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Geology of the Pacific Northwest



The following terms and their definitions come from glossaries found at:
  1. Iverson Software web site ("Geology")
  2. The Volcano World web site ("Volcanic and Geologic Terms")
  3. USGS/NPS web site ("Geologic Glossary")
  4. Alt, David, and Hyndman, Donald W., 1995, 3rd printing 1998, Northwest Exposures: A Geologic Story of the Northwest (Mountain Press Publishing Company, Missoula, Montana, pp.409-413)
  5. Chernicoff, Stanley, and Fox, Haydn A., 2000, Essentials of Geology (Houghton Mifflin Company, Boston, pp.G1-G14)
  6. Pellant, Chris, 1992, Rocks and Minerals (Dorling Kindersley, London, pp.250-251)
  7. Viens, Rob, 2001, Geology of the Pacific Northwest Field Handbook and Lecture Handouts (Bellevue Community College, Bellevue, Washington)


Absolute Dating:
The fixing of a geological structure or event in time, as by counting tree rings. See also relative dating.
A mineral whose presence in a rock is not essential to the proper classification of the rock.
Pyroclastic rocks that are formed from fragments of non-volcanic rocks or from volcanic rocks not related to the erupting volcano.
A descriptive term applied to igneous rocks with more than 60% silica (SiO2).
Active Volcano:
An erupting volcano or volcano that is not presently erupting but that has erupted within historical time and is considered likely to do so again in the future.
Alluvial Fan:
A triangular deposit of sediment left by a stream than has lost velocity upon entering a broad, relatively flat valley.
The second most abundant volcanic rock, it is medium dark in color and contains around 60 percent silica and moderate amounts of iron and magnesium. Andesites are found in large quantities in the Andes Mountains.
A hard, jet-black coal that develops from lignite and bituminous coal through metamorphism, has a carbon content of 92-98%, and contains little or so gas. Anthracite burns with an extremely hot, blue flame and very little smoke, but it is difficult to ignite and both difficult and dangerous to mine.
A fold that bends layered rocks up into an arch, the central part of which contains the oldest section of rock. See also syncline.
Fine-grained mineral. (See phaneritic.)
Fine particles of pulverized rock blown from an explosion vent. Measuring less than 1/10 inch in diameter, 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 magma).
Ashfall (Airfall):
Volcanic ash that has fallen through the air from an eruption cloud. A deposit so formed is usually well sorted and layered.
Ash Flow:
A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure. The mass of pyroclastics is normally of very high temperature and moves rapidly down the slopes or even along a level surface.
A zone of partially melted rock within the earth's mantle, the lower boundary of the lithosphere. Some tens of kilometers below the surface and of undefined thickness, the asthenosphere is a shell of weakness where plastic movements take place to permit pressure adjustments.
A body of rock that contains significant quantities of water that can be tapped by wells or springs.
A large mass of material or mixtures of material falling or sliding rapidly under the force of gravity. Avalanches often are classified by their content, such as snow, ice, soil, or rock avalanches. A mixture of these materials is a debris avalanche.


A dark-colored iron and magnesium rich volcanic rock or lava that makes up most of the ocean floor. It is the most abundant volcanic rock in the earth's crust.
The characteristic planar fabric of gneiss.
The complex assortment of granite, gneiss, and schist that forms the bulk of the continental crust.
A round or oval depression in the earth's surface, containing the youngest section of rock in its lowest, central part.
A massive discordant pluton with a surface area greater than 100 square kilometers, typically having a depth of about 30 kilometers. Batholiths are generally found in elongated mountain ranges after the country rock above them has eroded.
The division of sediment or sedimentary rock into parallel layers (beds) that can be distinguished from each other by such features as chemical composition and grain size.
Bedding Planes:
The top surface of a bed; .
The unstable, newly-formed front of a lava delta.
Bituminous Coal:
A shiny black coal that develops from deeply buried lignite through heat and pressure and which has a carbon content of 80-93%, making it a more efficient heating fuel than lignite.
A swelling of the crust of a lava flow formed by the puffing-up of gas or vapor beneath the flow. Blisters are about 1 meter in diameter and hollow.
Angular chunk of solid rock ejected during an eruption.
Fragment of molten or semi-molten rock, 2 1/2 inches to many feet in diameter, which is blown out during an eruption. Because of their plastic condition, bombs are often modified in shape during their flight or upon impact.
Braided Stream:
A network of converging and diverging streams separated from each other by narrow strips of sand and gravel.
A clastic rock composed of particles more than 2 mm in diameter and marked by the angularity of its component grains and rock fragments..
Brittle Failure:
The rupturing of a rock, a type of permanent deformation caused by great stress under relatively low temperature and pressure conditions. See also plastic deformation.


