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Glossary A-B


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.

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'.

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.


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.


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.


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. 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.


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.


Metamorphic rock rich in blue amphibole.

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).


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.

Rock types:
Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks