O

occluded front
A front in which a cold front overtakes a warm front.
ocean basin
Areas covered by ocean water. The three major ocean basins are the Pacific, Atlantic, and Indian.
ocean trench
The deepest parts of the ocean basins.
oceanic crust
The crust that underlies the oceans; thinner and denser than continental crust.
oceanic zone
The open ocean, where sunlight does not reach the seabed.
oil
A liquid fossil fuel from ancient dead organisms used for transportation and other products.
omnivore
An organism that consumes both producers and other consumers for food.
open cluster
Groups of up to a few thousand stars loosely held together by gravity.
orbit
To travel in a circular or elliptical path around another object.
orbiter
The main part of the space shuttle that has wings like an airplane.
ore
A type of rock that contains useful minerals.
ore deposit
A mineral deposit that contains enough minerals to be mined for profit.
organic
Something from living organisms.
original horizontality
Sedimentary layers that were deposited horizontally.
orogeny
A mountain building event, usually taking place over tens or hundreds of millions of years.
outcrop
Exposed rock formations that are attached to the ground.
outer planets
The four large planets beyond the asteroid belt in our solar system.
outgassing
The transfer of gases from Earth’s mantle to the atmosphere by volcanic eruptions.
over-consumption
Resource use that is unsustainable in the long term; obtaining many more products than people need.
overpopulation
When the population of an area exceeds its carrying capacity or when long-term harm is done to resource availability or the environment.
oxidation
Oxygen reacts with another element to create a metal oxide.
ozone
Three oxygen atoms bonded together in an O3 molecule. Ozone in the lower atmosphere is a pollutant but in the upper atmosphere protects life from ultraviolet radiation.
ozone hole
A region around Antarctica in which ozone levels are reduced in springtime because of the action of ozone-destroying chemicals.
ozone layer
A layer of the stratosphere where ozone gas is more highly concentrated.