Glossary Database

Select a Letter to view the associated terms.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

 Glossary Term: 

Definition:

 Obligate  restricted to a single mode of behavior or environmental condition (Morris 1992).
 Oblique  Neither perpendicular nor horizontal. A slanting direction; not direct.
 Oblique angle  An angle that is less than 90.
 Obsidian  A glassy igneous rock with a chemical composition equivalent to that of granite.
 Ocean basin  A low part of the lithosphere lying between continental masses. The rocks of an ocean basin are mostly basalt with a veneer of oceanic sediment.
 Ocean tide  Periodic rise and fall of the ocean level induced by gravitational attraction between the earth and moon in combination with earth rotation.
 Oceanic crust  The type of crust that underlies the ocean basins. It is generally less than 8 km thick, composed predominantly of basalt and gabbro. Its density is about 3.0 g/cm3. The velocities of compressional seismic waves traveling through it exceed 6.2 km/sec. Compare with continental crust.
 Oceanic ridge  A continuous ridge, or broad, fractured topographic swell, that extends through the central part of the Arctic, Atlantic, Indian, and South Pacific oceans. It is several hundred kilometers wide, and its elevation above the ocean floor is 600 m or more. The ridge marks a divergent plate boundary where new oceanic lithosphere is being formed.
 Oceanic trench  Narrow, deep depression in the seafloor representing the line of subduction of an oceanic lithospheric plate beneath the margin of a continental lithospheric plate; often associated with an island arc.
 Oil reservoir  Porous rock that can contain oil.
 Oil shale  Shale that is rich in hydrocarbon derivatives, In the United States, a significant oil shale is the Green River Formation, in the Rocky Mountain region.
 Oil trap  Impermeable rocks or structures that block the flow of oil and force it to accumulate into larger bodies.
 Olivine  An important silicate mineral with magnesium and iron [(Mg,Fe)2S04]
 Omnivorous  Eating any sort of food, esp. both animal and vegetable food (Guralnik 1970)
 Oolite  A limestone consisting largely of spherical grains of calcium carbonate in concentric spherical layers.
 Oolith  A small round accretionary grain of calcium carbonate, 0.25mm to2.0 mm in diameter.
 Ooze (marine geology)  Marine sediment consisting of more than 30% shell fragments of microscopic organisms.
 opercle  The large posterior bone of the gill cover.
 Opercular chamber  The closed cavity which covers the internal gills of the frog embryo, opening to the outside through a spiracle (Peters 1964).
 opercular flap  As in the sunfishes, a fleshy extension from the opercle.
 Ophiolite  A sequence of rocks characterized by ultramafic rocks at the base and (in ascending order) gabbro, sheeted dikes, pillow lavas, and deep-sea sediments. The typical sequence of rocks constituting the oceanic crust. We see ophiolites when they have been abducted onto the edge of continental crust.
 Ore  A natural combination of minerals from which economically important amounts of a metal or metals can be obtained.
 Ore deposit  A mass of rock containing metal (or some other commodity like diamonds) of sufficient abundance to be extracted at a profit.
 Organic  Designating any chemical compound containing carbon. Of or derived from living organisms.
 Organic sediment  A sediment deposited through biological means and rich in hydrocarbons, such as coal.
 origin of fins  The most anterior part of each fin at its base.
 Orogenesis  Mountain formation, as caused by volcanism, subduction, plate divergence, folding, or the movement of fault blocks.
 Orogenic  Pertaining to deformation of a continental margin to the extent that a mountain range is formed.
 Orogenic belt  A mountain belt.
 Orogeny  A major episode of mountain building.
 Oscillation  To vary regularly from high to low values, such as a light wave.
 Osmeterium  A Y-shaped gland located behind the head of some caterpillars which can be pushed out to emit a chemical disagreeable to potential predators.
 Ossified skull  Bony skull.
 Osteoderm  A bony deposit in the form of a plate or scale found in the dermal layers of the skin (Peters 1964).
 Outcrop  An exposure of bedrock.
 Outlet glacier  A tongue-like stream of ice, resembling a valley glacier, that forms where a continental glacier encounters a mountain system and is forced to move through a mountain pass in large streams.
 Outwash  Stratified sediment washed out from a glacier by meltwater streams and deposited in front of the end moraine.
 Outwash plain  The area beyond the margins of a glacier where meltwater deposits sand, gravel, and mud washed out from the glacier.
 Overland flow  A condition in which precipitation rate is faster than infiltration rate, and excess water runs over the surface of land.
 Overturned fold  A fold in which at least one limb has been rotated through an angle greater than 90 degrees.
 Oviparous  Laying eggs that develop and hatch outside the maternal body (Morris 1992).
 Oviposit  To lay eggs. The process of laying eggs is called oviposition.
 Ovulation  the discharge of an ovum from a vesicular follicle of the ovary; this usually occurs on the 14th day after the first day of the last menstrual period (Morris 1992).
 Oxbow lake  A lake formed in the channel of an abandoned river meander.
 Oxidation  1. Chemical combination of oxygen with another substance. 2. Decrease in the number of electrons in a compound via reaction with another compound (an oxidizing agent or oxidant).
 Oxide  A mineral compound characterized by the linkage of oxygen with one or more metallic elements, such a spinel, MgAl2O4.
 Ozone layer  The layer of ozone (O3) in the stratosphere that absorbs much ultraviolet radiation.