Science / Geology Glossary

A-Horizon: The uppermost layer of a soil, containing organic material and leached minerals.

Aa: A blocky and fragmented form of lava occurring in flows with fissured and angular surfaces.

Acidic Rock: An igneous rock that has a relatively high silica content. Examples are granite and rhyolite. Also see entries for basic, intermediate and ultrabasic rocks.

Acquisition: Obtaining the legal right to test a property for mineral resources and produce any that are discovered. The rights can be obtained by purchasing the entire property (surface and mineral righ . . . View Full Definition

Acre-Foot: The volume of water needed to flood one acre of land to a depth of one foot. Equivalent to 43,560 cubic feet, 1,233 cubic meters or 325,851 gallons. One of the most common units of measure u . . . View Full Definition

Acreage: An area, measured in acres, that is owned or controlled by one or more owners or lessees. “Gross Acreage” is the entire geographic area under control. “Net Acreage” is the gross acreage mult . . . View Full Definition

Algal Mat: A layered communal growth of algae observed in fossils an in present day tidal zones associated with carbonate sedimentation.

Alkali: Used in reference to materials that are rich in sodium and/or potassium.

Alkali Metal: A strongly basic metal like potassium or sodium.

Alluvial Fan: A fan-shaped wedge of sediment that typically accumulates on land where a stream emerges from a steep canyon onto a flat area. In map view it has the shape of an open fan. Typically forms in . . . View Full Definition

Alluvium: Unconsolidated terrestrial sediment composed of sorted or unsorted sand, gravel, and clay that has been deposited by water.

Amphibolite: Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. It is composed primarily of amphibole and plagio . . . View Full Definition

Andesite: A fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite. Picture of Andesite.

Angle Of Repose: The maximum angle that a soil, sediment or other loose material can be placed or accumulate and be stable. The angle of repose varies for different types of materials and different moisture . . . View Full Definition

Angstrom: A length of 10 to the minus tenth meter or one hundred millionth of a centimeter.

Angular Unconformity: An erosional surface that separates rock units of differing dips. The rocks below the surface were deposited, deformed and eroded. The younger rocks above then accumulated upon the erosional surface.

Anthracite: The highest rank of coal. By definition, a coal with a fixed carbon content of over 91% on a dry ash-free basis. Anthracite coals have a bright luster, break with a conchoidal fracture, a se . . . View Full Definition

Aquiclude: A subsurface rock, soil or sediment unit that does not yield useful quanties of water.

Aquifer: A subsurface rock or sediment unit that is porous and permeable. To be an aquifer it must have these traits to a high enough degree that it stores and transmits useful quantities of water.

Aquifer (Artesian): An aquifer that is bounded above and below by impermeable rock or sediment layers. The water in the aquifer is also under enough pressure that, when the aquifer is tapped by a well, the wate . . . View Full Definition

Aquifer (Confined): An aquifer that is bounded above and below by impermeable rock or sediment layers. There may or may not be enough pressure in the aquifer to make it an 'artesian aquifer'.

Aquifer (Unconfined): An aquifer that is not overlain by an impermeable rock unit. The water in this aquifer is under atmospheric pressure and is recharged by precipitation that falls on the land surface directly . . . View Full Definition

Arkose: A sandstone that contains at least 25% feldspar. Easily recognized because the feldspar grains are typically pink and angular in shape.

Arroyo: A steep-sided and flat-bottomed gulley in an arid region that is occupied by a stream only intermittently, after rains.

Artesian Well: A well that penetrates an aquiclude to reach an aquifer containing water under pressure. Thus water in the well rises above the surrounding water table.

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