Geologic Glossary

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- A -

A`a (pronounced "ah-ah" - a Hawaiian term), is lava that has a rough, jagged, spiny, and generally clinkery surface. In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense interior.

Occurs when more glacier ice is lost by melting and evaporation each year than is added by snowfall.

absolute age
The approximate age of a geologic event, feature, fossil, or rock in years. 'Absolute' ages are determined by using natural radioactive 'clocks'. The preferred term is radiometric age.

A process that adds part of one tectonic plate to a larger plate along a convergent (collisional) plate boundary.

A bright to gray-green member of the amphibole mineral family. In addition to silica, it contains calcium, magnesium, and iron. Actinolite is a non-hazardous relative of asbestos and is a common mineral in metamorphic rocks.

active volcano
A volcano that has erupted within historical time and is likely to do so again in the future.

Fine liquid or solid particles suspended in the atmosphere. Aerosols resulting from volcanic eruptions are tiny droplets of sulfuric acid -- sulfur dioxide that has picked up oxygen and water.

A horizon
The top layer of soil. Plant and other organic debris builds up in this layer. This is the part of the soil generally referred to as 'top soil'.

Volcanic ash that has fallen through the air from an eruption cloud. A deposit so formed is usually well sorted and layered. Also called ashfall.

alluvial fan
A fan-shaped pile of sediment that forms where a rapidly flowing mountain stream enters a relatively flat valley. As water slows down, it deposits sediment (alluvium) that gradually builds a fan.

Sand, gravel, and silt deposited by rivers and streams in a valley bottom.

A family of silicate minerals forming prism or needlelike crystals. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts, along with water. Hornblende always has aluminum and is a most common dark green to black variety of amphibole; it, forms forming in many igneous and metamorphic rocks. Actinolite has no aluminum; it and is needle-shaped and light green. Blue amphibole contains sodium and, of course, is bluish in color.

A rock made up mostly amphibole and plagioclase feldspar. Although the name amphibolite usually refers to a type of metamorphic rock, an igneous rock composed dominantly of amphibole can be called an amphibolite too.

Fine-grained, generally dark colored, igneous volcanic rock with more silica than basalt. Commonly with visible crystals of plagioclase feldspar. Generally occurs in lava flows, but also as dikes. The most common rock in volcanic arcs.

Literally, "without water". Refers to minerals or other materials which do not have water as an primary constituent.

A upward-curving (convex) fold in rock that resembles an arch. The central part contains the oldest section of rock.

annual snowline
A term used by glaciologists (scientists who study glaciers) for the boundary where the amount of snow loss from melting equals the amount of snow accumulation from snowfall (also called firn limit).

An igneous rock texture in which individual mineral grains are too small to be distinguished with the naked eye.

A light-colored igneous rock with the same mineral composition as granite: quartz, plagioclase feldspar, and potassium feldspar, but with a fine-grained, almost sugary texture.

Archean Eon
The time interval between 3800-2500 million years ago. The Archean is one of the Precambrian time intervals.

The science that focuses on the study of past human cultures.

arc rocks
Volcanic arc rocks.

A term used to describe clay-rich rocks.

argillic horizon
A clay-rich layer of soil. Clay often forms in overlying soil layers from the decomposition of feldspars and other minerals. The extremely fine clay particles are gradually carried down by water to accumulate into the argillic horizon.

Name used for unusually hard, fine-grained sedimentary rocks, such as shale, mudstone, siltstone, and claystone. Commonly black.

A region without earthquakes (seismic activity).

Fine particles of volcanic rock and glass blown into the atmosphere by a volcanic eruption.

See airfall.

ash flow
A pyroclastic flow consisting predominantly of ash-sized (less than 4 millimeters in diameter) particles. Also called a glowing avalanche if it is of very high temperature.

The uppermost layer of the mantle, located below the lithosphere. This zone of soft, easily deformed rock exists at depths of 100 kilometers to as deep as 700 kilometers.

atmospheric shock wave
Strong compressive atmospheric wave driven by volcanic ejecta.

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.

Augen are relatively large, eye-shaped mineral grains in certain types of metamorphic rocks, especially schist and gneiss. (Augen = eyes in German)

Masses of rock or ice that fall or slide suddenly under the force of gravity.

- B -

ballistic fragment
An explosively ejected rock fragment that follows a ballistic trajectory.

banded gneiss
See gneiss.