The Spanish word for cauldron, a caldera is a basin-shaped volcanic depression, at least a mile in diameter. Such depressions are formed when an eruption substantially empties the reservoir of magma beneath the cone's summit. Eventually the summit collapses inward, creating the caldera. Crater Lake occupies the best-known caldera in the Cascades.
Carbon-14 Dating:
A form of radiometric dating that relies on the 5730-year half-life of radioactive carbon-14, which decays into nitrogen-14, to determine the age of rocks in which carbon-14 is present. Carbon-14 dating is used for rocks from 100 to 100,000 years old.
Cenozoic Era:
The era that began 66 million years ago and is divided into two periods, the Tertiary (the introduction of mammals as well as distinct landforms and geographic features such as the Columbia Plateau and Cascade Mountains) and the Quaternary (the Ice Age and the introduction of humans).
Central Vent:
A central vent is an opening at the Earth's surface of a volcanic conduit of cylindrical or pipe-like form.
Central Volcano:
A volcano constructed by the ejection of debris and lava flows from a central point, forming a more or less symmetrical volcano.
Chemical Weathering:
The process by which chemical reactions alter the chemical composition of rocks and minerals that are unstable at the earth's surface and convert them into more stable substances. See also mechanical weathering.
Cinder Cone:
A volcanic cone built entirely of loose fragmented material (pyroclastics).
A steep-walled horseshoe-shaped recess high on a mountain that is formed by glacial erosion.
Grains of rock.
Being or pertaining to a sedimentary rock composed primarily from fragments of preexisting rocks or fossils.
The way certain minerals break along planes related to their internal atomic structure.
Composite Volcano:
A steep volcanic cone built by both lava flows and pyroclastic eruptions.
Compound Volcano:
A volcano that consists of a complex of two or more vents, or a volcano that has an associated volcanic dome, either in its crater or on its flanks. Examples are Vesuvius and Mont Pelee.
Compression Waves:
Earthquake waves that move like a slinky. As the wave moves to the left, for example, it expands and compresses in the same direction as it moves. Usage of compression waves.
A passage followed by magma in a volcano.
Curved or shell-like fracture in many minerals and some rocks.
A clastic rock composed of particles more than 2 mm in diameter and marked by the roundness of its component grains and rock fragments.
Contact Metamorphism:
Metamorphism caused by heat from a magmatic intrusion.
Continental Collision:
The convergence of two continental plates, resulting in the formation of mountain ranges.
Continental Crust:
Solid, outer layers of the earth, including the rocks of the continents. Usage of continental crust.
Continental Drift:
The theory that horizontal movement of the earth's surface causes slow, relative movements of the continents toward or away from one another.
The coming together of two lithospheric plates. Convergence causes subduction (when one or both plates is oceanic) and mountain formation (when both plates are continent). See also divergence.
The innermost layer of the earth, consiting primarily of pure metals such as iron and nickel. The core is the densest layer of the earth and is divided into the outer core, which is believed to be liquid, and the inner core, which is believed to be solid. See also crust and mantle.
Country Rocks:
The rock intruded by and surrounding an igneous intrusion.
A steep-sided, usually circular depression formed by either explosion or collapse at a volcanic vent.
A part of the earth's crust that has attained stability and has been little deformed for a prolonged period.
The upper surface of the lithosphere. Continental crust consists mostly of granite, gneiss, and schist; oceanic crust, of basalt.
Curtain of Fire:
A row of coalescing lava fountains along a fissure; a typical feature of a Hawaiian-type eruption.


