Photos of Pacific Coast, Cascades, Columbia
Plateau
Geology of the Pacific Northwest

Glossary

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A  (back to top)

absolute age
the age of a geologic material measured quantitatively, in units of time, usually in years.
active margin
the edge of a continent along which plate convergence or transform plate motion is occurring.
accreted terrane
a body of rock that originated elsewhere, on a different tectonic plate, and was then transferred to the continent at a subduction zone or along a transform fault. Evidence that a mass of rock is an accreted terrane includes: (1) it is bounded by faults; (2) it contains a sequence of rocks that record a geologic history not shared by the "native" rocks of the continent; (3) it contains fossils from exotic locales that do not match the fossils of the continent from the same geologic periods; (4) it contains paleomagnetism that indicates it was not at the latitude of the corresponding part of the continent at the time the rocks of the terrane formed.
accretionary complex
a zone of accreted terranes shoved up along the leading edge of the upper plate at a subduction zone. The Olympic Mountains in Washington State are considered a classic example of an accretionary complex.
accretionary prism
see accretionary complex.
accretionary wedge
see accretionary complex.
alluvial fan
a cone-shaped fan of sediment deposited by a stream where the stream leaves a steep slope and enters a relatively flat area.
alluvium
sediments deposited by a river or stream.
alpine glacier
a glacier on a mountain side or originating in mountains and confined to a valley.
amphibole
a group of silicate minerals that have two cleavages that do not meet at right angles.
amphibolite
a metamorphic rock consisting mostly of the dark amphibolite mineral called hornblende and the (usually white) feldspar mineral called plagioclase. Ampibolite is commonly formed from basalt that gets metamorphosed at moderate, and somewhat moist, conditions in the Earth's crust.
andesite
volcanic rock that originates as a lava flow and has an intermediate composition. Andesite is the most common rock type in composite cones. Therefore, andesite is the type of volcanic rock that is most characteristic of subduction zones.
angular unconformity
a type of unconformity in which stratified (layered) rocks beneath the unconformity are tilted or folded
anticline
a geologic fold that is convex upward.
anticline ridge
a ridge formed by an upward fold of rock layers. Most anticlines form within the crust and do not form a ridge at the surface. Anticlines in the Yakima Fold Belt are an exception -- they form ridges on the earth's surface.
aphanitic
a texture in igneous rocks in which the rock consists largely or entirely of minerals but they are too fine-grained to distinguish without magnification.
arenite
sandstone consisting mainly of quartz.
arête
an alpine ridge that is serrated (jagged like the edge of a saw blade), formed by the erosive action of glaciers on both sides of the ridge.
arkose
sandstone that consists largely of feldspar grains, along with quartz and other minerals. Arkose generally forms from eroded granite or gneiss.
ash fall
cooled, solidified ash erupted from a volcano that falls into place without flowing laterally.
ash flow
erupted ash and other pyroclastic debris that is flows across a landscape for some distance, suspended by heated gas coming out of the material.
ash flow tuff
tuff that originates from an explosive eruption of felsic magma, which falls to the ground with much gas and heat still coming out of it, flows across the landscape, and consolidates itself on the ground into a more or less solid tuff. Many ash flow-tuffs are large-volume, blanketing thousands of square miles of land, and are associated with the formation of a caldera. Ash flow tuffs are sometimes referred to as ignimbrites. Besides volcanic ash, ash flow tuffs may contain pieces of pumice, pieces of broken rock that were caught up in the eruption, and crystals that had already solidified in the magma before it erupted. All of these materials may become welded together in the solidified ash flow tuff. The yellow rock that gives Yellowstone National Park its name is an ash flow tuff.
asthenosphere
literally "weak sphere," the region of the mantle below the lithosphere. This region of the mantle undergoes plastic deformation, and is too weak to have the brittle deformation that causes earthquakes. (British spelling: aestheosphere.)
axis
in a fold, an imaginary line down the center of the fold. The fold axis is only represented on the surface or on a map of the earth's surface. A fold's axis line is not shown in a cross-section.

B  (back to top)

BP
years before present.
backarc
a region on the back side of a volcanic arc, opposite the subduction zone.
basal moraine
a relatively thin moraine deposited beneath the main body of a glacier, and compressed by the weight of the overlying ice. In the Puget Sound area, the basal moraine of the Vashon glaciation is referred to by locals as "hardpan" and forms a layer of compressed clay, sand and boulders beneath the soil.
basalt
volcanic rock that originates as a lava flow and has a mafic composition. Basalt is typically a dark, fine-grained rock. When it has no vesicles in it, basalt is somewhat denser than the average rock. Basalt comes from magma that originates from melting of the mantle. Divergent plate boundaries, which comprise the spreading ridges on the floor of the world's oceans, produce new oceanic lithosphere which contains, as its erupted volcanic component, ocean-floor basalt. Therefore, basalt of the type called mid-ocean ridge basalt is very abundant in oceanic crust. Basalt is also common on continents and in ocean islands formed at hot spots, such as Hawaii.
base level
the elevation of the place where a stream empties, such as the elevation where it joins a larger river, or the elevation of a lake that the stream empties into, or sea level if the stream empties into the ocean.
basin
1)a low area in which sediments are deposited. 2)A bowl-shaped or concave upward geologic structure.
batholith
a body of plutonic rock of more than 100 square km in area (as seen on a geologic map).
bay-mouth bar
a sand bar or sand spit that closes off the mouth of a bay.
bedding
visible layers of sediment.
bimodal volcanism
eruption of mafic and felsic volcanic material, closely spaced in time and place, with no intermediate volcanics.
biotite
an iron-rich, black mineral of the mica group. (Micas are silicate minerals with one perfect cleavage that naturally peels apart into thin sheets.)
blueschist
a type of schist containing abundant blue amphibole. Blue amphiboles only form in high-pressure, low-temperature conditions, as within a subducting plate that goes part way down a subduction zone.
braided stream
a stream with multiple channels that split apart and join together in many places. Braided streams tend to form in response to unusually large amounts of sediment entering the stream system.
breccia
clastic sedimentary rock that contains highly angular grains the size of gravel or larger.
brecciated
consisting largely or entirely of large, angular pieces.
brittle
the tendency to break if subjected to stress.

C  (back to top)

