by these reactions.

delta:  Landform protruding outward from a coastline and produced by sediment deposition where a stream enters a lake, reservoir, or sea.

density:  A measure of how compact a substance is, and mathematically defined by the mass divided by the volume of the substance.

deposit:  An accumulation of rocky material or sediment at rest, formed by a geologic process such as a volcanic eruption or deposition in a body of water.

diapir:  Blob of magma rising through the earth’s crust; generally having the shape of an up-side down teardrop.

diatom:  Marine or freshwater organism that builds its shell of siliceous material.

diatomite:  A light colored soft siliceous sedimentary rock, consisting chiefly of opaline frustules of the diatom.  Owing to its high surface area, absorptive capacity, and chemical stability, diatomite has a number of uses.  The term is generally reserved for deposits of commercial value.

dike:  A relatively thin sheet-like body of igneous rock that cuts through, in a generally vertical direction, older rock formations.  Dikes often form when narrow tabular magma sheets intrude previously erupted volcanic material, filling a fracture formed in the strata.

diorite:  Phaneritic igneous rock solidified from intermediate-composition magma.

dip:  Angle between an imaginary horizontal plane and the planar margin of a geologic feature; used with the measurement of strike to describe the orientation of any planar geologic feature such as a rock layer, fault, or margin of an igneous intrusion.

dip-slip fault:  Fault along which rocks move parallel to the dip direction of the fault plane.

disconformity:  An unconformity defined by a sharp erosional boundary between intervals of sedimentary or volcanic rocks where layers above and below the boundary are parallel to one another.

dissolved load:  The chemical ions dissolved in stream water.

distal:  Often used in geology when referring to the deposition of sediment or volcanic material at a considerable distance, e.g. tens of kilometers, from the source from which its constituents have been derived.

divergence:  The separation of ocean currents by horizontal flow in different directions from a common source, usually upwelling; also, the area in which this occurs.

divergent plate boundary:  Linear or curving zones where plates move apart from one another and new lithosphere forms.

dome:  A mounded extrusion of lava that, when erupted, was too viscous to flow far laterally and instead piled up over the erupting vent to form a mushroom-shaped cap.  When the lava mass is uplifted and consolidated lava fills the vent conduit, the resultant extrusion is called a plug dome.

dormant volcano:  A volcano that is presently quiet, but is expected to erupt again in the geologic future.

drainage basin:  Area from which a stream gathers water; can be used to describe the size of a stream; see also watershed.

dune:  A curving ridge of loose sediment, taller than 1 centimeter, which moves along with water or wind currents.

en echelon (faults):  Pattern of faulting in which individual fault segments within a fault zone are offset in a step-wise fashion.

End moraine:  A typically linear- to arcuate-shaped ridge of loose rock fragments and silt deposited by a glacier.

Eocene:  A geologic Epoch within the Tertiary Period.

eolian:  Pertaining to the wind; esp. said of such deposits as loess and dune sand, of sedimentary structures such as wind-formed ripple marks, or of erosion and deposition accomplished by the wind.

erosion:  The wearing-away of soil and rock by weathering, mass wasting, and the action of streams, glaciers, waves, wind, and underground water.

eruption:  The geologic process by which solid, liquid, and gaseous material is ejected onto the earth’s surface by volcanic activity.  Eruptions vary in style from the quiet outflow of molten rock (effusive type) to the violent expulsion of pyroclastic material (explosive type).

escarpment:  A long, more or less continuous cliff or relatively steep slope facing in one general direction, separating two level or gently sloping surfaces, and produced by erosion or faulting.  A steep, abrupt face of rock, marking the outcrop of a resistant layer occurring in a series of gently dipping softer strata; specifically, the steep face of a cuesta.

exotic terrane:  Blocks of crust added to a continent, such as most of far western North America, and that consist of rocks that do not resemble rocks of the same age in adjacent blocks or the rest of the continent.

extrusive (or volcanic) rock:  Igneous rock formed by eruption of lava flows and pyroclastic materials onto Earth’s surface.

facies:  The aspect, appearance, and characteristics of a rock unit, usually reflecting the conditions of its origin; features differentiating it from adjacent or associated units.

fault:  A fracture plane along which rock or regolith is displaced.

fault scarp:  Cliff or low step in the ground surface caused by displacement along a fault.