Volcanic rock (or lava) that characteristically is light in color and contains 62% to 69% silica and moderate amounts of sodium and potassium.
Debris Avalanche:
A rapid and unusually sudden sliding or flowage of unsorted masses of rock and other material. As applied to the major avalanche involved in the eruption of Mount St. Helens, a rapid mass movement that included fragmented cold and hot volcanic rock, water, snow, glacier ice, trees, and some hot pyroclastic material. Most of the May 18, 1980 deposits in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are from the debris avalanche.
Debris Flow:
A mixture of water-saturated rock debris that flows downslope under the force of gravity (also called lahar or mudflow).
Detachment Plane:
The surface along which a landslide disconnects from its original position.
A period of time in the Paleozoic Era that covered the time span between 400 and 345 million years.
The process by which soft sediment is turned into rock.
A breccia filled volcanic pipe that was formed by a gaseous explosion.
A sheetlike body of igneous rock that cuts across layering or contacts in the rock into which it intrudes.
The process by which two lithospheric plates, separated by rifting, move further apart, with soft mantle rock rising between them and forming new oceanic lithosphere. See also convergence.
A steep-sided mass of viscous lava extruded from a volcanic vent (often circular in plane view) and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome.
Dormant Volcano:
Literally, "sleeping" volcano. The term is used to describe a volcano which is presently inactive but which may erupt again. Most of the major Cascadevolcanoes are believed to be dormant rather than extinct.
Drainage Basin:
The area of land drained by a river system.


A movement within the earth's mantle.
Set of geologic features that are in an overlapping or a staggered arrangement (e.g., faults). Each is relatively short, but collectively they form a linear zone in which the strike of the individual features is oblique to that of the zone as a whole.
Material that is thrown out by a volcano, including pyroclastic material (tephra) and lava bombs.
The process by which solid, liquid, and gaseous materials are ejected into the earth's atmosphere and onto the earth's surface by volcanic activity. Eruptions range from the quiet overflow of liquid rock to the tremendously violent expulsion of pyroclastics.
Eruption Cloud:
The column of gases, ash, and larger rock fragments rising from a crater or other vent. If it is of sufficient volume and velocity, this gaseous column may reach many miles into the stratosphere, where high winds will carry it long distances.
Eruptive Vent:
The opening through which volcanic material is emitted.
Temporarily move people away from possible danger.
Mineral or rock formed by the evaporation of saline water.
Exotic Terrane:
A terrane that originated elsewhere but was transported by plate motion and attached, or accreted, to a continent's coast by collision.
Extinct Volcano:
A volcano that is not presently erupting and is not likely to do so for a very long time in the future. Usage of extinct.
The emission of magmatic material at the earth's surface. Also, the structure or form produced by the process (e.g., a lava flow, volcanic dome, or certain pyroclastic rocks).
Extrusive Rock:
An igneous rock formed from lava that has flowed out onto the earth's surface, characterized by rapid solidification and grains that are so small as to be barely visible to the naked eye.


A crack or fracture in the earth's surface, along which the rocks on either side have shifted. Movement along the fault can cause earthquakes or–in the process of mountain-building–can release underlying magma and permit it to rise to the surface.
Fault Scarp
A steep slope or cliff formed directly by movement along a fault and representing the exposed surface of the fault before modification by erosion and weathering.
A large family of aluminum silicate minerals, together the most abundant in the continental crust. The major kinds of feldspar are plagioclase and orthoclase.
An igneous rock having abundant light-colored minerals.
Fire fountain:
See also: lava fountain
Elongated fractures or cracks on the slopes of a volcano. Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.
Flank Eruption:
An eruption from the side of a volcano (in contrast to a summit eruption.)
Produced by the action of of flowing water.
A bend that develops in an initially horizontal layer of rock, usually caused by plastic deformation. Folds occur most frequently in sedimentary rocks.
The arrangement of a set of minerals in parallel, sheet-like layers that lie perpendicular to the flattened plane of a rock.
A body of rock identified by lithic characteristics and stratigraphic position and is mappable at the earth's surface or traceable in the subsurface.
Any record of past life preserved in the crustal rocks, such as bones and shells, footprints, excrement, and borings.
The manner of breaking due to intense folding or faulting. There are four kinds of fracture: conchoidal (shell-like curves with sharp edges); fibrous (asbestos-like); rough (like pyrite); and smooth (like kaolinite).
A vent or opening through which issue steam, hydrogen sulfide, or other gases. The craters of many dormant volcanoes contain active fumaroles.


Geothermal Energy:
Energy derived from the internal heat of the earth.
Geothermal Power:
Power generated by using the heat energy of the earth.
Glacial Till:
Drift that is deposited directly from glacial ice and, therefore, not sorted.
A metamorphic rock characterized by the segregation of different minerals into distinct layers, resulting in a rock with a banded appearance.
An elongate crustal block that is relatively depressed between two fault systems.
A pink-colored, felsic, plutonic (intrusive) rock that contains potassium and usually sodium feldspar, and has a quartz content of about 10%. Granite is commonly found on continents but virtually absent from the ocean basins.
Smaller mineral grains in a porphyritic igneous rock; also called a matrix. See also phenocrysts.
A type of seamount that has a platform top. Named for a nineteenth-century Swiss-American geologist.


The resistance of a mineral to scratching.
Harmonic Tremor:
A continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault.
Heat transfer:
Movement of heat from one place to another.
Material is made up of a heterogeneous mix of different rock types. Instead of being composed on one rock type, it is composed of fragments of many different rocks.
The time period from 10,000 years ago to the present. Also, the rocks and deposits of that age.
Hot Spot:
An isolated volcano that has no apparent relationship to plate boundaries.
Molecules that are made up entirely of hydrogen and carbon.
Hydrothermal Reservoir:
An underground zone of porous rock containing hot water.


Igneous Rock:
A rock made from molten (melted) or partly molten material that has cooled and solidified.
A measure of the effects of an earthquake at a particular place. Intensity depends not only on the magnitude of the earthquake, but also on the distance from the epicenter and the local geology.
The process of emplacement of magma in pre-existing rock. Also, the term refers to igneous rock mass so formed within the surrounding rock.
Intrusive Rock:
An igneous rock formed by the entrance of magma into preexisting rock.


A surface of fracture in a rock.
Pyroclastic material derived directly from magma reaching the surface.


A solid, waxy, organic substance that forms when pressure and heat from the Earth act on the remains of plants and animals. Kerogen converts to various liquid and gaseous hydrocarbons at a depth of 7 or more kilometers and a temperature between 50 degrees and 100 degrees.
An area surrounded by a lava flow.


A body of igneous rocks with a flat bottom and domed top. It is parallel to the layers above and below it.
A torrential flow of water-saturated volcanic debris down the slope of a volcano in response to gravity. A type of mudflow. Usage of lahar. For a larger discussion on lahars, click here.
Magma which has reached the surface through a volcanic eruption. The term is most commonly applied to streams of liquid rock that flow from a crater or fissure. It also refers to cooled and solidified rock.
Lava Dome:
Mass of lava, created by many individual flows, that has built a dome-shaped pile of lava.
Lava Flow:
An outpouring of lava onto the land surface from a vent or fissure. Also, a solidified tongue like or sheet-like body formed by outpouring lava.
Lava Fountain:
A rhythmic vertical fountainlike eruption of lava.
Lava Lake (Pond):
A lake of molten lava, usually basaltic, contained in a vent, crater, or broad depression of a shield volcano.
Lava Shields:
A shield volcano made of basaltic lava.
Lava Tube:
A tunnel formed when the surface of a lava flow cools and solidifies while the still-molten interior flows through and drains away.
A soft, brownish coal that develops from peat through bacterial action, is right in kerogen, and has a carbon content of 70%, which makes it a more efficient heating fuel than past.
Of or pertaining to stone.
A sedimentary rock composed mostly of calcite, the mineral form of calcium carbonate.
Turned into rock.
The rigid crust and uppermost mantle of the earth. Thickness is on the order of 60 miles (100 km). Stronger than the underlying asthenosphere.
The reflection of light from the surface of a mineral.


An igneous rock composed chiefly of one or more dark-colored minerals.
Molten rock beneath the surface of the earth.
Magma Chamber:
The subterranean cavity containing the gas-rich liquid magma which feeds a volcano.
Pertaining to magma.
A numerical expression of the amount of energy released by an earthquake, determined by measuring earthquake waves on standardized recording instruments (seismographs.) The number scale for magnitudes is logarithmic rather than arithmetic. Therefore, deflections on a seismograph for a magnitude 5 earthquake, for example, are 10 times greater than those for a magnitude 4 earthquake, 100 times greater than for a magnitude 3 earthquake, and so on.
The middle layer of the earth, lying just below the crust and consisting of relatively dense rocks. The mantle is divided into two sections; the lower mantle has greater density than the upper mantle. See also core and crust.
The solid matter in which a fossil or crystal is embedded. Also, a binding substance (e.g., cement in concrete).
A mixture of rocks and terranes formed by tectonic movements, sedimentary sliding, or a combination of such processes.
Mechanical Weathering:
The process by which a rock or mineral is broken down into smaller fragments without altering its chemical makeup. See also chemical weathering.
Mesozoic Era:
That era of Earth's history 245 million years ago during dinosaurs coexisted with mammals, flowering plants, and birds for 179 million years. The era includes the Triassic, Jurassic, and Cretaceous periods.
Metamorphic Differentiation:
Theprocess by which minerals from a chemically uniform rock separate from each other during metamorphism and form individual layers within a new metamorphic rock.
Metamorphic Rock:
A rock that has undergone chemical or structural changes caused by heat, pressure, or a chemical reaction.
The process by which conditions within the earth alter the mineral content, chemical composition, and structure of solid (igneous, sedimentary, and metamorphic) rocks without melting them.
A single, large mass of glacial till that accumulates, typically at the edge of a glacier.
A flowage of water-saturated earth material possessing a high degree of fluidity during movement. A less-saturated flowing mass is often called a debris flow. A mudflow originating on the flank of a volcano is properly called a lahar.


North Cascades:
A region of northwestern Washington that was added to North America as a series of islands landed against the western edge of the continent.


A black or dark-colored volcanic glass, usually composed of rhyolite.
Oceanic Crust:
The earth's crust where it underlies oceans.
A slice of oceanic crust incorporated into a continent, generally as part of a trench filling.
A kind of feldspar, a pink or white potassium aluminum silicate mineral. It typically abounds in pale igneous or metamorphic rocks that also contain quartz.


Paleozoic Era:
That era of Earth's history between 570-245 million years ago during which marine life, insects, amphibians, reptiles, and forests appeared. The single super-continent Pangaea broke apart four different times.
Pangaea (pronounced pan-JEE-uh) was a supercontinent that included most of the earth's continental crust in two continents, Gondwana and Laurasia during Permian and Triassic times. Then it broke into the modern continents.
A soft, brown mass of compressed, partially decomposed vegetation that forms in a water-saturated environment and has a carbon content of 50%. Dried peat can be burned as fuel.
Igneous rocks in which the molecular proportion of aluminum oxide is less than that of sodium and potassium oxides combined.
Coarse-grained mineral. (See aphanitic).
Larger mineral grains within the fine-grained groundmass of a porphyritic igneous rock.
An explosive volcanic eruption that results from the interaction of surface or subsurface water and magma.
A foliated metamorphic rock that develops from slate and is marked by a silky sheen and medium grain size.
Pillow basalt:
Basalt that flowed out into long cylinders as it erupted under water. When seen exposed in section in a roadcut or cliff, the cylinders suggest a pile of pillows..
Pillow lava:
Interconnected, sack-like bodies of lava formed underwater.
A vertical conduit through the Earth's crust below a volcano, through which magmatic materials have passed. Commonly filled with volcanic breccia and fragments of older rock.
Pit Crater:
A crater formed by sinking in of the surface, not primarily a vent for lava.
A kind of feldspar that contains sodium and calcium. Plagioclase (pronounced PLA-jee-o-CLASE) is white or greenish, and typically occurs in medium to dark igneous or metamorphic rocks, most abundantly in those that contain little or no quartz.
Capable of being molded into any form, which is retained.
Plastic Deformation:
A permanent change in the shape or volume of a rock caused by great stress under higher temperatures and pressures than that which produce brittle failure.
A segment of the lithosphere. The lithosphere is divided into a dozen or more plates of widely various sizes that move about on the earth's surface.
Plate Tectonics:
The theory that the earth's crust is broken into about 10 fragments (plates) which move in relation to one another, shifting continents, forming new ocean crust, and stimulating volcanic eruptions.
A epoch in Earth's history from about 2-5 million years to 10,000 years ago. Also refers to the rocks and sediment deposited in that epoch.
Solidified lava that fills the conduit of a volcano. It is usually more resistant to erosion than the material making up the surrounding cone, and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.
Plug Dome:
The steep-sided, rounded mound formed when viscous lava wells up into a crater and is too stiff to flow away. It piles up as a dome-shaped mass, often completely filling the vent from which it emerged.
A large igneous intrusion formed at great depth in the crust.
Originating in various ways or from various sources.
A common igneous texture represented by two distinct grain sizes.
All geologic time from the beginning of Earth history to 570 million years ago. Also refers to the rocks that formed in that epoch.
A form of rhyolite so full of minute gas bubbles that it is essentially a glass foam. The light-colored volcanic rock is formed by the expansion of gas in erupting lava.
Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.
Pyroclastic Flow:
Lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass shards) that can move at high speed (50 to 100 miles an hour.) The term also can refer to the deposit so formed.