A dark, fine-grained, extrusive (volcanic) igneous rock with a low silica content (40% to 50%), but rich in iron, magnesium and calcium. Generally occurs in lava flows, but also as dikes. Basaltic lava can flow quickly and easily move >20 kilometers from a vent. The low viscosity typically allows volcanic gases to escape without generating enormous eruption columns. Basaltic lava fountains and fissure eruptions, however, still form explosive fountains hundreds of meters tall. Common minerals in basalt include olivine, pyroxene, and plagioclase. Basalt is erupted at temperatures between 1100 to 1250° C. Basalt makes up most of the ocean floor and is the most abundant volcanic rock in the Earth's crust.

base level
The level (elevation) at which a stream or river can erode no more, usually sea level.

base surge
Turbulent, low-density cloud of rock debris and water and (or) steam that moves over the ground surface at high speed. Base surges are generated by explosions.

A depression in the Earth's surface that collects sediment.

Basin and Range province
This province extends from eastern California to central Utah, and from southern Idaho into the state of Sonora in Mexico. Within the Basin and Range province the Earth's crust and uppermost mantle have been stretched, creating large faults. Along these faults linear mountain ranges were uplifted and flat valleys down-dropped, producing the distinctive topography of the Basin and Range province.

Very large mass of intrusive (plutonic) igneous rock that forms when magma solidifies at depth. A batholith must have greater than 100 square kilometers (40 square miles) of exposed area. See pluton, stock.

A layer of sediment or sedimentary rock.

Parallel layers of sediment or sedimentary rock (beds) that can be distinguished from each other by characteristics such as grain size and chemical composition.

Sedimentary layers in a rock. The beds are distinguished from each other by grain size and composition, such as in shale and sandstone. Subtle changes, such as beds richer in iron-oxide, help distinguish bedding. Most beds are deposited essentially horizontally.

The solid rock that lies beneath soil and other loose surface materials.

In North America, 1,000,000,000

A common rock-forming mineral of the mica family. Biotite is a black or dark brown silicate rich in iron, magnesium, potassium, aluminum, and, of course, silica. Like other micas, it forms flat book-like crystals that peal apart into individual sheets on cleavage planes.

black sand beach
The famous "black sand" beaches of Hawaii were created virtually instantaneously by the violent interaction between hot lava and sea water.

Tephra is the general term now used by volcanologists for airborne volcanic ejecta of any size. Historically, however, various terms have been used to describe ejecta of different sizes. Fragments larger than about 2.5 inches are called blocks if they were ejected in a solid state and volcanic bombs if ejected in semi-solid, or plastic, condition.

Trees felled by a volcanic blast.

Metamorphic rock rich in blue amphibole.

Tephra is the general term now used by volcanologists for airborne volcanic ejecta of any size. Historically, however, various terms have been used to describe ejecta of different sizes. Fragments larger than about 2.5 inches are called blocks if they were ejected in a solid state and volcanic bombs if ejected in semi-solid, or plastic, condition. Volcanic bombs undergo widely varying degrees of aerodynamic shaping, depending on their fluidity, during the flight through the atmosphere. Based on their shapes after they hit the ground, bombs are variously described, in graphic terms, as "spindle or fusiform,", "ribbon", "bread-crust", or "cow-dung".

borrow pit
A pit or excavation area used for gathering earth materials (borrow) such as sand or gravel.

Any loose rock (sediment) larger than 256 millimeters (10 inches).

bread-crust bombs
See bombs.

Rock made up of angular fragments of other rocks held together by mineral cement or a fine-grained matrix. Volcanic breccia is made of volcanic rock fragments, generally blown from a volcano or eroded from it. Fault breccia is made by breaking and grinding rocks along a fault.

- C -

A descriptive term used for rocks and other earth materials that have an abundance of calcium carbonate (CaCO3). For example, a calcareous sandstone has up to 50% calcium carbonate.

calcic horizon
A soil layer at least 15 cm thick that has been enriched with calcium carbonate (CaCO3).

Mineral made of calcium carbonate (CaCO3). Generally white, easily scratched with knife. Most seashells are made of calcite or related minerals. This is the lime of limestone.

Large, generally circular, fault-bounded depression caused by the withdrawal of magma from below a volcano or volcanoes. Commonly, the magma erupts explosively as from a giant volcano and, falling back to Earth as volcanic ash, fills the caldera so formed. Calderas are different from craters, which are smaller, circular depressions created primarily by explosive excavation of rock during eruptions.

A sedimentary rock made mainly of calcium carbonate (CaCO3). Limestone and dolomite are common carbonate sedimentary rocks.

carbonic acid
A mild acid formed when water and carbon dioxide chemically combine in the atmosphere and soil.This acid is a very important component in the development of cave decorations (speleothems).

A natural opening in the ground extending beyond the zone of light and large enough to permit the entry of an average human.

cave system
A cave or caves having a complex network of interconnected chambers and passages that constitute an underground drainage system.

cavernous weathering
A combination of chemical and mechanical weathering processes act on rock surfaces to produce hollows and caverns. This is also called honeycomb weathering.