felsic:  Describes igneous rocks composed mostly of quartz, sodium-rich plagioclase, and potassium feldspars, and the magmas that these rocks crystallize from; derived from the words feldspar and silica.

fissure (eruption):  A volcanic eruption that occurs along a narrow fissure or line of closely spaced fractures in the earth’s crust.

flank:  The side or slope of a positive topographic feature (a hill, a mountain); that area of a fold between adjacent fold hinges.  It may be planar or curved.

flood basalt:  The high-volume fissure eruptions of basaltic lava with lower than average viscosity that covers extremely large areas, such as the Columbia Plateau Basalts.

floodplain:  The land surface adjacent to a stream channel that is constructed by stream erosion and deposition and that is inundated during floods.

flow breccia:  A type of lava flow, usually of silicic composition, in which fragments of solidified or partly solidified lava, produced by explosion of flowage, have become welded together or cemented by the still fluid parts of the same flow.

footwall:  The body of rock or regolith that exist below a fault plane.

fumarole:  A vent or opening through which issues steam or other volcanic gases, such as carbon dioxide or hydrogen sulfide; often results in the formation of densely indurated “pipes” within more loosely consolidated pyroclastic deposits.

fumarolic activity:  Gas emissions that may be accompanied by an increase in temperature of the gases issuing from a vent.

fume:  A gaseous volcanic cloud not containing tephra.

gabbro:  Phaneritic, mafic igneous rock.

geologic time scale:  Established chronological order of time intervals in geologic history.

geology:  The science of the origin, composition, structure, and history of Earth.

geothermal energy:  Energy derived from the earth’s internal heat.

glacial erosion:  Reduction of the earth’s surface as a result of grinding and scouring by glacier ice armed with rock fragments, together with the erosive action of meltwater streams.

glacial lobe:  A large tongue-like protrusion from the margin of an ice cap or ice sheet; an outlet glacier.

glacial maximum:  The time or position of the greatest advance of a glacier, or of glaciers (such as the greatest extent of Pleistocene glaciation).

glacial minimum:  The time or position of the greatest retreat of a glacier.

glacial scour:  The eroding action of a glacier, including the removal of surficial material and the abrasion and polishing of the bedrock surface by rock fragments dragged along by the ice.

glacial till:  A poorly sorted, unstratified deposit of subrounded rock fragments, sand, and silt formed by a glacier.

glaciated:  Said of a formerly glacier-covered land surface, esp. one that has been modified by the action of a glacier or an ice sheet.

glaciation:  The formation, movement, and recession of glaciers or ice sheets.  A collective term for the geological processes of glacial activity and the resulting effects on the earth’s surface.  A climatic episode during which extensive glaciers developed, attained a maximum, and receded.

glacier:  An accumulation of snow and ice that is thick enough to flow under its own weight.

graben:  A block of crust usually displaced downward along adjacent and parallel normal faults.

granite:  Phaneritic igneous rock solidified from a felsic-composition magma.

ground moraine:  A typically hummocky mound of loose rock fragments and silt deposited by a glacier.

half-life:  The time interval required for half of the radioactive parent-isotope atoms to decay to form an equal number of daughter-isotope atoms.

hanging valley:  A tributary glacial valley whose mouth is high above the floor of the main valley, the discordance being due to the greater erosive power of the trunk glacier.  A tributary stream valley whose mouth is notably higher than the floor of the main valley, as a result of more rapid deepening of the latter.

hanging wall:  A body of rock or regolith that exists above a fault plane.

headwall:  A steep slope at the head of a valley; especially the rock cliff at the back of a cirque.

Holocene:  The 10,000 to 12,000 year span of time that has elapsed since the end of Pleistocene time; the most recent division of geologic time in the Quaternary Period and the one in which we now live.

horn:  A high pyramidal peak with steep sides formed by the intersection walls of three or more cirques, e.g. the Matterhorn.

horst:  Block of crust displaced upward along normal faults.

hot spot:  A volcanic center, 100 to 200 km across and persistent for at least a few tens of millions of years, that is thought to be the surface expression of a rising plume of hot mantle material.  Hot spots are not linked with arcs, and may or may not be associated with oceanic ridges.  Some 200 late Cenozoic hot spots have been identified.

hummock:  A knoll or mound above a level surface.

hummocky moraine:  An area of knob-and-kettle topography that may have been formed either along a live ice front or around masses of stagnant ice.