The mineral form of silica, silicon dioxide. Quartz occurs in many forms, most typically in transparent crystals that look like grains of glass. Although feldspar is volumetrically more abundant, no other mineral occurs in as many kinds of rocks as quartz.
The period of Earth's history from about 2 million years ago to the present; also, the rocks and deposits of that age.


Radiometric Dating:
A variety of methods by which absolute ages for minerals and rocks can be obtained by studying the ratio betwen stable (or radioactive) daughter products and their parent elements. See also carbon-14 dating.
Regional metamorphism:
Occurs when rocks undergo increased temperatures and pressures. It is associated with the formation of mountain belts along convergent plate boundaries, where rocks may be buried to great depths (10-20 km). The additional pressure causes tabular minerals in the rock to grow parallel to each other and perpendicular to the direction of pressure (stress), generating a mineral alignment termed foliation.
Relative Dating:
The fixing of a geologic structure or event in a chronological sequence relative to other geologic structures or events. See also absolute dating.
The vertical difference between the summit of a mountain and the adjacent valley or plain.
The interval of time between volcanic eruptions.
An extrusive rock intermediate in composition between dacite and rhyolite.
Volcanic rock (or lava) that characteristically is light in color, contains 69% silica or more, and is rich in potassium and sodium.
Ridge, Oceanic:
A major submarine mountain range.
Richter scale:
A logarithmic scale that measures the amount of energy released during an earthquake on the basis of the amplitude of the highest peak recorded on a seismogram. Each unit increase in the Richter scale represents a 10-fold increase in the amplitude recorded on the seismogram and a 30-fold increase in energy released by the earthquake.
The tearing apart of a plate to form a depression in the earth's crust and often eventually separating the plate into two or more smaller plates.
Rift System:
The oceanic ridges formed where tectonic plates are separating and a new crust is being created; also, their on-land counterparts such as the East African Rift.
Rift Zone:
A zone of volcanic features associated with underlying dikes. The location of the rift is marked by cracks, faults, and vents.
Ring of Fire:
The regions of mountain-building earthquakes and volcanoes which surround the Pacific Ocean.


A metamorphic rock characterized by the parallel arrangement of platy or elongate minerals (micas and amphiboles).
The planar fabric of schist.
A bomb-size (> 64 mm) pyroclast that is irregular in form and generally very vesicular. It is usually heavier, darker, and more crystalline than pumice.
Sea-Floor Spreading:
The mechanism by which new seafloor crust is created at oceanic ridges and slowly spreads away as plates are separating.
A submarine volcano.
An instrument that records vibrations of the earth (seismic waves).
Scientists who study earthquake waves and what they tell us about the inside of the Earth. Usage of seismologist.
An instrument that measures motion of the ground caused by earthquake waves. Usage of seismometer.
A dark green rock that forms as peridotite reacts with water, typically beneath the oceanic crust.
The motion of surfaces sliding past one another.
Shear Waves:
Earthquake waves that move up and down as the wave itself moves. For example, to the left. Usage of shear waves.
Shield Volcano:
A gently sloping volcano in the shape of a flattened dome and built almost exclusively of lava flows.
A chemical combination of silicon and oxygen.
A tabular body of intrusive igneous rock, parallel to the layering of the rocks into which it intrudes.
An opening formed by a collapse in the roof of a lava tube.
A fine-grained, metamorphic rock that develops from shale and tends to break into thin, flat sheets.
Slaty Cleavage:
The tendency of a metamorphic rock to split along very regular parallel planes (not sedimentary bedding planes, however).
Spatter Cone:
A low, steep-sided cone of spatter built up on a fissure or vent. It is usually of basaltic material.
Spatter Rampart:
A ridge of congealed pyroclastic material (usually basaltic) built up on a fissure or vent.
Specific Gravity:
The density of a mineral divided by the density of water.
Horn-like projections formed upon a lava dome.
A cone shaped deposit of minerals hanging from the roof of a cavern.
The study of rock strata, especially of their distribution, deposition, and age.
The layering of sediment.
A volcano composed of both lava flows and pyroclastic material.
Sediment accumulated in horizontal layers; also called beds.
The color of a mineral in the powdered form.
Strike-Slip Fault:
A nearly vertical fault with side-slipping displacement.
Strombolian Eruption:
A type of volcanic eruption characterized by jetting of clots or fountains of fluid basaltic lava from a central crater.
The sinking of an oceanic plate edge as a result of convergence with a plate of lesser density. Subduction often causes earthquakes and creates volcano chains.
A ring-shaped cloud of gas and suspended solid debris that moves radially outward at high velocity as a density flow from the base of a vertical eruption column accompanying a volcanic eruption or crater formation.
A fold that warps layered rocks down into a trough. See also anticline.


