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A glassy volcanic rock, formed when felsic lava cools rapidly. Although obsidian is dark in color, it is composed mainly of silicon dioxide (SiO2), and its dark color is a result of impurities such as iron and magnesium.
Obsidian is extremely brittle and breaks with very sharp edges. It was valuable to Stone Age cultures for its use as cutting implements or arrowheads.


A member of the group of whales (cetaceans) such as sperm whales and killer whales, which have baleen for feeding upon some combination of fish, squid, and mammals. The earliest odontocetes occur in the Oligocene.


See petroleum


A geologic time interval spanning from about 34 to 23 million years ago. It is an epoch of the Paleogene.


An iron -magnesium silicate mineral ((Mg,Fe)2SiO4) that is a common constituent of magnesium-rich, silica-poor igneous rocks.

oolite (adj. oolitic)

A sedimentary rock, especially limestone, consisting of tiny (< 2 millimeters [0.787 inches]) spherical grains made of concentric layers of calcium carbonate (ooids), sometimes preserved within a limy mud matrix. Ooids typically grow when calcium carbonate precipitates as the ooids roll around in intertidal supersaturated seawater.


A silicate gemstone lacking a rigid crystalline structure (and therefore a "mineraloid" as opposed to a mineral ). It forms when silica-rich water precipitates in fissures of almost any type of rock, as well as occasional organic matter.


A section of the Earth’s oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often thrust onto continental crustal rocks. Ophiolites are often formed during subduction —as oceanic crust is subducted, some of the deep-sea sediments overlying the crust, the oceanic crust itself, and sometimes rock from the upper mantle, can be scraped off the descending plate and accreted to the continental crust.


A geologic time period spanning from 485 to 443 million years ago. During the Ordovician, invertebrates dominated the oceans and fish began to diversify.
The Ordovician is part of the Paleozoic.


A type of rock that contains minerals with valuable elements, including metals, that are economically viable to extract.


An extinct ungulate (hoofed animal) related to modern camels. Oreodonts lived in woodlands and grasslands throughout North America during the Oligocene and Miocene.


A mountain-building event generally caused by colliding plates and compression of the edge of the continents. Orogeny is derived from the Greek word oro, meaning "mountain."

orographic precipitation

Rainfall caused when wind pushes a mass of humid air up the side of an elevated land formation like a mountain. As the air rises, it cools, and the moisture precipitates out.


A sandstone composed nearly entirely of well-rounded quartz grains cemented by silica.

Ouachita Orogeny

The late Paleozoic mountain building event that resulted in the folding and faulting of strata currently exposed in the Ouachita Mountains. The mountain range extends through Arkansas, Oklahoma, and the Marathon uplift region of West Texas.
See also: orogeny

outwash plain

Large sandy flats created by sediment-laden water deposited when a glacier melts. Outwash sediments are also called fluvial material.


A stream meander in the shape of a “U,” named after the U-shaped collar of an ox yoke. An oxbow may curve sufficiently to bend back and connect with itself; in this case the stream will straighten and the curved bow may become isolated as an “oxbow lake.”

oxidation, oxide

A chemical reaction involving the loss of at least one electron when two substances interact; most often used to describe the interaction between oxygen molecules and the substances they come into contact with. Oxidation causes effects such as rust and cut apples turning brown.


A soil order ; these are very old, extremely leached and weathered soils with a subsurface accumulation of iron and aluminum oxides. Commonly found in humid, tropical environments.