hyaloclastite: Volcaniclastic rock derived from explosive eruptions of magma upward through water-saturated rock, sediment, and/or regolith.

ice age:  A loosely used syn. of glacial epoch, or time of extensive glacial activity; specif. the latest of the glacial epochs, also known as the Pleistocene Epoch.

ice cap:  A dome-shaped or plate-like cover of perennial ice and snow, covering the summit area of a mountain mass so that only isolated peaks emerge through it, or covering a flat landmass such as an Arctic island; spreading outwards in all directions, due to its own weight; and having an area of less than 50,000 sq km.  An ice cap is considerably smaller than an ice sheet. 

igneous rock:  Rock derived from the solidification of magma.  Igneous rocks that congeal beneath the earth’s surface are called plutonic, while those formed on the surface are volcanic.

ignimbrite:  A highly silicic volcanic rock formed by the eruption of large-volume ash flows.  Dense clouds of incandescent rock fragments erupted at extremely high temperatures settle and congeal so that individual particles are fused together.  See also tuff.

intrusion:  An igneous rock formation created when subterranean magma invades a body of surrounding rocks and then solidifies.

intrusive rock:  An igneous rock body formed underground when magma is injected into an older body of rock.

isotope:  One of two or more atoms of the same element that have the same number of protons but different numbers of neutrons.

joint:  A surface of fracture or parting in a rock, without displacement; the surface is often planar and may occur with parallel joints to form a joint set.

kettle:  A depression in glacial drift, esp. in outwash and a kame field, formed by the melting of a detached block of stagnant ice that was buried in the drift.  It often contains a lake or swamp.

kipuka:  Island-like landform of higher ground not covered by a recent lava flow, often retaining its original vegetation.

lacustrine:  Generally pertaining to lake-derived processes, deposits, or landforms.

lahar:  Indonesian term describing the rapid flow of water and loose debris the down steep slopes of volcanoes.

lapilli:  Round to angular rock fragments measuring 0.1 to 2.5 inches in diameter that are erupted explosively in either a solid or molten state; from the Latin word for “little stones.” 

lateral moraine:  A typically linear-shaped ridge of loose rock fragments and silt deposited by a glacier.

lava:  Magma erupted on the earth’s surface; the term most commonly applies to streams of molten rock flowing from a volcanic vent, but also refers to solidified volcanic rock.

lavacicle:  Stalactite or stalagmite formed on the roof or floor of a recently evacuated lava tube (from the cooling of dripping lava).

lava cone:  A volcanic cone built of lava flows, usually basaltic, that were very mobile at the time of eruption; it resembles a miniature shield volcano.

lava dome:  A dome-shaped mountain of solidified lava in the form of many individual flows, generally formed by the extrusion of highly viscous, silicic lava.

lava flow:  Molten material extruded at, and flowing away from, a volcanic vent.

lava lake:  A large pool of molten rock in a crater or other volcanic depression.  The term also refers to a lake of solidified lava.

lava tube:  A cave or tunnel formed inside an active lava flow and often preserved in solidified flows.  Tubes can form in several ways, commonly when lava spatter roofs (arches and solidifies) over a surface of molten rock, or when the outer crust of a lava stream cools and solidifies while the molten interior continues to flow and drain away, leaving behind a long, hollow tube.

lithic:  A sediment or sedimentary rock composed of the solid fragments of preexisting rock.

lithic ash:  Volcanic ash derived from the explosive pulverizing of previously existing rock.

lithic pyroclastic flow:  A pyroclastic flow containing mainly rock derived from previously erupted material.

lithification:  The process of transforming loose sediment into sedimentary rock by compaction and cementation.

lithosphere:  The outer rigid, but brittle, shell of the solid earth consisting of the crust and uppermost mantle.

lobe:  A glacial lobe; also, a tongue-like extension of glacial drift beyond the main drift area.

loess:  A blanket deposit of buff-colored calcareous silt, homogeneous, nonstratifed, weakly coherent, porous, and friable windblown dust of Pleistocene age.

maar:  A broad, low relief volcanic crater that is caused by a phreatomagmatic eruption, an explosion caused by hot lava or magma coming into contact with groundwater saturated sediment or rock (or more rarely water); a maar characteristically fills with water to form a relatively shallow crater lake.

mafic:  Describes igneous rocks composed mostly of olivine, pyroxene, and calcium-rich plagioclase feldspar (basalt, gabbro) that have relatively high magnesium and iron (ferric) contents and a relatively low silica content.