calcite
a carbonate mineral with the chemical formula CaCO3.
calcium carbonate
any mineral made of calcium ion, Ca2+, chemically bonded with the carbonate ion, CO32-, so that its chemical formula is CaCO3. The most common carbonate mineral is calcite. Another calcium carbonate mineral is aragonite. Calcite and aragonite have the same chemical formula but different crystal lattice structures. Calcium carbonate minerals are a subclass of the carbonate minerals.
carbonate mineral
a mineral with a chemical formula showing one or more metal ions bonded with the carbonate ion, CO32-. Examples of carbonate minerals include calcite, CaCO3, dolomite, CaMg(CO3)2, magnesite, MgCO3, and rhodocrosite, MnCO3.
carbonate rock
rock made almost entirely of carbonate minerals, usually either the mineral calcite or the mineral dolomite. Examples of carbonate rock are limestone (a sedimentary rock made almost entirely of calcite), dolostone (a sedimentary rock made mostly of dolomite), and marble (a metamorphic rock consisting almost entirely of either calcite or dolomite).
caldera
a wide volcanic crater. Many calderas form as the aftermath of a cataclysmic, explosive volcanic eruption that creates a widespread ash flow, after which the earth collapses back into the evacuated magma chamber. Another common occurrence of calderas is at the crest of shield volcanoes, where a circular zone of the mountain sinks downward like a piston as large volumes of liquid magma come pouring out from beneath.
carbon-14
a radioactive isotope of carbon. The half-life of carbon-14 is 5730 years. Carbon-14 is created in the atmosphere by cosmic rays and maintains an approximately constant background level in the atmosphere, surface waters and living things. Once a living organism dies, the carbon-14 built into its tissue decays without being replaced. This allows the age of dead materials (such as dead trees, charcoal, and mummies) to be determined by analyzing how much carbon-14 is left. Beyond 10 to 12 half-lives (about 50,000-70,000 years) there is not enough carbon-14 left to measure, so carbon-14 is only useful for determining the ages of very young geologic materials.
Cascadia Subduction Zone
the subduction zone of the Pacific Northwest, extending from Cape Mendocino in northern California to Vancouver Island in British Columbia. It is associated with the Cascadia Volcanic Arc.
Cascadia Volcanic Arc
the volcanic arc associated with the Cascadia Subduction Zone. It extends from Lassen Peak in northern California to Mount Garibaldi in southern British Columbia, Canada. The Cascadia Volcanic Arc follows the Cascade Range from northern California to Canada. It is important to realize, however, that most of the peaks in the Cascade Range are not volcanoes. The composite cone of the Cascadia Volcanic Arc include Mount Shasta in California, Mount Hood in Oregon, and, in Washington state, Mount St. Helens, Mount Adams, Mount Rainier, Glacier Peak and Mount Baker.
chalcedony
rock that consists of ultramicroscopic crystals of quartz. Chalcedony is a hard rock with conchoidal fracture. Light-colored chalcedony is called chert. Red chalcedony is called jasper. Black chalcedony is called flint. Chalcedony is also be classified as a mineral, since it consists of just quartz.
chemical sedimentary rock
rock formed from sediment that precipitated from solution in water. Chemical substances that are dissolved in water and settle out of the water in solid form become chemical sedimentary rocks. Examples include limestone and rock salt.
chemical sediment
sediment that forms by precipitation of chemical elements that were dissolved in water.
chert
chemical sedimentary rock made of sub-microscopic (ultrafine) quartz. Chert commonly originates from the siliceous remains of dead plankton that accumulate in layers on the deep ocean floor. Chert is a hard, tough rock that commonly has a gray color and conchoidal fracture (like broken glass).
cinder cone
a small, pyroclastic volcano formed from tephra (bits of volcanic material hurled up into the air) that fountained above a single vent and fell right back down, piling up into a cone made of loose material. The tephra in a cinder cone consists of some combination of volcanic ash, lapilli, blocks or bombs, and usually has a mafic composition. Most cinder cones are less than 300 m (1,000 ft) tall. Many composite cones and shield volcanoes have cinder cones on their flanks or nearby, small bumps compared to the much larger shield volcano or composite cone.
cinder cone
a small volcanic cone made of tephra that usually has a mafic composition.
cirque
a bowl-shaped, steep-walled amphitheater created by glacial erosion at the upper end of an alpine or valley glacier. Many of the cirques in the mountains of the Pacific Northwest have not had a glacier in them since the end of the Pleistocene Epoch. Many cirques contain a small lake, of the type known as a tarn.
clast
a grain of sediment, usually consisting of a piece of rock or a piece of mineral. Clasts range in size from microscopic clay particles to boulders as large as a house.
clastic
consisting of broken or eroded pieces of rocks, minerals or tephra.
clastic sediment
sediment consisting of broken or eroded pieces of pre-existing rocks or minerals.
clay
a clastic particle less than 0.004mm (4 micrometers) in diameter. Also, a group of silicate minerals with a single cleavage that forms layered crystals of extremely small size.
cleavage (mineral)
a direction in which a mineral breaks along flat surfaces, due to a direction of weaker bonding in the crystal lattice structure.
cleavage (rock)
flat surfaces into which a rock breaks.
coal
black, soft rock made of carbon. Coal originates from the compressed, lithified remains of dead leaves, stems and trunks of plants that grew in a swampy environment.
cobble
a clastic particle 64 to 256 mm in diameter.
colonnade
a layer of vertical columns, many of them six-sided, that commonly form the base of a lava flow. Colonnades occur in nearly all of the Columbia River Basalt flows of the Pacific Northwest. In some lava flows, the colonnade may be above the base, or even at the top of a flow, and thus some flows appear to have more than one colonnade. In some Columbia River Basalt colonnades, the columns are several meters (many feet) thick and tens of meters (over 50 feet) tall, though in many cases they are smaller.
composite cone
a tall, cone-shaped, complex volcano consisting largely of intermediate volcanic rocks such as andesite or dacite. Eruptions of intermediate lavas, explosive eruptions of tephra, addition of lesser volcanoes to the larger edifice during growth, such as small shield volcanoes (usually early in eruptive history if part of a composite cone), cinder cones, or lava domes erupting and becoming enveloped in or part of the volcano, along with self-intrusions that form dikes and sills, add up together to form a composite cone. Composite cones are the characteristic volcanoes of volcanic arcs, which occur along subduction zones at the edge of continents. Composite cones are the characteristic volcanoes of island arcs, which occur along subduction zones where oceanic lithosphere is going beneath oceanic lithosphere. To summarize the last two sentences, chains or arcs of composite cones are always present at subduction zones, whether in the ocean (island arc) or along the edge of a continent (volcanic arc). Composite cones of the Cascadia volcanic arc in the Pacific Northwest include Lassen Peak and Mount Shasta in northern California, Mount McGlaughlin, Mount Mazama (Crater Lake), Mount Thielsen, the Three Sisters, Mount Bachelor, Mount Washington, Mount Jefferson and Mount Hood in Oregon, Mount Adams, Mount Saint Helens, Mount Rainier, Glacier Peak, and Mount Baker in Washington, and Mount Garibaldi and Mount Meager in British Columbia. Most of these volcanoes are over 10,000 feet (~3,000 m) in elevation above sea level, but are built on top of the already-elevated platform of the Cascade Mountains so the volcanoes themselves are on the order of 4,000-9,000 feet (1,200-2,800 m) tall from their volcanic base levels. Along with abundant andesite, dacite and tephra, composite cones may also erupt lesser amounts of basalt and rhyolite. That is why they are called composite cones - they are a composite of many types of volcanic rock. Composite cones are also called stratovolcanoes.
compression
pushing together. A region of the crust undergoing tectonic compression, for example in response to subduction and terrane accretion, tends to form anticlines, synclines, and thrust or reverse faults.
conchoidal fracture
a style of fracture that produces sharp edges and scalloped-shaped cavities; a thick piece of standard glass exhibits conchoidal fracture when broken.
conglomerate
a sedimentary rock that contains abundant, rounded pebble, cobble, or boulder-sized particles.
contact
a contact is surface at which two different types of rock touch each other (or two different formations of rock touch each other). On a geologic map, a contact between different rock types is usually shown as a simple line.
contact metamorphism
metamorphism caused by heat from a nearby igneous intrusion. Contact metamorphic rocks are generally not foliated.
continent-continent convergent plate boundary
a convergent plate boundary at which two continents collide with each other. Characterized by a high, wide, non-volcanic mountain range.
continental basement
the deeper layers of continental crust beneath sedimentary and volcanic rocks. It consists of metamorphic and plutonic rocks.
continental crust
the crust that comprises the continents. Compared to oceanic crust, continental crust is much thicker, is less dense, and, in igneous terms, has an intermediate average composition. Granite is common in continental crust, and rare in oceanic crust.
continental glacier
a glacier large enough to cover a large portion of a continent. Continental glaciers are not confined to a single valley; instead, they may blanket whole landscape, from horizon to horizon.
contour interval
the vertical spacing between adjacent contour lines.
contour line
on topographic maps, a line of constant elevation above mean sea level.
convergent plate boundary
a plate boundary where the two plates are moving toward each other. There are three types of convergent plate boundaries: (1) ocean-ocean, where both plates carry oceanic crust, and one plate subducts beneath the other to form a island arc (a volcanic arc in the ocean, such as the Aleutian Islands of Alaska); (2) ocean-continent, where a plate carrying oceanic crust subducts beneath a plate carrying continental crust; the Cascadia Subduction Zone is an ocean-continent subduction zone; (3) continent-continent, where two plates carrying continental crust collide with each other and normal subduction does not occur; the Himalayan Mountains in India, the greatest mountain range in the world, is a continent-continent convergent plate boundary where the Indian sub-continent is colliding with the rest of the Asian continent. Continent-continent convergent plate boundaries do not have subduction zones, nor do they have volcanic arcs.
coulee
a term used locally in the inland Pacific Northwest to refer to a flat-bottomed valley with steep or vertical walls. Many coulees are dry, with no stream running through them.
core
the region of the earth's interior below a depth of 2900 km, composed of liquid (outer core) and solid (inner core) metallic iron-nickel.
craton
the old, stable portion of a continent.
creep
in tectonics, creep refers to the gradual deformation of rocks under tectonic stress. Creep contrasts with earthquakes, which are abrupt deformations of rocks that had built up tectonic stress.
cross-bedded
a sedimentary bed that slopes at an angle between the main horizontal beds.
cross-section
a view of the structure or layering of the crust beneath the earth's surface, as seen looking sideways at a vertical slice of the crust.
crust
the chemically defined outer layer of the earth, which consists of low-density rock that has average composition ranging from mafic (oceanic) to intermediate (continental). The chemical and mineral composition of the crust distinguish it from the ultramafic rock of the underlying mantle.
crystal lattice structure
the geometric arrangement of atoms and the bonds that hold the atoms together within a mineral.

