Glossary Starting with the letter: O

Ocean Basin

The area of the seafloor from the base of the continental margin (generally at the beginning of the continental rise) to the mid-ocean ridge.

Oceanic Crust

Part of the crust that forms the upper part of the rigid lithosphere, the outer layer of the Earth. Oceanic crust is formed at mid-ocean ridges and is denser and typically thinner than continental crust. Oceans are underlain by oceanic crust.

Oceanic Trench

A narrow, steep-sided, elongated depression of the deep-sea floor.


The study of the ocean, including the physical properties of the ocean such as the currents and waves (physical oceanography), the chemistry of the ocean (chemical oceanography), the geology of the seafloor (marine geology), and the organisms that carve their niche within the ocean realm (marine biology and marine ecology).

Outer Core

The upper or outermost part of the Earth’s core extending from 2,900 to 5,100 kilometers below the surface. It is believed to be liquid because it significantly reduces P-wave (compressed waves) velocities and does not transmit S-waves (shear waves).


A primitive theropod dinosaur that developed from the carnivorous raptor theropods during the Late Cretaceous Period. The Oviraptor’s skull was light in weight and its bones thin, much like modern birds. It had a short, thick, parrot-like beak and ran on its hind legs.


A process in which a chemical element loses electrons.


A chemically active bluish gas that is made of molecules of three oxygen atoms (O3). In the stratosphere, ozone acts as a protective barrier for Earth’s surface by absorbing much of the potentially damaging ultraviolet radiation that comes from the Sun. In the troposphere, ozone acts as a harmful pollutant.

Ozone Layer

A layer in the stratosphere that contains about ninety percent of Earth’s ozone. The ozone layer occurs approximately twenty-five kilometers (sixteen miles) above the surface of Earth.