magma:  A liquid formed by partial or complete melting of rock that may erupt at the surface as lava or pyroclastic material.

magma chamber:  An accumulation of magma in the upper mantle or crust.

magnetic pole:  A region where the strength of the earth’s magnetic field is greatest and where the magnetic lines of force apparently enter or leave the earth (the earth has a distinct North and South pole just as a bar-magnet).

magnetization:  The magnetic moment per unit volume.  The magnetization of a rock is the sum of its two types: induced magnetization and remnant magnetization.

mantle:  The solid layer between the crust and the outer core, composed mainly of silicon, magnesium, and oxygen; subdivided in descending order into the lithosphere, asthenosphere, and mesosphere based on the density (or packing) of its constituent crystalline matter.

mass wasting:  A general term for the downslope movement of soil and rock material under the direct influence of gravity.

matrix:  The finer-grained, sometimes glassy material that surrounds larger crystals in igneous rock, the larger crystals are sometimes called phenocrysts.

meandering:  Tendency of a stream channel to gradually shift position across a valley as a result of simultaneous erosion and deposition on opposite stream banks over time.

medial moraine: A glacial moraine derived from the merger of two lateral moraines from coalescent valley glaciers.

Milankovitch Cycles: Refers to three astronomical variations in Earth’s orbit that induce changes in the amount of solar radiation received by Earth; generally considered major forcing factors in climate change.

mineral:  A naturally occurring solid with a definite, or only slightly variable chemical composition, and an ordered atomic structure formed mostly, but not entirely, by inorganic processes.

Miocene:  A geologic Epoch during the Tertiary Period.

moraine:  A typically linear- to arcuate-shaped ridge or hummocky mound of loose rock fragments and silt deposited by a glacier; often associated with a more specific depositional process or location and referred to as an end, ground, lateral, recessional, or terminal moraine.

mudflow:  A water-saturated mass of rock debris that travels downslope as a liquid under the pull of gravity.  A major potential hazard at many Cascade volcanoes, mudflows (or lahars, when specifically referring to mudflows generated from volcanic debris) typically originate in two ways: by the sudden collapse of areas of hydrothermally altered rock high on the volcanic cone or by melting snow and ice during an eruption.  Both types commonly travel tens of miles down valleys beyond the volcano.

nivation:  Frost action and mass-wasting beneath a snowbank.  The work of snow and ice beyond the limits of glacier action.

normal fault:  A fault formed where the hanging-wall block (the block overhead) moves downward compared to the foot-wall block (the block at your feet).

nucleus (atomic):  The center of the atom.  The nucleus contains one or more protons, which are particles with a positive electrical charge, and usually one or more neutral neutrons.

nuee ardent:  French term for a “glowing cloud” of hot gas and volcanic ash that typically rises above and extends beyond the margins of a pyroclastic flow because of its lower density.  See also pyroclastic surge.

nunatak: An often rocky portion of a ridge or mountain peak protruding above the level of surrounding glacial ice.

obsidian:  A dense, black, glassy volcanic rock largely devoid of bubbles or mineral crystals.  It is formed from highly silicic magma, typically by the collapse of frothy, pumiceous rhyolite.  Prominent domes or flows of obsidian occur at the Newberry and Medicine Lake volcanoes.

olivine:  A green or brown orthorhombic mineral of mafic, ultramafic, and low-silica igneous rocks (gabbro, basalt, peridotite, dunite): it crystallizes early from a magma, weathers readily at the earth’s surface, and metamorphoses to serpentine.

outcrop:  That part of a geologic formation or structure that appears at the surface of the earth; also, bedrock that is covered by surficial deposits such as alluvium.

outwash:  Sand and gravel deposited by meltwater streams in front of the end moraine or the margin of an active glacier.

outwash plain:  A broad, gently sloping sheet of outwash deposited by meltwater streams flowing in front of or beyond a glacier; a broad body of outwash.

oxidation:  Chemical reaction between a substance and oxygen that produces new substances; a type of weathering process.

pahoehoe:  Hawaiian word for basaltic lava with a smooth, ropy, or billowy crust related to its slightly lower than average viscosity.  Pahoehoe flows commonly contain lava tubes and may transition into aa lavas downslope as they cool and degas.

palagonite:  A yellow or orange isotropic mineraloid formed by hydration and devitrification of basaltic glass.  See also: palagonite tuff.