D  (back to top)

dacite
volcanic rock that forms from solidified lava and has chemical and mineral composition between rhyolite and andesite.
daughter product
the stable isotope (type of atom) that a radioactive parent isotope changes into after it undergoes radioactive decay.
debris avalanche
a landslide consisting of a tumbling mixture of broken rock pieces. In some debris avalanches, along with all the tumbling pieces of rock, some larger slabs of rock may slide.
debris flow
a water-saturated landslide or a flood saturated with sedimentary debris.
delta
a deposit of alluvium at the mouth of a stream or river, where the river empties into a base-level body of water. Rivers carry sediments suspended in the river water, and also transport sediment rolled and bounced along the bottom of the river channel. Where the river empties into its base-level body of water, its current slows down and it loses its ability to carry sediments, so the sediments are deposited at the mouth of the river, where they can accumulate to form a delta.
deposition
in geology, usually refers to accumulation of sediment.
depositional environment
the physical setting in which sediments are deposited. A depositional environment encompasses the shape of the earth's surface, the climate, the life forms, and the agent of transport and deposition (water or wind currents).
detachment fault
a low-angle normal fault that separates volcanic and sedimentary rocks of the upper crust (in the hanging wall) from granitic and metamorphic rocks of the deeper crust (in the footwall). Detachment faults are associated with metamorphic core complexes.
diatom
a type of algae with an intricate "shell" (actually called a frustule) made of hydrated silica. Most types of diatoms live in either fresh or marine water, where their remains (frustrules) may accumulate on the bottom and form a biogenic chemical sediment made of silica.
diatomite
a chemical sediment made of diatom frustrules. Also called diatomaceous earth. Diatomite is white, powdery, and siliceous. Because of the intricate holes and tunnels in the diatom frustrules, diatomite has a porosity that is useful in certain types of filtration processes.
dike
a narrow igneous intrusion that cuts across the layers of rock it intrudes.
diorite
an intermediate plutonic rock. Diorite is like granite, except that it lacks quartz and pink feldspar and contains more dark minerals.
dip
the angle of slope. Dip is zero for a horizontal slope, such as a flat-lying sandstone bed. Dip is 90° for a a vertical slope. Dip usually refers to geological surfaces such as the surfaces of sedimentary beds. Most dips are between zero and 90°.
dip-slip fault
a fault that is not vertical, and along which the hanging wall has moved either up-dip (thrust fault) or down-dip (normal fault) relative to the footwall
disconformity
a type of unconformity in which the strata (layers of sedimentary rock) beneath the unconformity are parallel to the layer(s) of rock above the unconformity (compare to angular unconformity)
Disturbed Belt
a zone of rock that lies east of the Overthrust Belt, along the boundary between the Great Plains and the northern Rocky Mountains characterized by anticlines, synclines and minor thrust faults.
divergent plate boundary
a plate boundary along which the two plates are spreading or separating away from each other. The mid-ocean spreading ridges are divergent plate boundaries, and are the sites where new oceanic crust and lithosphere are created by the spreading process.
dolomite
a carbonate mineral with the chemical formula CaMg(CO3)2.
dolostone
a chemical sedimentary rock made of the mineral dolomite.
dome
a convex upward geologic structure, like an upside-down bowl.
drumlinoid
a ridge or elongate hill consisting of deposits of glacial drift that formed beneath a flowing ice sheet. Drumlinoids are oriented with their long axes parallel to the direction of glacial flow. In the Puget Sound region, including the city of Seattle, there are many drumlinoid hills or ridges that tend to have their long axes oriented north-south. A drumlinoid is a deposit built up of glacial drift, and therefore should not be confused with such glacial landforms as roche moutonnées, which are created by the erosion of bedrock.
ductile
having the ability to bend or change shape, without breaking, if subjected to stress.
dune
see sand dune
dune field
many sand dunes in a single area. Dune fields are places where sand dunes, which are formed by wind, accumulate. A combination of three factors are required for dune field formation: a prevailing wind, a source of plentiful sand, and a relatively low, flat area where the sand dunes can accumulate. Most dune fields in the Pacific Northwest are either (a) behind beaches and sandy sea cliffs, where the prevailing wind is onshore, or (b) in areas that had a lot of sand-rich sediment left nearby in Pleistocene glacial drift. While a dune field is active, the sand dunes gradually migrate in the direction of the prevailing wind. If the sand supply runs out, a dune field may become inactive and start to become covered by bushes and trees, "freezing" the dunes in place.

E  (back to top)

earthquake
the abrupt failure and shifting of large masses of rock along a fault within the earth.
elastic
the property of changing shape under stress but returning to the original shape once the stress is released.
entablature
a layer of a solidified lava flow with closely spaced, curving, and irregular joints. Columbia River Basalt flows commonly contain one (or more) entablatures, which contrast with the colonnade. It is common for an entablature to form the upper part of a Columbia River Basalt lava flow. However, not all Columbia River Basalt flows have entablatures as their main upper layer.
eolian
blown or deposited by the wind. (British spelling: aeolian.)
erode
to remove disintegrated rock material.
erosion
the process of removing disintegrated rock material. The main agents of erosion are water, ice, wind and gravity.
erratic
see glacial erratic.
esker
a sinuous (curving) ridge of glacial outwash, deposited beneath a stream that was confined to a tunnel underneath a glacier
estuary
a zone at the mouth of a river where fresh water mixes with salt water.
exotic terrane
an accreted terrane that shows evidence of originating far from its present location.

