Glossary

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O Horizon -  Topmost layer of most soils. It is composed mainly of plant litter and humus.
Oblique Aerial Photograph -  Photograph taken from a non-perpendicular angle from a platform in the atmosphere.
Obliquity -  Tilt of the Earth's polar axis as measured from the perpendicular to the plane of the Earth's orbit around the Sun. The angle of this tilt varies from 22.5 to 24.5° over a 41,000 year period. Current obliquity is 23.5°
Obsidian -  Glassy dark colored volcanic rock. Usually composed of rhyolite.
Occluded Front -  A transition zone in the atmosphere where an advancing cold air mass sandwiches a warm air mass between another cold air mass pushing the warm air into the upper atmosphere.
Occlusions -  A transition zone in the atmosphere where an advancing cold air mass sandwiches a warm air mass between another cold air mass pushing the warm air into the upper atmosphere.
Ocean Basin -  Part of the Earth's outer surface that is comprised of the ocean floor, mid-oceanic ridges, continental rise, and continental slope. The ocean basins are filled with saline water that makes up the oceans.
Ocean Currents -  Large scale horizontal flow of ocean water that is persistent and driven by atmospheric circulation.
Ocean Floor -  Flat plain found at the bottom of the ocean. The ocean floor represents the surface of the oceanic crust. The ocean floor lies between the mid-oceanic ridges and the trenches, usually 5,000 to 7,000 meters below the ocean surface. Also called the abyssal plain.
Ocean Trench -  Deep depression found at the edge of the ocean floor. Represents area of tectonic plate subduction.
Oceanic Crust -  Basaltic portion of the Earth's crust that makes up the ocean basins. Approximately 5 to 10 kilometers thick. See sima layer.
Oceanic Plate -  A rigid, independent segment of the lithosphere composed of mainly basalt that floats on the viscous plastic asthenosphere and moves over the surface of the Earth. The Earth's oceanic plates are an average 75 kilometers thick and were formed less than several hundred million years ago at one of the Earth's mid-oceanic ridges. Also see continental plate.
Oceanography -  The scientific study of phenomena found in the world's oceans.
Oceans -  A body of saline water found occupying all or part of the Earth's ocean basins. There are five recognized oceans: the Atlantic, the Southern Ocean, the Pacific Ocean, the Indian Ocean, and the Arctic Ocean.
Oil -  Hydrocarbon based liquid commonly found in the pores of sedimentary rocks of marine origin.
Oligotrophic Lake -  Lake with a low supply of nutrients in its waters. Also see eutrophic lake and mesotrophic lake.
Onshore-Offshore Transport -  The up and down movement of sediment roughly perpendicular to a shoreline because of wave action.
Open Sea -  That part of the ocean that extends from the continental shelf. Compare with coastal zone.
Open Talik -  Is a form of localized unfrozen ground (talik) in an area of permafrost. It is open to the ground surface but enclosed to permafrost below and at its sides.
Opisometer -  Mechanical device for measuring non-linear distances on maps.
Ordovician -  Geologic period that occurred roughly 438 to 505 million years ago. During this period, the first fish and fungi species appear.
Organic -  (1) Relating to an organism. (2) Derived from an organism.
Organic Matter -  Mass of matter that contains living organisms or non-living material derived from organisms. Sometime refers to the organic constituents of soil. Also see soil organic matter.
Orogenesis -  The process of mountain building through tectonic forces of compression and volcanism.
Orogenic Belt -  A major range of mountains on the continents.
Orographic Precipitation -  Is precipitation that forms when air is forced to rise because of the physical presence of elevated land. As the parcel rises it cools as a result of adiabatic expansion at a rate of approximately 10° Celsius per 1,000 meters until saturation. The large amounts of precipitation along the west coast of Canada are due mainly to this process.
Orographic Uplift -  Uplift of an air mass because of a topographic obstruction. Uplift also causes the cooling of the air mass. If enough cooling occurs condensation can occur and form into orographic precipitation.
Orthographic Projection -  Map projection that presents the Earth's surface in two-dimensions as if it were being observed from a great distance in space. Distortion of areas and angles becomes greater as you move from the center of the projection to its edges.
Outcrop -  Area of exposed bedrock at the Earth's surface with no overlying deposits of soil or regolith.
Outer Core -  Outer region of the Earth's core. It is believed to be liquid nickel and iron and has a density of about 11 grams per cubic centimeter. It surrounds the inner core and has an average thickness of about 2,250 kilometres.
Outgassing -  The release of gas from cooling molten rock or the interior of the Earth. Much of the atmosphere's gaseous constituents, like water vapor, nitrogen, and argon, came from outgassing.
Outwash -  Glaciofluvial sediments deposited by meltwater streams at the edge of a glacier.
Outwash Plain -  A flat or gentle sloping surface of glaciofluvial sediments deposited by meltwater streams at the edge of a glacier. Usually found in close spatial association with moraines.
Overbank Flow -  Movement of flood waters outside a stream channel during period of high discharge.
Overland Flow -  The topographic movement of a thin film of water from precipitation to lower elevations. With time, this water will begin to organizing its flow into small channels called rills. The rills converge to form progressively larger channels until stream channels are formed. Occurs when the infiltration capacity of an area's soil has been exceeded. Also called sheet flow or runoff.
Overthrust Fault -  Fault produced by the fracturing of rock in a fold because of intense compression.
Overturned Fold -  A fold in rock layers where one limb is pushed past the perpendicular. This results in both limbs having dips in the same direction.
Oxbow Lake -  Is portion of abandoned stream channel filled with stagnant water and cut off from the rest of the stream. Oxbow lakes are created when meanders are cut off from the rest of the channel because of lateral stream erosion.
Oxidation -  (1) Chemical attachment of free oxygen to other elements and compounds. One of the types of chemical weathering. (2) Loss of an electron during a chemical reaction from one atom to another.
Ozone -  Tri-atomic oxygen that exists in the Earth's atmosphere as a gas. Ozone is highest in concentration in the stratosphere (10-50 kilometers above the Earth's surface) where it absorbs the Sun's ultraviolet radiation. Stratospheric ozone is produced naturally and helps to protect life from the harmful effects of solar ultraviolet radiation. Over the last few decades levels of stratospheric ozone have been declining globally, especially in Antarctica. Scientists have determined that chlorine molecules released from the decomposition of chlorofluorocarbons are primarily responsible for ozone destruction in the stratosphere.It is also abundant near the the Earth's surface in highly polluted urban centers. In these areas, it forms as a by product of photochemical smog, and is hazardous to human health.
Ozone Hole -  Is a sharp seasonal decrease in stratospheric ozone concentration that occurs over Antarctica in the spring. First detected in the late 1970s, the ozone hole continues to appear as a result of complex chemical reaction in the atmosphere that involves CFCs.
Ozone Layer -  Atmospheric concentration of ozone found at an altitude of 10 to 50 kilometers above the Earth's surface. This layer is important to life on the Earth because ozone absorbs harmful ultraviolet radiation.
Ozonosphere -  Another name for the ozone layer. Atmospheric concentration of ozone found at an altitude of 10 to 50 kilometers above the Earth's surface. This layer is important to life on the Earth because ozone absorbs harmful ultraviolet radiation.