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ablation zone

The front part of a glacier, where ice is lost due to melting and calving.

Acadian Orogeny

A Devonian mountain-building event involving the collision of the east coast of North America and the accreted terrane of Avalon. The event caused metaphorphism, folding, and faulting in an area from New York to Newfoundland; sediments eroded from the mountains accumulated in thick strata, the Catskill Delta, in the Appalachian Basin of New York and Pennsylvania.
See also: orogeny


A class of extinct fish, bearing a superficial resemblence to sharks, and sharing features with both bony and cartilaginous fish. Acanthodian skin was covered with tiny, spiny scales.

accretion, accrete

The process by which a body of rock increases in size due to the addition of further sedimentary particles or of large chunks of land, such as terranes.

accretionary prism

A pile of sediments and ocean crust, scraped off a descending plate during subduction, and piled onto the overlying continental crust.

accumulation zone

The highly elevated part of a glacier, where annual snow accumulation outpaces snow loss.

acid rain

Rain or other precipitation that contains high amounts of sulfuric and nitric acid. It occurs when sulfur dioxide and nitrogen oxide react with water, oxygen, and other chemicals in the atmosphere to form these acidic compounds. Acid rain can cause damage to trees, soils, and entire ecosystems, as well as accelerating the decay of human works such as paint and building materials.

active plate boundary, active plate margin

The boundary between two plates of the Earth’s crust that are colliding, pulling apart, or moving past each other.
See also: plate tectonics

adaptive radiation

Process in which many new species evolve, adapting to vacant ecological niches in a relatively short interval of geological time. Examples occur across a range of scales, from the diversification of numerous species from a single species (e.g., Galapagos finches) to the diversification of higher taxa into previously unoccupied environments or into niches vacated through mass extinction (e.g., mammals after the extinction of dinosaurs).


Pertaining to, caused by, or carried by the wind. Aeolian sediments are often polished, giving them a “frosty” appearance.
The name comes from Aeolus, the Greek god of wind.


Tiny solid or liquid particles in the air. Examples include dust, smoke, mist, and human-made substances such as particles emitted from factories and cars.


A crystalline silicate rock with a colorful banded pattern. It is a variety of chalcedony ( quartz ). Agates usually occur as nodules in volcanic rock.


Crushed stone or naturally occurring unlithified sand and gravel, used for construction, agriculture, and industry. Aggregate properties depends on the properties of the component rock. Rock quarried for crushed stone includes, for example, granite and limestone.


A soil order ; these are highly fertile and productive agricultural soils in which clays often accumulate below the surface. They are found in humid and subhumid climates.

alkalic basalt

A fine-grained dark basaltic lava containing a high proportion of silicates, and relatively high in sodium and potassium.
See also: basalt, lava, silica

Alleghanian Orogeny

A Carboniferous to Permian mountain-building event involving the collision of the eastern coast of North America and northwestern coast of Africa (then part of Gondwana ). The event caused a combination of metaphorphism, folding, and faulting in an area from Alabama to Newfoundland. The orogeny resulted in the Appalachian and Allegheny Mountains, which are heavily eroded remnants of the original mountains formed by the event.

alluvium, alluvial

A layer of river-deposited sediment.


A metallic chemical element (Al), and the most abundant metal in the Earth’s crust.
Aluminium has a low density and an excellent ability to resist corrosion. Structural components made from the metal and its alloys are commonly used in the aerospace industry, transportation, and household goods.


A yellow or yellowish-brown hard translucent fossil resin that sometimes preserves small soft-bodied organisms inside.

ammonoid, ammonite

A group of extinct cephalopods belonging to the Phylum Mollusca, and possessing a spiraling, tightly-coiled shell characterized by ridges, or septa.


The group of tetrapods distinguished from amphibians by the development of an egg capable of maturing entirely out of water. Amniotes include the reptiles, birds, and mammals.


A group of dark-colored silicate minerals, or either igneous or metamorphic origin.


A type of tetrapod vertebrate whose skull has no openings near the temple. Anapsids are the most primitive subclass of reptile.


A fine-grained extrusive volcanic rock, with a silica content intermediate between that of basalt and dacite.


A soil order ; these are highly productive soils often formed from volcanic materials. They possess very high water- and nutrient-holding capabilities, and are commonly found in cool areas with moderate to high levels of precipitation.