F  (back to top)

fault
a more or less planar surface within the earth along which the rocks on either side have shifted position relative to the rocks on the other side. Most faults are inactive. Inactive faults stopped producing earthquakes long ago in geologic time and remain as a scar in the crust.
fault-block mountain range
a range of mountains uplifted along major faults. Fault-block mountain ranges are distinguished from volcanic arcs and accretionary prisms by not having chains of composite cone volcanoes and not being at the leading edge of a subduction zone. The mountain ranges of the Rocky Mountains and the Basin and Range province are fault-block mountain ranges. The faults along the margins of mountain ranges in the Rocky Mountains are thrust or reverse faults in some cases, normal faults in other cases. The faults along the borders of mountain ranges of the Basin and Range province are all normal faults.
feldspar
a group of silicate minerals that are the most common type of mineral in the crust, even more common than quartz. Feldspars combine aluminum and silica with an either potassium, sodium or calcium. Feldspars form fairly hard, shiny crystals. Specific types of feldspars include orthoclase (a potassium feldspar that commonly has a pink color), Na-plagioclase (usually pearly white), and Ca-plagioclase (pearly gray).
felsic
an igneous composition (the chemical composition of a magma or igneous rock) that is relatively high in silica content and low in iron and magnesium. Felsic magma is highly viscous, and thus is more likely to erupt explosively than other magma compositions.
fissure eruptions
eruptions that occur when lava comes pouring out of a crack in the surface of the Earth.
flood basalts
high-volume, widespread flows of mafic magma that solidified into basalt flows, in some cases covering thousands of square miles. Flood basalts usually originate from fissure eruptions, cracks in the earth that rapidly disgorge huge volumes of lava. The Columbia River Basalts comprise one of the world's great flood basalt provinces. Some of the Columbia River basalt flows originated near the border of Idaho, covered much of eastern Washington and north central Oregon, and flowed down through the Columbia River gorge beyond the Cascade Range to cover parts of the Portland area and reach the Pacific Ocean, all in a single lava flow.
floodplain
a flat plain within a river valley that extends sideways for some distance from the river channel, and barely rises above the elevation of the river channel. True to their name, floodplains are covered by floodwaters when the river reaches a high stage of water discharge. A flood is a natural event in the dynamic of a river.
fold
a geologic structure formed by plastic deformation in which the rock has been bent. Anticlines and synclines are the basic types of folds.
foliated
having layers that formed during metamorphism.
foliation
a layer or style of layers that formed in a metamorphic rock
footwall
the body or rock that lies beneath a fault
forearc basin
a large basin that lies between the accretionary prism and the volcanic arc of a subduction zone. In the Pacific Northwest, the Puget-Willamette lowlands are a forearc basin, separating the accretionary prism of the coast ranges from the Cascade Range volcanic arc.
formation
a sequence of strata of closely related rocks that formed over a limited interval of geologic time, which can be consistently recognized and mapped (usually sedimentary strata, but some formations are volcanic or metamorphic; a body of intrusive igneous rock may be given a name and be mapped but cannot formally qualify as a formation)
 
frothy
an igneous texture in which half or more of the volume of the rock consists of holes.
fusulinid
a single-celled animal that lived by floating in ocean water and had a hard "shell" that could be fossilized. Fusulinids existed in the late Paleozoic.

G  (back to top)

Ga
billions of years ago.
gabbro
mafic plutonic rock. Gabbro is like granite, except that it is richer in iron and magnesium and consists entirely of dark minerals.
geologic map
a map that shows the distribution of rock or sediment units that are exposed on the surface of the earth, using different colors or patterns for the different units of rock or sediment. A geologic map usually depicts geologic structures as well, such as folds and faults.
geotherm
the rate at which temperature increases as depth in the earth increases; the higher the geotherm, the higher the heat flow.
glacial drift
all the sediments deposited by a glacier. This includes glacial till, which is deposited directly by glacial ice, and sediments deposited in the vicinity of the glacier by glacial meltwater.
glacial erratic
a boulder transported by a glacier and left sitting on the ground after the glacier retreats.
glacial outwash
sand and gravel beds deposited by water flowing from a glacier.
glacial till
sediments deposited directly from glacial ice. Glacial till is generally unsorted (contains sediment clasts of all sizes) and unbedded (does not have distinctive internal layers). Another characteristic of glacial till is that some of the boulders it contains have flattened sides marked with straight grooves, signs of being dragged along the base of the glacier.
glacial trough
a steep-sided valley formed by the erosive action of a valley glacier. With their steep valley walls and rounded valley floor, glacial troughs generally have a U-shaped profile. This contrasts with the typical V-shaped profile of a valley eroded entirely by a stream or river.
glacier
a body of ice on land that forms from accumulated snow and flows under the force of its own weight.
glassy
as an igneous texture, glassy means made of glass; as a luster, glassy means reflecting light as glass does.
gneiss
regional metamorphic rock with foliation in the form of stripes, the stripes being layers that have different mineral composition, for example layers of quartz and feldspar alternating with layers of biotite and amphibole. Gneiss forms at high metamorphic grade (high temperature).
graben
a geologic structure in which a body of rock drops downward between two normal faults.
gradient
slope; or, the amount of change in elevation over a unit of horizontal distance, such as how many feet of elevation a stream loses in a horizontal distance of one mile.
granite
felsic plutonic rock that consists mostly of feldspars and quartz. Granite has a coarse-grained igneous texture and, along with the pink and white feldspars and clear quartz, is usually speckled with dark minerals such as biotite or hornblende.
granodiorite
plutonic rock with a chemical and mineral composition between granite and diorite. Compared to true granite, granodiorite contains less quartz and pink feldspar (orthoclase) and, usually (though not necesarily), more dark minerals than granite.
graywacke
sandstone consisting of a mixture of feldspar, volcanic rock debris, and clay, plus or minus quartz. Graywacke sediments come from volcanic regions such as island arcs.
ground moraine
see basal moraine

H  (back to top)

half life
the length of time it takes for half of a sample of atoms of a specific radioactive isotope to break down and turn into stable daughter products.
hanging wall
the body of rock that lies above a fault
hanging valley
a valley that ends abruptly at a drop-off into a deeper valley.
hardness
in minerals, hardness is gauged by which materials scratch which materials, or are scratched in return.
heat flow
the rate at which heat flows through a substance (see geotherm)
hogback ridge
a ridge formed by erosion of the edge of a tilted layer of resistant rock.
horn
a steep, sharp peak that was eroded by glaciers on several sides.
hornblende
a black amphibole with a shiny luster. Hornblende is a silicate mineral that is common in rocks that form at high temperatures such as granite, diorite, or amphibolite.
horst
a geologic structure in which a body of rock rises upward between two normal faults.
hotspot
a place where high amounts of heat and large quantities of magama rise to the earth's surface from deep in the mantle. Hotspots come from sources beneath the moving tectonic plates, can cause volcanism in the middle of a tectonic plate, and can leave a volcanic trail as the overlying tectonic plate moves across the hotspot.
hydrothermal fluid
a hot, water-rich fluid in the Earth that transports chemical elements and causes the growth of new minerals in the rocks. Many ore deposits are deposited by hydrothermal fluids. Circulation of high-temperature hydrothermal fluid in the upper crust is commonly driven by the heat from magma and volcanic activity. Lower-temperature, but still warm, hydrothermal fluids may also circulate in the crust, depositing lower-temperature minerals.
 