palagonite tuff:  An indurated deposit of glassy basaltic ash in which the constituent particles are largely altered to palagonite.

paleo:  A combining form meaning old or ancient.

paleomagnetism:  The study of natural remnant magnetization in order to determine the intensity and direction of the earth’s magnetic field in the geologic past.

paleosols:  A buried soil; a soil of the past.

parabolic dune:  Crescent shaped dune with an over-all shape that resembles the graph of a parabola with a depression (blowout) in the center and the two ends of the parabola pointing in the upwind direction

parent isotope:  Radioactive isotope that decays through time to a daughter isotope.

paternoster lake:  One of a linear series of small lakes occupying depressions in a glacial valley, connected by streams, rapids, or waterfalls.

Pelean eruption:  Explosive eruption producing pyroclastic flows and surges, commonly associated with dome growth and collapse.  Named for the 1902 eruption of Mont Pelee, a volcano on the Caribbean island of Martinique that killed 30,000 people, this type of activity is typical of Glacier Peak, Lassen Peak-Chaos Crags, and Mounts Shasta, St. Helens, and Hood.

period:  The fundamental division of time on the geologic time scale, their boundaries defined by the presence of key fossils.

permeable:  Said of a rock or sediment that allows water, oil, or gas to move through it at an appreciable rate via supercapillary openings.

phenocrysts:  Large mineral crystals in a fine-grained igneous rock.

phreatic eruption:  A violent steam explosion that produces little or no new lava, typically ejecting solid fragments of pre-existing rock from the volcanic edifice.  Triggered by the conversion of groundwater into steam by an underground heat source, small phreatic eruptions characterize the opening stage of many volcanic episodes, including those at Lassen Peak in 1914 and St Helens in 1980.

phreatomagmatic: A phreatic eruption that does produce some magma.

pillow basalt: Mafic lavas erupted in water that often form aggregated, pillow-like structures.

pillow lava:  A general term for those lavas displaying pillow structure and considered to have formed under water; such lava is usually basaltic or andesitic.

plagioclase:  A group of feldspar silicate minerals containing abundant alumina.  Plagioclase minerals are among the commonest rock-forming minerals.

plateau:  A relatively elevated area of comparatively flat land which is commonly limited on at least one side by an abrupt descent to lower ground; specif. an extensive land area more than 150-300 m above the adjacent country or above sea level.  It is higher than a plain and more extensive than a mesa.

playa:  A dry lake bed in an internally drained basin that is often characterized by a dry mud-cracked surface or a dusty salt flat of evaporate minerals.

plate tectonics:  Theory that Earth’s lithosphere is not seamlessly continuous but is broken into discrete pieces that move slowly relative to one another and change in size over geologic time.

Pleistocene:  The division of geologic time immediately preceding the Holocene and lasting from about 1.8 million to 10,000 or 12,000 years ago characterized by the development of large continental ice sheets throughout the Northern Hemisphere and by the formation of ice caps and valley glaciers in mountain ranges, including the Cascades, Rockies, and Sierra Nevada. 

plinian eruption:  A violently explosive eruption that produces an unusually high ash plume carrying tephra far into the stratosphere, commonly with associated pyroclastic flows.  Named after Pliny the Younger, who described the catastrophic eruption of Mount Vesuvius in AD 79.

Pliocene:  The division of geologic time immediately preceding the Pleistocene and lasting from about 5.3 million to about 1.8 million years ago.  Volcanism associated with the rise of the modern Cascade Range began during this time.

plucking:  A process of glacial erosion by which blocks of rock are loosened, detached, and borne away from bedrock by the freezing of water in fissures.

plug dome:  A volcanic dome characterized by an upheaved, consolidated mass filling the conduit.

plume:  A persistent pipe-like body of hot material moving upward from the earth’s mantle into the crust.  Its surface expression may be a hot spot.

pluvial lake:  A lake that exists only during a period of climatic cooling and lower than normal evaporation (often coincident with higher than normal rainfall); typically refers to ice-age lakes that formed in what are today mostly dry, enclosed drainage basins in the western United States. 

point bar:  A place where sediment accumulates on the inside of a bend in a stream, where the water flow is slow and shallow; these features typically form in low gradient, meandering streams, alternating from bank to bank in a down stream direction

porphyritic: An aphanitic igneous rock containing many phenocrysts; the dual grain sizes imply two distinct cooling periods, the first involved slow cooling of the magma at depth to form large crystals, the second involved rapid cooling of the remaining magma at or near the surface to form a fine-grained matrix.