A plutonic igneous rock made mostly of plagioclase feldspar. Most anorthosite rocks were formed in the Proterozoic eon (Precambrian).


A dense, shiny coal that has a high carbon content and little volatile matter. Anthracite is as much as 95% carbon. Found in deformed rocks, anthracite is the cleanest burning of the three types of coal, because it contains the highest amount of pure carbon.


Caused or created by human activity.


A layer of rock folded (bent) along an axis, concave side down (i.e., in an upside down "U" or "V" shape). Thus rocks at the center of the anticline, along the fold (crest), are lifted up relative to the rest of the layer.


A lustrous gray metallic element (Sb), mainly found in nature as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient times, when it was used in cosmetics. Today, the largest applications for the element are as an alloying material for lead and tin, and for plates in lead - acid batteries.

Antler Orogeny

A period of mountain building that deformed rocks in a belt extending from the California - Nevada border northward into Idaho. The Antler Orogeny began began in the late Devonian and continued into the Carboniferous.
See also: orogeny

Appalachian Basin

An inland basin, formed by the Taconic and Acadian mountain-building events. The crust was downwarped as a result of the colliding plates, and the basin was later filled with an inland sea.


A water-bearing formation of gravel, permeable rock, or sand that is capable of providing water, in usable quantities, to springs or wells.


A thin ridge of rock with an almost knife-like edge, formed when two glaciers erode parallel valleys.


A member of a group of primitive whales that lived during the Eocene and Oligocene epochs. The earliest members of the group are from the Indo-Pakistan region and were only partially aquatic.


A vase-shaped organism with a carbonate skeleton, generally believed to be a sponge. Archaeocyathids were the first important animal reef builders, originating in the early Cambrian. They were very diverse, but went extinct by the end of the Cambrian. Archeocyathids are often easiest to recognize in limestones, by their distinctive cross-sections.


A geologic time interval that extends from 4 billion to 2.5 billion years ago. It is part of the Precambrian.


A soil order ; these are formed in very dry (arid) climates. The lack of moisture restricts weathering and leaching, resulting in both the accumulation of salts and limited subsurface development. They are commonly found in deserts.


A channel that releases pressure from an aquifer, allowing the aquifer’s internal pressure to push the water up to the surface without the aid of a pump.


An invertebrate animal, belonding to the Phylum Arthropoda, and posessing an external skeleton (exoskeleton), body segments, and jointed appendages.
Arthropods include crustaceans, arachnids, and insects, and there are over a million described arthropod species living today. Trilobites are a major group of extinct arthropods.


A fibrous silicate mineral that is resistant to heat, flames, and chemical action. As a very slow conductor of heat, asbestos was once commonly used as a fireproofing material and electrical insulation. Concerns over its health effects on the lungs have led to its removal from most common uses.


A black, sticky, semi-solid, and viscous form of petroleum.


A thin, semifluid layer of the Earth, below the outer rigid lithosphere, forming much of the upper mantle. The heat and pressure created by the overlying lithosphere make the solid rock of the asthenosphere bend and move like metal when heated. The layer is thought to flow vertically and horizontally with circular convection currents, enabling sections of lithosphere to subside, rise, and undergo lateral movement.


A layer of gases surrounding a planet. Earth’s atmosphere protects living organisms from damage by solar ultraviolet radiation, and it is mostly composed of nitrogen. Oxygen is used by most organisms for respiration. Carbon dioxide is used by plants, algae, and cyanobacteria for photosynthesis.


A circular or horseshoe-shaped coral reef, surrounded by deep water and enclosing a shallow lagoon. The rim of the coral often extends above the water, sometimes creating a small beach or sandbar. Atolls tend to form as reefs grow around the rim of extinct volcanoes. As the volcanic peak erodes and the volcano subsides beneath the water’s surface, the reef grows upward and a lagoon is formed within the reef. Atolls range in diameter from 1 kilometer (0.6 miles) to more than 130 kilometers (81 miles).
See also: caldera, erosion, reef, scleractinian coral


An early Paleozoic microcontinent offshore of what is now the eastern coast of North America. Avalon collided with and became the eastern edge of North America during the Acadian Orogeny.