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ice sheet
A glacier that is much larger in extent than an alpine glacier or a valley glacier. Ice sheets are glaciers that can cover whole landscapes, including mountain ranges; the largest ice sheets can cover most of a continent. Greenland and Antarctica are mostly covered by ice sheets today. During the Pleistocene epoch, much of northern North America and Northern Europe were covered by ice sheets several times. Ice sheets, like all glaciers, consist of flowing ice, ice in motion.
igneous rocks
rocks that form from magma, lava or tephra.
immature stream
a stream with little or no floodplain and a high abundance of waterfalls or rapids.
interglacial
an interval of time when continental glaciers have receded and the climate is, at least for a short while, warmer.
intermediate
an igneous composition (the chemical composition of a magma or igneous rock) that has a moderate silica content and moderate amounts of iron and magnesium.
intrusion
(igneous intrusion) see pluton.
intrusive
see plutonic
island arc
a volcanic arc at subduction zone where a plate with oceanic crust is subducting beneath another plate that carries oceanic crust. Island arcs are chains of composite cone volcanoes that form an archipelago, such as the Aleutian Islands in Alaska. Island arcs form along subduction zones in the ocean.
island arc crust
the crust that comprises island arcs. Compared to continental crust, island arc crust is slightly thinner. Compared to oceanic crust, island arc crust is thicker and less dense. In igneous terms, island arc crust has an intermediate average composition.
isotope
an atom with a specific number of neutrons in its nucleus; each chemical element has several isotopes.

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joint
in rocks, a joint is a fracture along which the rocks have separated slightly.

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kame
a body of glacial outwash deposited alongside glacial ice. Once the glacier has melted away, a kame deposit may stand out as a hill of sand and gravel.
kettle
a bowl-shaped depression in glacial drift, formed when a stranded piece of glacial ice slowly melts as glacial sediment accumulates around it.
kettle lake
a lake that occupies a kettle.

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lahar
a volcanic mudflow, a flowing slurry of volcanic debris mixed with water from melted snow, ice or rain. Lahars can flow down river valleys into the lowlands far from the volcano, causing far-flung devastation.
lapilli
pieces of tephra between 0.5 and 5 cm across.
lateral moraine
a ridge of till that forms along the side of a valley glacier.
lava
liquid, molten rock on the surface of the earth. Along with liquid, molten rock, lava may also contain crystals and dissolved gas.
lava dome
a steep-sided domes formed by the pile-up of viscous, felsic magma. It is not uncommon for a composite cone to erupt explosively, forming a crater, and then to erupt viscous lava into the crater, forming a lava dome. The lava that forms the lava dome may be from the same magma that erupted explosively, but after venting most of its gas in the explosive eruption, the remaining magma has lost its explosiveness and thus erupts as lava flows.
lava plateau
an elevated region, lacking in mountain ranges, where the upper crust consists almost entirely of volcanic rocks.
limestone
a chemical sedimentary rock made predominantly of the mineral calcite.
lithification
the process of turning into rock; becoming rock
lithify
to turn into rock; to become rock
lithosphere
the rigid outer layer of the earth which comprises the tectonic plates and consists of the rigid upper mantle and the crust. The rigid physical behavior of the lithosphere distinguishes it from the weak, ductile asthenosphere underneath it.
littoral drift
all the sediments moved by wave action in a shoreline-beach system.
loess
accumulated silt that was blown and deposited by wind. Loess tends to form fertile soil.
lodgment till
the till that composes a basal moraine.
longshore current
a current along a shore, parallel to the shore, driven by prevailing wind and waves.
luster
the manner in which the surface of a solid reflects light.

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Ma
millions of years ago.
mafic
an igneous composition (the chemical composition of a magma or igneous rock) that is relatively high in iron and magnesium content and low in silica. Mafic magma has a relatively low viscosity, and thus is less likely to erupt explosively than other magma compositions.
magma
liquid, molten rock inside the earth. Along with liquid, molten rock, magma may also contain crystals that have already solidified in the magma, and gas dissolved in the magma.
mantle
layer of the earth between the crust and the core. It contains most of the earth's volume and consists of dense iron, magnesium and silica-rich rocks.
marble
metamorphic rock made of calcite or dolomite.
mature stream
a stream with a wide floodplain and a meandering channel.
meandering stream
a stream with a channel that curves back and forth as it winds along a floodplain.
metamorphic core complex
a dome-like uplift of metamorphic and plutonic rock from deeper in the crust, which has had the shallow-crust overlying rock detach and move aside along detachment faults
metamorphism
the process of changing into a new rock type by recrystallization of minerals due to heat, pressure, or both.
metamorphic rock
rock that forms by recrystallization of a pre-existing rock due to changes in temperature, pressure, or fluids in the rock.
mid-ocean spreading ridge
a ridge system along a divergent plate boundary on the ocean floor. New oceanic crust forms there from magma that rises from the mantle into the ridge system, and the growing tectonic plates spread away from the ridge in opposite directions.
migmatite
a rock that is a pervasive mixture of metamorphic rock and plutonic rock. The metamorphic rock is high-grade or medium-to-high grade metamorphic rock, such as gneiss, schist, or amphibolite. The plutonic rock usually appears granitic, having a phaneritic texture with abundant feldspar and quartz. The metamorphic and plutonic rocks in a migmatite are repeatedly mixed, interpenetrating or alternating on a scale of centimeters or meters (inches or feet).
mineral
an inorganic solid with a specific chemical composition and a regular crystal lattice structure.
Mohs hardness scale
a scale used for expressing mineral hardness in reference to minerals numbered from 1 (softest) to 10 (hardest) as follows: 1-talc, 2-gypsum, 3-calcite, 4-fluorite, 5-apatite, 6-feldspar, 7-quartz, 8-topaz, 9-corundum, 10-diamond.
moraine
a deposit of glacial till. There are several specific types of moraine: terminal moraine; lateral moraine; basal moraine.
mud
a mixture of silt and clay.
mud cracks (syneresis cracks)
cracks that form when a wet layer of clay-rich sediment dries and shrivels
muscovite
a clear or silvery-white mineral of the mica group. (Micas are silicate minerals with one perfect cleavage that naturally peels apart into thin, flexible sheets.)

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normal fault
a dip-slip fault along which the hanging wall has moved down-dip. Normal faults result from the crust undergoing tension, or being pulled apart by tectonic forces. Grabens are bounded by normal faults.
nonconformity
a type of unconformity in which the rocks underneath the unconformity are plutonic rock and/or metamorphic rock

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oblique fault
a fault along which there are two significant components of motion--dip-slip (up or down) and strike-slip (horizontal).
obsidian
volcanic glass.
ocean-continent convergent plate boundary
a convergent plate boundary at which a plate carrying oceanic crust subducts beneath the margin of a continent. Characteristic features include an oceanic trench, an accretionary complex, a forearc basin, and a volcanic arc.
ocean island
an island built of mafic volcanic rocks on oceanic above a mantle hotspot. The Hawaiian Islands are ocean islands.
ocean-ocean convergent plate boundary
a convergent plate boundary at which a plate carrying oceanic crust subducts beneath the margin of another plate that carries oceanic crust. Characteristic features include an oceanic trench and a volcanic arc.
oceanic crust
the type of crust that makes up the floor of the ocean basins. Compared to continental crust, oceanic crust is very thin (typically about 5 km thick) and more dense, and has a mafic composition.
oceanic trench
a deep trench on the floor of the ocean at the outer edge of a subduction zone.
olivine
a silicate mineral, rich in iron or magnesium, that has no cleavage and usually has a green color.
opal
an amorphous mixture of silica and water; precious forms of opal have layers of silica-water spheres which diffract light into colors.
ophiolite
a sequence of rocks including ultramafic rock, gabbro, mafic dikes, pillow basalt, and oceanic sediments. An ophiolite is a section of oceanic lithosphere accreted to a tectonic plate.
orogen
a place in earth's crust where orogeny is occurring or has occurred. The rocks in an orogen are highly deformed.
orogeny
a mountain-building event that affects the geology of a large region and lasts millions of years.
orthoclase
one of the feldspar minerals, with a Mohs hardness of 6, pearly luster and two cleavages that intersect at a right angle. Orthoclase comnmonly has a pink color and is found in granite.
outwash
deposits of alluvium from high-energy meltwater streams flowing from the margins of a glacier. Outwash tends to be cross-bedded layers of sand and gravel.
outwash plain
a plain underlain by glacial outwash.
overlap formation
a sedimentary formation deposited over the top of more than one accreted terrane. It can be deduced that the terranes must have accreted before the overlap sediments formed.
Overthrust Belt
an area of the northern Rocky Mountains, in parts of Wyoming, Montana, and Alberta, characterized by major thrust faults along which thrust sheets moved many miles to the east from their original positions.

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palagonite
an orange-colored mixture of clays and iron-bearing minerals, formed from the breakdown of basaltic glass.
paleomagnetism
magnetism preserved in rocks since the time the rocks formed.
Pangaea
a supercontinent consisting of all the other continents joined together. Pangaea existed during the Permian and Triassic periods.
parent isotope
a radioactive isotope (type of atom) that decays into a stable daughter product.
passive margin
an edge of a continent where there are no plate boundaries. Passive margins of continents tend to have gradual slopes, wide continental shelves, and accumulation of similar sediment layers over very broad areas.
pegmatite
extremely coarse-grained granitic rock. Pegmatite, like granite, commonly consists mainly of quartz and feldspars, but the crystals average more than a centimeter (nearly half an inch) across. Rare pegmatites have crystals up to 100 cm (several feet) in length, and some pegmatites contain rare minerals. Pegmatites are plutonic igneous rocks, and commonly occur as dikes in or near granitic plutons.
pebble
a clastic particle 2 mm to 64 mm in diameter.
phaneritic
an igneous texture in which the minerals throughout the rock are large enough to easily see with the naked eye. Characteristic of plutonic rocks.
phenocryst
a larger, easily visible crystal in an otherwise fine-grained igneous rock.
phyllite
regional metamorphic rock similar to slate except that the foliation surfaces are shinier and more wrinkled. Forms at slightly higher metamorphic grade (higher temperature) than slate.
pillow basalt
basalt piled up in the approximate shape and size of pillows, with each pillow having a glassy or very-fine-grained rind. Commonly, the spaces between pillows contain volcanic glass that was splintered and altered by interaction with water, forming an orange-colored material known as palagonite. Pillow basalts form when mafic lava erupts underwater. The lava freezes on the outside against the cold water, but keeps flowing on the inside, bulging out into pillow-like shapes which break off from the main flow. These pillows can accumulate into a large volume of pillow basalts. Pillow basalts serve as an indicator that the lava was erupted into a body of water. Much of the oceanic crust consists of basalt, most of which erupted onto the seafloor to form pillow basalts. Pillow basalt can also form when mafic lava erupts on land and flows into a lake or into the ocean. In some places, there are layers of pillow basalt in the Columbia River Basalts, showing that the basalt flows locally flowed into lakes, spalling into pillows as it entered the lake.
plagioclase
a type of feldspar in which the alkali element is either sodium (Na), calcium (Ca), or a mixture of the two alkalies. Plagioclase is as common as quartz in the earth's crust.
plastic
the property of deforming by gradually yet permanently changing shape without breaking.
plate tectonic theory
the theory that the earth's lithosphere consists of moving plates.
plate tectonics
movement of earth's lithospheric plates.
plunging fold
an anticline or syncline with an axis that is tilted from the horizontal.
pluton
a body of plutonic rock. Includes batholiths and dikes.
plutonic
having to do with intrusion of magma within the earth. Also called intrusive. Contrasts with volcanic, which has to do with extrusions of magma onto the earth's surface as lava or tephra.
plutonic rocks
rocks that form from intrusions of magma within the earth. Examples include granite and gabbro. Plutonic rocks are a sub-category of igneous rocks.
porphyritic
a texture in igneous rocks consisting of larger crystals set in a finer-grained matrix. For example, the volcanic rock andesite commonly has a porphyritic texture, in which case it is called porphyritic andesite.
protolith
the type of rock that a rock was before it got metamorphosed. For example, if a body of limestone gets metamorphosed within the Earth to become marble, then you would say the protolith of the marble was the limestone.
pumice
felsic or intermediate volcanic rock that has a frothy texture, so that more than half of its volume is void space. Because it usually has a low iron content, most pumice has a white or light color.
pyroclastic
having to do with material that is launched through the air by an explosive volcanic eruption. See see tephra.
pyroxene
a silicate mineral with two cleavages that meet at 90° and commonly has a dark or green color.

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quartz
a very common mineral that is relatively hard and tends to fracture like broken glass. Quartz is a silicate mineral, made of the elements silicon and oxygen.
quartzite
metamorphic rock made of quartz.

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radioactive
having unstable atomic nuclei, which will eventually break down and change into stable nuclei.
radiocarbon
carbon-14.
radiometric
measured on the basis of the proportion of radioactive parent isotopes and stable daughter products.
rapakivi
a texture in granitic rocks in which pink, potassium feldspar cores are mantled by white, Na-plagioclase rims. Occurs in some granites that are unusually high in alkalies (potassium and sodium).
regional metamorphism
metamorphism in which a large volume of crust is subjected to stress and heat, forming foliated metamorphic rocks.
regression
a retreat of the sea from the continent, exposing land to the atmosphere.
resistant
in the context of erosion in geology, resistant refers to a layer of rock or body of rock being resistant to erosion. Resistant rock tends to stand out as a ridge, hill, or cliff above less resistant rock.
reverse fault
a high-angle thrust fault. A reverse fault is a steeply dipping thrust that has formed in an area where the crust is undergoing compression. In a reverse fault, the hanging wall has moved upward relative to the footwall.
rhyolite
felsic volcanic rock that originates as a lava flow. Rhyolite commonly has a white or pink color. Rhyolite comes from magma that originates in the crust.
ribbon chert
layers of sedimentary chert.
rip-up clast
a sediment clast that originated as mud (clay) that was ripped up by an energetic flow of water and redeposited as pieces of sediment in the overlying layer
roche moutonnée
a glacially eroded, asymmetric landform consisting of a hill or promontory of rock that was smoothed into a streamlined shape on the side the glacier flowed from (the "stoss" side), and plucked by glacial ice into a steep slope or cliff on the downflow-side (the "lee" side).