proglacial:  Immediately in front of or just beyond the outer limits of a glacier or ice sheet, generally at or near its lower end; said of lakes, streams, deposits, and other features produced by or derived from the glacier ice.

pumice:  A highly porous volcanic rock formed of glassy, silicic, magmatic froth blown from a vent.  Usually light colored, pumice is so full of tiny bubbles that it is typically buoyant enough to float in water.

pumice flow:  A type of pyroclastic flow in which a large proportion of the fragments are of pumice.

pyroclastic:  A class of fragmental volcanic rocks formed when explosions break apart magma and vent rocks and eject the resulting rock fragments  and liquid drops and blobs that quickly quench into glass and fall to the ground.

pyroclastic flow:  A turbulent avalanche of incandescent rock fragments and hot gas that travels downslope like a heavy fluid; it may be composed either of frothy pumice or of dense lithic (nonporous) rock debris, such as the hot fragments from a collapsing lava dome.

pyroclastic rock:  Volcanic rock formed of lithified pyroclastic material.

pyroclastic surge:  A highly mobile cloud of hot gas and ash, less dense than a ground-hugging pyroclastic flow that it typically accompanies.  Surges are capable of surmounting topographical barriers and spreading laterally over large areas.

Quaternary:  The youngest geologic period (encompassing Pleistocene and Holocene time), which began about 1.8 million years ago and includes the present time.

radioactive decay:  The process by which unstable (radioactive) isotopes transform to new elements by a change in the number of protons and neutrons in the nucleus.

radiocarbon dating:  The calculation of the age of geologic deposits by any of the methods based on nuclear decay of natural radioactive elements in carbonaceous material, such as charcoal or wood from trees or other plants found in layers of volcanic ash.

radiometric dating: The calculation of the age of geologic deposits by any of the methods based on nuclear decay of natural radioactive elements; the process requires that a geologist accurately “daughter” isotope (the isotope that the parent decays into), as well as the rate of decay of parent to daughter.

ramp:  The steepened segment of a thrust fault, especially where a bedding thrust or decollement changes from a stratigraphically lower to a higher bed.

rampart:  A narrow ridge, up to a few meters in height, built by rocky debris accumulating at the foot of a perennial snowbank on a steep slope.  A crescentic or ring-like ridge of pyroclastics around the top of a volcano.

recessional moraine:  A typically linear- to arcuate-shaped ridge or hummocky mound of loose rock fragments and silt deposited during a temporary stillstand of a glacier in overall retreat; essentially a minor end moraine.

regolith:  Unconsolidated remains of weathered rock overlying solid rock, some of it transported as sediment and some of it remaining as fragments found above or near the source bedrock.

reverse fault:  A fault inclined at an angle steeper than 45 degrees and formed where the hanging wall rock moves upward compared to the footwall.

rift zone:  A linear spreading-center along a divergent plate boundary where crustal plates are separating and moving apart.  Magma is erupted along active rifts, creating new crust, while hot, plastic asthenosphere cools to form new lithosphere at depth; the crust and lithosphere are generally mafic in composition.

rhyolite:  Aphanitic igneous rock solidified from felsic magma.

rhyodacite: Aphanitic igneous rock solidified from felsic magma with a composition between dacite and rhyolite.

roche moutonnees: A glacially sculptured knob of bedrock, with its long axis oriented in the direction of ice movement, an upstream (stoss) side that is gently inclined, rounded, and hackly; see whaleback.

saltation: Sediment transport in which particles are moved forward in a series of short leaps or bounces, e.g. sand grains bounding downstream in a current not turbulent enough to retain them in suspension.

scarp:  An abbreviation for escarpment.  A long line of cliffs produced by movement along an earthquake fault or by erosion (usually by flowing water).

scree: An often steeply sloping accumulation of loose rock fragments.

scoria:  Glassy fragments of dark-colored volcanic rock, less porous than pumice, commonly produced by jets of semiliquid, gaseous, basaltic magma shot into the air during formation of a cinder cone.

sedimentary rock:  Rock consisting of the lithified particulate and precipitated dissolved products of the weathering and erosion of older rocks.

sedimentary structure:  A physical feature, such as cross-bedding or ripple marks, produced in sediment at the time it is deposited, or shortly after deposition and before lithification into rock.