One of the processes that work together to turn sediment into sedimentary rock (lithification). Mineral-laden water percolates through sediment with open pore spaces. The spaces are gradually filled by minerals precipitating from the water, binding the grains together.

Cenozoic Era
The time span between 66.4 million years ago to the present. he Cenozoic contains the Tertiary and the Quaternary periods.

chemical sedimentary rock
Sedimentary rock composed of minerals that were precipitated from water. This process begins when water traveling through rock dissolves some of the minerals, carrying them away from their source. Eventually these minerals are redeposited, or precipitated, when the water evaporates away or when the water becomes over-saturated.

chemical weathering
The process that changes the chemical makeup of a rock or mineral at or near the Earth's surface. Chemical weathering alters the internal structure of minerals by the removing and/or adding elements. Compare with mechanical weathering.

A very fine-grained sedimentary rock made of quartz. Usually made of millions of globular siliceous skeletons of tiny marine plankton called radiolarians. Black chert is called flint.

Family of platy silicate minerals containing various amounts of magnesium, iron, aluminum, water, and small amounts of other elements. Some mineralogists include chorites in the mica family because the crystals form small flakes. Commonly green.

A bubbly (vesicular) volcanic rock fragment that forms when molten, gas-filled lava is thrown into the air, then solidifies as it falls.

cinder cone
A volcanic cone built almost entirely of ejected cinders, loose volcanic fragments, ash, and pumice (pyroclastics or tephra).

A fragment of a pre-existing rock or fossil embedded within another rock.

A sedimentary rock composed of fragments (clasts) of pre-existing rock or fossils. (=Detrital sedimentary rocks)

A family of platy silicate minerals that commonly form as a product of rock weathering. Also, any particle smaller than 1/256 of a millimeter in diameter.
More Information: Sedimentary - Clays

The tendency of a mineral to break along weak planes.

Loose particles of rock or mineral (sediment) that range in size from 64 - 256 millimeters in diameter. Cobbles are a size of gravel larger than pebbles, but smaller than boulders.

Occurs when the weight of overlying material compresses more deeply buried sediment. Along with cementation, this process converts sediments to solid rock.

composite volcano
See stratovolcano.

conduit (volcanic)
A subterranean passage through which magma reaches the surface during volcanic activity.

Rock layers that were deposited in sequence without episodes of erosion between deposition of layers. .

A sedimentary rock rock made of rounded rock fragments, such as pebbles, cobbles, and boulders, in a finer-grained matrix. To call the rock a conglomerate, some of the consituent pebbles must be at least 2 mm (about 1/13th of an inch) across.

contact metamorphism
Metamorphism caused by heat from an igneous intrusion.

continental collision
Convergence of two continental plates. Such a convergence between the Indian and Eurasian plates is responsible for producing the Himalayas.

continental crust
The rigid, outer layer of relatively low density rock that makes up the continents.

continental drift
A hypothesis proposed by Alfred Wegener suggesting that the continents are not stationary, but have 'drifted' through time. Plate tectonics is the name for the theory that provided the evidence necessary to support Wegener's hypothesis.

continental volcanoes
In the typical "continental" environment, volcanoes are located in unstable, mountainous belts that have thick roots of granite or granitelike rock. Magmas, generated near the base of the mountain root, rise slowly or intermittently along fractures in the crust. During passage through the granite layer, magmas are commonly modified or changed in composition and erupt on the surface to form volcanoes constructed of nonbasaltic rocks.

contour lines
Parallel lines used on topographic maps to show the shape and elevation of the land. They connect points of equal elevations.

Cordilleran ice sheet
Ice cap that grew in western North America during the Pleistocene Epoch. It began growing first in Canada, eventually covering much of British Columbia, Alaska, the northern U.S., and parts of several western states.

convergent plate boundary
A boundary in which two plates collide. The collision can be between two continents (continental collision), an relatively dense oceanic plate and a more buoyant continental plate (subduction zone) or two oceanic plates (subduction zone).

The innermost layer of the Earth, made up of mostly of iron and nickel. The core is divided into a liquid outer core and a solid inner core. The core is the most dense of the Earth's layers.

The depression produced by a meteorite impact or a steep-sided, usually circular depression formed by either explosion or collapse at a volcanic vent.

The relatively stable nucleus of a continent. Cratons are made up of a shield-like core of Precambrian Rock and a buried extension of the shield.

The rocky, relatively low density, outermost layer of the Earth.

Growth of minerals (crystalline solids) from a liquid or gas.
Continue to glossary entries D through I

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