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sand
clastic sediment grains between 1/16 mm and 2 mm across.
sand dune
a ridge or hill of sand deposited by wind.
sand spit
a long sand bar, built up above wave level, and attached to a shoreline at one end. Like sandy beaches, sand spits are a form of littoral drift--sediment deposited by longshore currents. Sand spits form in response to an abundant supply of sand, regular longshore currents driven by wind and waves parallel to a shoreline, and a bend or point in the shoreline that becomes the base of the spit. It is common for a sand spit to extend from the land across part of the mouth of a bay.
sandstone
clastic sedimentary rock consisting of grains of sand that were compacted and cemented together. The mineral grains need not be quartz to qualify as sand, they simply have to be the size of sand grains.
schist
regional metamorphic rock with foliation that is due to parallel flat minerals, especially mica minerals such as biotite or muscovite, with crystals big enough to see with the naked eye.
scoria
mafic volcanic rock that has a frothy texture, so that more than half of its volume is void space. Because of its high iron content, scoria is usually rust-red or black in color.
sea stack
a tower of bedrock in the sea adjacent to a shoreline, remnant of an eroded point of land.
sediment
granular or dissolved material deposited on the surface of the earth.
sedimentary rocks
rocks that form from accumulated sediment.
sedimentary structure
a characteristic shape or pattern in beds of sediment resulting from a particular means of deposition.
serpentinite
a metamorphic rock consisting largely of the soft, dark-green mineral serpentine. Serpentinite forms by metamorphism of ultramafic rock, such as rock from the earth's mantle.
shale
layered, clastic sedimentary rock made of clay, or of very fine grains of sediment.
shear
being pushed side-by-side in opposite directions. A region of the crust undergoing horizontal shear, for example along a transform plate boundary, tends to form a strike-slip fault.
shield volcano
a broad, gently sloping volcano that, in profile, has the shape of a shield laid upon the surface of the earth. Lava flow after lava flow of low-viscosity, mafic lava builds up a basaltic shield volcano. Shield volcanoes are the largest type of volcano on Earth, as exemplified by Mauna Loa on the big Island of Hawaii. Shield volcanoes in the Pacific Northwest include Medicine Lake Volcano in northern California, Newberry Volcano (Paulina Peak) in Oregon, and Simco Volcano in south Central Washington, on the Yakama Indian Reservation near the town of Goldendale. Because shield volcanoes are so much wider than they are tall, and have gentle rather than steep slopes, they do not fit the stereotype of what a large volcano looks like.
silica
SiO2, which comes in several solid forms, including such minerals as quartz and coesite and as an amorphous (non-mineral) solid.
silicate
the name of a large group of minerals, all of which have a crystalline structure built around silicon atoms combined with oxygen atoms. The silicate minerals compose most of the earth's crust and mantle, and comprise thousands of minerals.
sill
an igneous intrusion that is parallel to the layers of sediment or layers of volcanic rock that it intrudes.
silt
clastic sediment grains between 1/256 mm and 1/16 mm across.
siltstone
clastic sedimentary rock made of silt grains that were compacted and cemented together. Siltstone is similar to sandstone, except that it consists of finer grains.
slate
fine-grained regional metamorphic rock that splits apart into perfectly flat layers. Slate forms at low metamorphic grade (low-temperature).
spatter cone
a small volcanic landform that results from lava spattering up in a small fountain and piling up in blobs of semi-molten material that accumulate into a solid, roughly cone-shaped form. Spatter cones are commonly only a few feet or a few tens of feet tall. A lava flow coming down the flank of a shield volcano or composite cone may spawn a subsidiary spatter cone.
steptoe
a hill or butte of rock that is part of a deeper layer of older rock, and sticks up through layers of younger rock or sediment.
stitching pluton
a pluton that cuts across (intrudes) a terrane and adjacent rock, indicating that the terrane was tectonically accreted to the adjacent rock before the intrusion occurred.
strain
deformation (change of shape) in response to stress.
strata
layers (plural). Usually refers to sedimentary layers or beds. A stratum is a single layer.
stratigraphy
the study of sequences of layered rocks and their history.
stratovolcano
the type of large, cone-shaped volcano, such as Mount Rainier and Mount Saint Helens in Washington state, that is produced by subduction of oceanic lithosphere. Most stratovolcanoes on Earth occurs as part of an arc or chain of stratovolcanoes along the length of a subdcution zone. See composite cone for more information.
stream channel
the conduit through which the water of a stream flows and in which it is confined most of the time.
 
stream terrace
an elevated, flat-topped bench along the side of a stream valley. May be a remnant of an earlier, higher-level floodplain.
stream valley
a valley formed by a stream or river eroding the land it flows over. Most valleys on earth were created by stream erosion. Some valleys in which streams flow were created by tectonic events and later filled with a stream that continued to shape its own valley. Over time, streams grade (tend to level) their valleys, eroding the headwaters and steep areas down, widening their valleys, and depositing sediment in the flatter, wider parts of their valleys. More "mature" streams have wide floodplains and gentle gradients.
 
stress
a physical force that would push, pull, or rotate an object.
striations
parallel lines or grooves on a flat surface of a rock or mineral.
strike and dip
strike and dip are a way of representing the three-dimensional orientation of a planar surface on a two-dimensional map. The strike is the compass direction of a horizontal line on the plane. All the horizontal lines on a plane are parallel, so they all have the same characteristic compass direction. The dip is the angle at which the plane slopes downhill from the horizontal, at its maximum slope, which is at right angles (90º) from the strike. The combined strike and dip symbol shows which way the beds (or other flat surfaces) are tilted at a particular place. See the Focus Page on Geologic Maps.
strike-slip fault
a vertical or nearly vertical fault along which the rocks have moved horizontally in opposite directions on either side
stromatolite
a structure created by films of photosynthetic bacteria, which trap layers of sediment that build up into distinctive shapes. Stromatolites come in several shapes, such as stacked mounds and cabbage-like structures
subduction
the process by which a lithospheric plate descends into the mantle. A subducting plate has oceanic crust on it, much thinner and denser than continenal crust. This allows it to descend into the Earth - to subduct - beneath the margin of the adjacent plate.
subduction earthquakes
earthquakes that occur on subducting plates within subduction zones. The deepest and most powerful earthquakes in the world are subduction earthquakes.
subduction zone
a tectonic zone where a plate with oceanic crust is going downward beneath the margin of an adjacent plate. Characteristics of subduction zones include an oceanic trench, subduction earthquakes, accretionary prisms, forearc basins and volcanic arcs. The Cascadia Subduction Zone is unusual in that it does not appear to have an oceanic trench, and has not had a great subduction earthquake in historic time.
superpose
In geology, a layer of rock or sediment is superposed (on top of) another. If the layer of rock is sedimentary, it is inferred by the principle of superposition that the layer is younger than what it is beneath it (unless there is evidence that the rocks in the area have been turned upside down, or evidence that the contact between the layer and the rock beneath it is a fault).
 
syncline
a geologic fold which is convex downward.