A slope formed a the base of a steeper slope, made of fallen and disintegrated materials.
Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.
The collection, preparation, petrographic description, and approximate dating of tephra.
A rock formation or assemblage of rock formations that share a common geologic history. A geologic terrane is distinguished from neighboring terranes by its different history, either in its formation or in its subsequent deformation and/or metamorphism. Terranes are separated by faults. See exotic terrane.
A disorderly mixture of debris of all sizes deposited directly from glacial ice.
The angle between the slope of a part of a volcano and some reference. The reference may be the slope of the volcano at some previous time.
Transform Motion:
The movement of two adjacent lithospheric plates in opposite directions along a parallel line at their common edge. Transform motion often causes earthquakes.
Low amplitude, continuous earthquake activity often associated with magma movement.
A great sea wave produced by a submarine earthquake, volcanic eruption, or large landslide.
Rock formed of pyroclastic material.
Tuff Cone:
A type of volcanic cone formed by the interaction of basaltic magma and water. Smaller and steeper than a tuff ring.
Tuff Ring:
A wide, low-rimmed, well-bedded accumulation of hyalo-clastic debris built around a volcanic vent located in a lake, coastal zone, marsh, or area of abundant ground water.
A doming or small mound on the crest of a lava flow caused by pressure due to the difference in the rate of flow between the cooler crust and the more fluid lava below.


Igneous rocks made mostly of the mafic minerals hypersthene, augite, and/or olivine.
A substantial break or gap in the geologic record where a rock unit is overlain by another that is not next in stratigraphic sucession, such as an interruption in continuity of a depositional sequence of sedimentary rocks or a break between eroded igneous rocks and younger sedimentary strata. It results from a change that caused deposition to cease for a considerable time, and it normally implies uplift and erosion with loss of the previous formed record.


The opening at the earth's surface through which volcanic materials issue forth. Usage of vent.
A small air pocket or cavity formed in volcanic rock during solidification.
A measure of resistance to flow in a liquid (water has low viscosity, while honey has a higher viscosity.)
Having a relatively high resistance to flow (the flow is slow, like honey).
Volcanic Arc:
A generally curved linear belt of volcanoes above a subduction zone, and the volcanic and plutonic (intrusive) rocks formed there.
Volcanic Complex:
A persistent volcanic vent area that has built a complex combination of volcanic landforms.
Volcanic Cone:
A mound of loose material that was ejected ballistically.
Volcanic Neck:
A massive pillar of rock more resistant to erosion than the lavas and pyroclastic rocks of a volcanic cone.
Roman god of fire and the forge, after whom volcanoes are named.
A type of eruption consisting of the explosive ejection of incandescent fragments of new viscous lava, usually on the form of blocks.
The set of geological processes that result in the expulsion of lava, pyroclastics, and gases at the earth's surface.
The solid structure created when lava, gases, and hot particles escape to the earth's surface through vents. Volcanoes are usually conical. A volcano is "active" when it is erupting or has erupted recently. Volcanoes that have not erupted recently but are considered likely to erupt in the future are said to be "dormant." A volcano that has not erupted for a long time and is not expected to erupt in the future is "extinct."


Water Table:
The surface between where the pore space in rock is filled with water and where the pore space in rock is filled with air.
The process by which rocks, at the earth's surface, break down. See chemical weathering and mechanical weathering.
Western Cascades:
The chain of Cascade volcanoes that became extinct about eighteen million years ago.


A crystal that resembles a phenocryst in igneous rock but is foreign to the body of rock in which it occurs.
A foreign inclusion in an igneous rock.

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