shield volcano:  A broad, gently sloping volcanic edifice built almost exclusively by the quiet effusion of thin, fluid, basaltic lava flows.  Named for their supposed resemblance to an ancient Icelandic warrior’s shield laid down flat with the curved side upward.

silicic:  Said of a silica-rich igneous rock or magma.  The amount of silica is usually said to constitute at least 65 percent or two-thirds of the rock.  In addition to the combined silica in feldspars, silicic rocks generally contain free silica in the form of quartz.  Granite and rhyolite are typical silicic rocks.

silicic lava:  Lava rich in silica (over 62-64 percent) and having a relatively low melting point (about 1,500 degrees Fahrenheit); silicic magma typically emerges as a stiff, viscous mass and does not flow long distances, commonly piling up over the erupting vent to form a steep-sided lava dome.

silica tetrahedron:  The basic building block of the silicate mineral crystal structure, consisting of four oxygen atoms surrounding and bonded to a silicon atom.

sill:  A tabular, commonly horizontal or near-horizontal igneous intrusion that usually forms by injection of magma between sedimentary layers.

silt:  A detrital particle finer than fine sand and coarser than clay, commonly in the range of 1/16 to 1/256 mm.  A loose aggregate of rock or mineral particles of silt size, commonly with a high content of clay minerals.  Mud or fine earth suspended in water.

slump:  A type of slide where rock or regolith move by rotation along a curved surface of rupture.

spatial: Of or referring to distance or area.

stalactite:  A conical or cylindrical speleothem developed downward from the roof of a cave by precipitation of minerals from dripping water; usually composed of calcite.

stalagmite:  A conical or cylindrical speleothem developed upward from the floor of a cave by precipitation of minerals from dripping water; usually composed of calcite.

star dune:  Sand dune consisting of curving ridges of sand that radiate from the center and resulting from highly variable wind directions crossing large areas of readily eroded sand.

stillstand:  Stability of an area of land, as a continent or island, with reference to the earth’s interior or mean sea level, as might be reflected by a relatively unvarying base level or erosion between periods of crustal movement.   

stratigraphy:  Referring to the study of the facies and depositional environments associated with sedimentary rocks, or the facies and depositional environments themselves.  

stratovolcano: See composite cone

stream terrace:  A raised bench of fluvial sediments representing an old floodplain now incised by the modern, active stream system.

striation:  The straight scratches or grooves incised on a rock surface by rock fragments carried in a flowing glacier.

strike-slip fault:  A fault where rocks are displaced by horizontal movement along the strike direction of the fault plane.

Strombolian eruption:  A type of volcanic eruption characterized by jetting of clots or “fountains” of fluid basaltic lava from a central crater.

subduction: The process by which a denser plate of crust and lithosphere descends beneath a neighboring plate into the deeper mantle at a convergent plate boundary.

suspended load:  The fine-grained sediment intimately mixed with the water and flowing above the bed of a stream.

spatter:  Liquid or plastic fragments of lava blown from a vent.

spit:  An elongate sediment ridge formed by longshore currents and that projects from headlands in the direction of the longshore current.  Wave refraction around the end of the spit causes sediment deposition along the tip and landward side of the spit and may form a prominent hook-shaped beach.

talus:  Rock fragments, usually coarse and angular, lying at the base of a cliff or steep slope from which they have been derived; also, the heap or mass of such broken rock, considered as a unit.

tarn:  see cirque lake.

tectonic:  Referring to movement or deformation of part of the earth’s crust, such as the uplift that results in mountain building (orogenesis).

temporal: Of or referring to passage of time.

tephra: Pyroclastic material that is thrown into the air above a volcano.  Tephra can range in size from fine dust and ash to lava blocks tens of feet in diameter.  The term, first used by Aristotle, is the Greek word for “ash”.

terminal moraine:  A typically arcuate-shaped ridge of loose rock fragments and silt deposited by a glacier at its terminus.

terminus:  The outer margin or extremity of a glacier.

terrain: Topographic variation in a landscape.

terrane:  An obsolescent term applied to a rock or group of rocks and to the area in which they crop out.  The term is used in a general sense and does not imply a specific rock unit.