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talus
rock debris piled up at the base of a cliff or steep, rocky slope.
tectonic
having to do with faults, earthquakes, plate motions, or deformation of the crust.
tectonic plate
a segment of the lithosphere (crust plus rigid upper mantle) that moves like a giant slab across the face of the earth. There are three tectonic plates involved in the Pacific Northwest today: the North American Plate, the Juan de Fuca Plate, and the Pacific Plate.
tension
pulling apart. A region of the crust undergoing extension will tend to have normal faults.
tephra
volcanic material that erupted explosively and flew through the air for some distance before landing on the ground. Tephra results from pyroclastic eruptions, which means explosive volcanic eruptions that hurl material into the air. There are several types of pyroclastic material, or tephra, including volcanic blocks, volcanic bombs, lapilli, and volcanic ash
terminal moraine
terminal moraine - a moraine that accumulates at the end (terminus) of a glacier. Terminal moraines tend to be hummocky (hummocks are lumpy little hills), with random boulders (glacial erratics) strewn across them, and ponds or marshes in low spots.
terrane
a group of related rocks that formed together in one area, do not show any relationship to the other rocks around them, and are separated from the rocks around them by faults. Same as accreted terrane.
Tethys Sea
a large embayment of the ocean between the northern part of Pangaea (Laurasia) and the southern part of Pangaea (Gondwanaland). The Tethys Sea existed from the late Paleozoic era through the Mesozoic era. After Gondwonaland merged with Laurasia to form the single supercontinent Pangaea, the Tethys Sea was a huge embayment on what we would now call the east side of Pangaea. This Tethys Seaevolved physically with the moving plates and continents and was a zone of very high biological productivity and the site of key stages in the evolution of some types of marine organisms. The Tethys Sea extended across areas now occupied by the Mediterranean Sea, the Himyalas, southern Asia, Asia Minor, and North Africa. Most oil in the Middle East comes from the Tethys Sea.
thrust fault
a dip-slip fault along which the hanging wall has moved up-dip. Steep thrust faults (with a dip of greater than 30°) are sometimes called reverse faults. Thrust faults (and reverse faults) result from the crust undergoing compression, being pushed together by tectonic forces.
till
an unsorted, unbedded mixture of sediment deposited directly from a glacier.
topographic map
a map that shows the three-dimensional shape of the land surface, most commonly by using contour lines.
transform plate boundary
a plate boundary along which the two plates are moving side-by-side, in opposite directions parallel to the boundary. The San Andreas Fault in California is a transform plate boundary. All transform plate boundaries are strike-slip faults. Many of the world's strike-slip faults, however, are not transform plate boundaries.
transform fault
a strike-slip fault that is a transform plate boundary.
transgression
advance of the sea onto the continent, inundating land with sea water.
transport
in the context of sediments, refers to movement of sediment by flowing water or by wind. In a stream, sediment is transported by being bounced and slid along the bed of the stream, by suspension in flowing water, or by being dissolved in flowing water.
tsunami
an extremely long-wavelength water wave created by an abrupt geologic disturbance, such as an earthquake that shakes the ocean floor or a major volcanic eruption that explodes through the surface of the ocean. The tsunami from the 1964 earthquake in southern Alaska killed 16 people on the coast of Oregon and California.
tuff
volcanic rock consisting of volcanic ash, plus or minus other tephra.
tuffaceous
an igneous texture in which the rock consists largely or entirely of volcanic ash.
turbidite
a sequence of sedimentary beds deposited as the result of a turbidity currrent, an underwater landslide in which sediment mixes with water as it spills down a slope into deeper water. Turbidites tend to have a bottom layer of sand (sandstone), grading upward to a top layer of fine clay (shale).

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ultramafic
an igneous composition (the chemical composition of a magma or igneous rock) that is extremely high in magnesium content and low in silica. Ultramafic magma does not normally erupt from inside the earth, because in liquid form it is only stable at very high temperatures, higher than exist in the earth's crust. The mantle itself, on average, has an ultramafic composition. Most ultramafic rocks found in the crust are interpreted to have moved there from the mantle, either as stray pieces of mantle rock caught up in an eruption of basalt or gas from the upper mantle, or as unusual igneous intrusions from the upper mantle, or by larger pieces of the solid mantle being forced up into the crust along faults by tectonic forces.
unbedded
lacking in sedimentary strata; unlayered.
unconformity
an eroded or long-exposed surface on which sedimentary or volcanic strata were deposited.
underfit stream
a stream that occupies a valley and floodplain too large to have been created by the stream. An underfit stream valley was formed during an earlier part of the drainage history, such as when a now vanished glacier was melting and discharging large volumes of water and sediment into the valley.
unsorted
in a sediment or sedimentary rock, clastic grains of all sizes mixed together indiscriminately.

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valley glacier
a glacier that occupies a valley. Most alpine glaciers (glaciers in the mountains) are valley glaciers. Valley glaciers create such erosional glacial landforms as cirques, arêtes and glacial troughs.
varve
a thin layer of sediment that forms in a lake that freezes over during the winter and receives a lot of water during the summer. Each varve consists of two layers, a layer of quartz-rich sand, silt, or silty clay from spring and summer runoff, and a layer of dark clay that accumulates while the lake is frozen.
varves
thin, coupled layers of sediment: a fine, quartz-rich silt layer and a dark, clay-rich layer. Varves are thought to originate in large lakes that freeze over in the winter, when the water becomes stagnant and microscopic organisms die and settle to the bottom, resulting in a layer of fine clay darkened by organic carbon, followed by a summer, ice-free season when water pours in from glaciers melting nearby, carrying quartz-silt out across the lake to form the second layer of the varve. This repeats year after year, so that each varve represents a year.
vesicle
a hole in a volcanic rock created by the rock solidifying around a gas bubble.
vesicular
applies to volcanic rocks containing holes (vesicles) that were formed by gas bubbles when the rock was a lava or ash.
viscosity
how much a liquid (or solid) resists flowing. Ketchup has a higher viscosity than water. A "thick" milkshake has a higher viscosity than a "thin" milkshake. A felsic magma generally has a much higher viscosity than a mafic magma.
volcanic
igneous material erupted onto the earth's surface or into the atmosphere, or processes related to such eruptions.
volcanic arcs
arc-shaped or linear chains of composite cone volcanoes, generally associated with subduction zones.
volcanic ash
fine-grained (<0.5 cm) tephra. It mainly consists of fine pieces of volcanic glass, which form by solidification and splintering of small pieces of magma during an explosive eruption. It may also contain pieces of crystals, from minerals that had already solidified in the magma before it erupted.
volcanic blocks
large (>5 cm) pieces of tephra that were solid during the eruption. They tend to have blocky or jagged shapes.
volcanic bombs
large (>5 cm) pieces of tephra that landed while still molten or semi-molten. They tend to have shapes that were streamlined while passing through the air, or else have a sort of splattered shape from hitting the ground while soft.
volcanic breccia
volcanic rock that contains angular clasts the size of gravel or larger.
volcanic fissure
a crack in the earth's surface through which lava or tephra erupts.
volcanic mudflow
see lahar
volcanic rocks
rocks formed from lava or tephra erupted at the earth's surface. Examples include basalt, andesite, rhyolite, tuff and pumice. Volcanic rocks are a sub-category of igneous rocks.

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waterfall
a vertical sections of a stream channel, where water plummets over a precipice. Waterfalls are temporary features, geologically speaking. Given enough time, a stream will erode its waterfalls away and develop a smoothly graded profile. Therefore, waterfalls indicate that some geological activity has occurred recently enough that the streams still have not recovered and established graded profiles. Commonly in the Pacific Northwest the recent event that led to a waterfall was the coming and going of large glaciers during the Pleistocene Epoch, which ended only about 10,000 years ago. Valley glaciers erode their valleys into steep walls, cirques and steps, so there may be numerous waterfalls in the valley after the glacier has departed.
wave-cut terrace
a horizontal surface cut into bedrock at sea level by wave erosion.
weather
to undergo physical or chemical decomposition at the earth's surface upon exposure to air, water, or ice.

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Geology of the Pacific Northwest
Glossary
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 10/08/18