Glossary Database

Select a Letter to view the associated terms.

 Glossary Term: 


 A horizon  The topsoil layer in a soil profile that commonly contains organic matter.
 aa flow  A lava flow with a surface typified by angular, jagged blocks. Contrast with pahoehoe flow.
 aa lava  Lava flow that has a rough, jagged appearance.
 Abdomen  One of the three main regions comprising an insect. It is the region in the rear.
 abdominal  Pertaining to the belly or abdomen. Pelvic fins are abdominal if situated well behind the pectoral fins.
 Ablation  Reduction of a glacier by melting, evaporation, iceberg calving, deflation.
 Abrasion  The mechanical wearing away of a rock by friction, rubbing, scraping, or grinding.
 Absolute time  Geologic time measured in a specific duration of years (in contrast to relative time, which involves only the chronological order of events).
 Abyssal  Pertaining to the great depths of the oceans, generally 1000 fathoms (2000 m) or more below sea level.
 Abyssal floor  The deep, relatively flat surface of the ocean floor located on both sides of the oceanic ridge. It includes the abyssal plains and the hills.
 Abyssal hills  The part of the ocean consisting of hills rising as much as 1000 m above the surrounding floor. They are found seaward of most abyssal plains and occur in profusion in basins isolated from continents by trenches, ridges, or rises.
 Abyssal plains  Flat areas of the ocean floor, having a slope of less than 1:1000. Most abyssal plains lie at the base of a continental rise and are simply areas where abyssal hills are completely covered with sediment.
 accessory scale  An enlarged, scale-like flap or fleshy appendage in the axil of the pelvic or pectoral fins.
 Accreted terrane  A landmass that originated as an island arc or a microcontinent that was later added onto a continent.
 Accretionary prism  1. A wedge-shaped body of faulted and folded material scraped off subducting oceanic crust and added to an island arc or continental margin at a subduction zone. 2. Mass of deformed trench sediments and ocean floor sediments accumulated in wedge like slices on the underside of the overlying plate above a plate undergoing subduction.
 Acute angle  Angle that measures less than 90 degrees.
 adipose fin  Small fleshy fin without rays located on back between dorsal and caudal fins.
 Aestivate  To pass the summer in a certain manner or condition, often in a dormant or torpid state (Morris 1992). It is when animals, especially desert animals, go into a state of torpor or hibernation in response to very dry conditions or hot temperatures. They usually emerge for a period of activity in the fall of the year.
 Aftershock  An earthquake that follows a larger earthquake. Generally, many aftershocks occur over a period of days or even months after a major earthquake.
 Agate  A variety of cryptocrystalline quartz in which colors occur in bands; commonly deposited in cavities in rocks.
 Aggregate  A mass or body of rock particles, mineral grains, or both.
 alevins  Newly hatched, incompletely developed fishes (usually salmonids) still in nest or inactive on bottom, living off stored yolk.
 Algae  1.) Simple, plantlike organisms, one-celled or many-celled, containing chlorophyll, found in water or damp places. 2.) Mostly small aquatic plants that give water a green or brown color.
 Allochthonous  Rocks or materials formed someplace other than in their present site; of foreign origin.
 Alluvial  deposits of silt or other material, made by running water
 Alluvial fan  A fan-shaped deposit of sediment built by a stream where it emerges from an upland or a mountain range into a broad valley or plain. Alluvial fans are common in arid and semi-arid climates but are not restricted to them.
 Alluvium  A general term for any sedimentary accumulations deposited by comparatively recent action of rivers. It thus includes sediment laid down in river beds, flood plains, and alluvial fans.
 Altitude  The vertical distance between a point and a reference datum surface such as sea level.
 Altricial  refers to mammals born in a fairly undeveloped state; the young require close parental care for a period of time. Rabbits are born in an altricial state, whereas, hares are precocial.
 Altruism  The principle or practice of unselfish concern for or devotion to the welfare of others. 2) Altruistic
 Amber  Glass-like rock formed by the lithification of tree sap.
 Amorphous solid  A solid in which atoms or ions are not arranged in a definite crystal structure. Examples: glass, amber, obsidian.
 Amphibole  An important rock-forming mineral group of mafic silicates. Amphibole crystals are constructed from double chains of silicon-oxygen tetrahedra. Example: hornblende.
 Amphibolite  A metamorphic rock consisting mostly of amphibole and plagioclase feldspar.
 Anadromous  A management program for anadromous steelhead and chinook salmon which provides for harvest opportunity, where possible, on known hatchery origin fish while preserving and rebuilding wild and natural fish.
 anadromous/anadromous fish  Fish that leave the freshwater where they were hatched and migrate to the ocean to live their adult life. They then return to the same freshwater tributaries where they were hatched to spawn, thereby completing their life cycle.
 anal fin  The single median fin lying behind the anus, between the posterior end of the abdomen and the anterior end of the caudal peduncle.
 Anapsis  Refers to reptilian skull in which there are no temporal openings. A comparative common condition in fossil forms; in living reptiles, known only in turtles (Peters 1964).
 Andesite  A fine-grained igneous rock composed mostly of plagioclase feldspar and from 20% to 40% pyroxene, amphibole, or biotite, but no quartz or K-feldspar. It is abundant in mountains bordering the Pacific Ocean, such as the Andes Mountains of South America, from which the name was derived.