Tertiary:  The geologic Period covering the span of time between about 65 million and 1.8 million years ago, encompassing from oldest to youngest, the Paleocene, Eocene, Oligocene, Miocene, and Pliocene Epochs.

thalweg:  The zone of highest velocity in a stream channel; also the deepest or best navigable portion of the channel.

tholeiite:  A basalt characterized by the presence of orthopyroxene and or pigeonite in addition to clinopyroxene and calcic plagioclase.  Olivine may be present.  The term is derived from Tholey, Saarland, Germany.

thrust fault:  A fault dipping at an angle less than 45 degrees and formed where the hanging wall rocks move upward compared to the footwall.

till:  An unconsolidated, poorly sorted, and unstratified debris consisting of subangular to subrounded clasts from boulders to gravel, sand, and mud that was eroded, transported, and deposited by a glacier; till often forms moraines, but can also be deposited as a blanket of loose material on the landscape.

topography:  The general configuration of a land surface, including its relief and the position of its natural and man-made features.

transform plate boundary:  Zones where lithospheric plates slide past one another with neither creation nor destruction of lithosphere.

transverse dune:  A linear or curving sand dune with a crest oriented perpendicular to the prevailing wind direction; forms where calcite precipitates from water around springs.

tributary stream:  Any stream that contributes water to another stream.

tuff:  Fine-grained igneous rock composed mostly of volcanic ash, but which often contains lapilli and rock fragments.  Welded tuff is a rock formed from pyroclastic material hot enough to fuse or weld together when emplaced.

valley train:  A long, narrow body of outwash, deposited by meltwater streams far beyond the terminal moraine or the margin of an active glacier and confined within the walls of a valley below the glacier; it may or may not emerge from the mouth of the valley to join an outwash plain.

varve:  A sedimentary lamina or sequence of laminae deposited in a body of still water within one year’s time; specif. a pair of layers seasonally deposited in a glacial lake.  A glacially-produced varve normally includes a lower “summer” layer consisting of light-colored sand or silt, which grades upward into a thinner “winter” layer, consisting of clayey, often organic, dark sediment.  Counting and correlation of varves have been used to measure the ages of Pleistocene glacial deposits.

vent:  An opening, typically cylindrical in form, in the earth’s surface through which volcanic material is ejected.

vesicular:  Referring to pores or tiny cavities in a volcanic rock, formed by the development of gas bubbles in the liquid magma.  Some pumice and scoria is vesicular enough to resemble a porous sponge.

viscosity:  A liquid’s resistance to flow; in a magma, viscosity is largely determined by temperature (heat content), gas content, and the chemical composition of the molten material, particularly its silica content.

vitric:  Referring to volcanic material consisting mainly of glassy matter, such as vitric ash, which is at least 75 percent glass.

volcanic arc:  A curved (arcuate) line of volcanoes paralleling a subduction zone.

volcanic ash:  Fine pyroclastic matter (under 2 mm in diameter).  The term usually refers to unconsolidated material, but is sometimes also used for its consolidated counterpart, or tuff.

volcanic bomb:  A blob of lava that was ejected while viscous and received a rounded shape while in flight.  It is larger than 64 mm in diameter and that may or may not have a matrix.

volcanic chain:  Linear arrangement of a number of volcanoes, apparently associated with a major geologic feature such as a fault or subduction zone.

volcanic conduit:  The channel way that brings volcanic material up from depth; the vent.

volcanic cone:  A conical hill of lava and/or pyroclastics that is built up around a volcanic vent.

volcanic dome:  A steep-sided protrusion of viscous lava squeezed out from a volcano, forming a dome-shaped or bulbous mass above and around the vent.  Older lavas may be lifted by the pressure of new lava rising from below.  The structure generally develops inside a volcanic crater or on the flank of a large volcano, and is usually much fissured and brecciated.

volcanism:  The process by which magma and its associated gases rise into the crust and are extruded onto the earth’s surface and into the atmosphere.

weathering:  The process of deterioration of rocks at Earth’s surface; more systematically defined as the response of the solid geosphere (crustal rocks) at its interface with the atmosphere, hydrosphere, and biosphere to its physical and chemical decomposition into loose particles, while dissolving some minerals and producing new ones.

whaleback:  A large mound or hill having the general shape of a whale’s back, esp. a smooth elongated ridge of desert sand having a rounded crest and ranging widely in size (about 300 km long, 1-3 km wide, and perhaps 50 m high).  It forms a coarse-grained platform or pedestal built up and left behind by a succession of longitudinal dunes along the same path.