 Angular unconformity  An unconformity in which younger sediment or sedimentary rocks rest on the eroded surface of tilted or folded older rocks.
 Anion  A negatively charged ion.
 annulus (plural annuli)  A mark or marks formed on a fish scale or bone each year.
 Anomaly  A deviation from the norm or average.
 Anorthosite  A coarse-grained intrusive igneous rock composed primarily of calcium-rich plagioclase feldspar.
 Anthropomorphic  regarding animals as possessing human qualities (Morris 1992)
 Antibodies  a proteins, produced as a result of the introduction of an antigen, that has the ability to combine with the antigen that caused its production (Morris 1992).
 Anticline  1. A fold in which the limbs dip away from the hinge. After erosion, the oldest rocks are exposed in the central core of the fold. 2. A fold, generally convex upward, whose core contains stratigraphically older rocks.
 Antlers  antlers are found on members of the deer family (Cervidae). They consist of bone and are shed each year. New antlers grow each spring and are covered by skin (called velvet) which has blood vessels that carry nutrients to the growing antler. In the fall of the year the skin dies and is rubbed off on bushes and saplings. Antlers are typically found only on males, except in the caribou.
 anus  The external opening of the intestinal tract.
 Aphanitic texture  A rock texture in which individual crystals are too small to be identified without the aid of a microscope. In hand specimens, aphanitic rocks appear to be dense and structureless.
 aquaculture  Husbandry of aquatic organisms.
 Aquifer  A permeable stratum or zone below the Earth's surface through which groundwater moves.
 Arboreal  Pertaining to animals that live primarily in trees (Morris 1992)
 Arc magma  arc magma is magma associated with a volcanic island arc which is associated with a subduction zone. See volcanic island arc.
 Arch  1.)An arch-shaped landform produced by weathering and differential erosion. 2.)A bridge of bone in the skull, such as zygomatic arch (Peters 1964).
 Archean eon  A part of Precambrian time beginning 3.8 billion years ago, corresponding to the age of the oldest known rocks on Earth, and ending 2.5 billion years ago. View timeline of geologic events.
 Arcuate  Curved or bent in the shape of a bow.
 Arete   A rocky, sharp-edged ridge or spur, commonly present above the snowline in rugged mountains sculptured by glaciers.
 Arid climate  Subtype of the dry climates that is extremely dry and supports little or no vegetation cover. Evaporation exceeds precipitation on the average throughout the year.
 Arke  A narrow, sharp ridge separating two adjacent valley glaciers.
 Arkose  A sandstone containing at least 25% feldspar.
 Artesian water  Groundwater confined in an aquifer and under pressure great enough to cause the water to rise above the top of the aquifer when it is tapped by a well. See aquifer diagram.
 Artesian-pressure surface  The level to which water in an artesian system would rise in a pipe high enough to stop the flow. See aquifer diagram.
 articulated  Attached by means of a movable joint; jointed.
 Ash  Volcanic fragments the size of dust particles.
 Ash flow  A turbulent blend of unsorted pyroclastic material (mostly fine-grained) mixed with high-temperature gases ejected explosively from a fissure or crater.
 Ash-flow tuff  A rock composed of volcanic ash and dust, formed by deposition and consolidation of ash flows.
 Assemblage  A group of fossils or minerals. A group of fossils that occur at the same stratigraphic level are called a fossil assemblage. A group of minerals formed in a specific pressure/temperature zone are called a mineral assemblage.
 Assimilation  The process by which hot magma incorporates or dissolves the surrounding solid country rock.
 Asteroid  A small, rocky planetary body orbiting the Sun. Asteroids are numbered in the tens of thousands. Most are located between the orbit of Mars and the orbit of Jupiter. Their diameters range downward from 1000 km. Compare to meteorite.
 Asthenosphere  The weak zone inside Earth directly below the lithosphere, from 10 to 200 km below the surface. Seismic velocities are distinctly lower in the asthenosphere than in adjacent parts of Earth's interior. The material in the asthenosphere is therefore believed to be soft and yielding to plastic flow.
 Asymmetric  Irregular, uneven; without symmetry; having no center or axis where something can be divided evenly.
 Asymmetric fold  A fold (anticline or syncline) in which one limb dips more steeply than the other.
 Atmosphere  The mixture of gases surrounding a planet. The Earth's atmosphere consists chiefly of oxygen and nitrogen, with minor amounts of other gases. Synonymous with air.
 Atoll  A ring of low coral islands surrounding a lagoon.
 Atom  The smallest unit of an element. Atoms are composed of protons
 Atomic number  The number of protons in the nucleus of an atom. It uniquely defines an element.
 Atomic weight  The mass of one atom of an element
 Augite  A dark mineral of the pyroxene group
 Authigenic  Formed or generated in place. Said of rocks or minerals formed at the spot where they are now found.
 Autotomy  The ability of certain lower animals, such as lizards and starfish, to cast off injured body parts, such as the tail and, usually, to regenerate new ones (Morris 1992).
 Autotomy plane  The zone or septum of soft tissue which passes through a caudal vertebrae, along which breakage takes place in autotomy (Peters 1964).
 axillary process  A small flap of skin situated at the base of the pectoral or pelvic fin.
 Axis  1. Crystallography. An imaginary line passing through a crystal around which the parts of the crystal are symmetrically arranged. 2. Geophysics. A straight line about which a planet or moon rotates or spins.