Glossary of Terms: O

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.

 

Ocean

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.

 

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 Current

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.

 

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.

 

Ocean Trench

Deep depression found at the edge of the ocean floor. Represents area of tectonic plate subduction.

 

O Horizon

Topmost layer of most soils. It is composed mainly of plant litter and humus.

 

Oil

Hydrocarbon based liquid commonly found in the pores of sedimentary rocks of marine origin.

 

Old Growth Forest

Climax forests dominated by late successional species of trees that are hundreds to thousands of years old. Examples include virgin uncut forests of Douglas fir, western hemlock, giant sequoia, and coastal redwoods located in western North America. Also see second-growth forest.

 

Oligotrophic Lake

Lake with a low supply of nutrients in its waters. Also see eutrophic lake and mesotrophic lake.

 

Olivine

Common silicate mineral found in rocks formed from mafic magma. Its chemical composition varies between Mg₂SiO₄ and Fe₂SiO₄.

 

Omnivore

Heterotrophic organism that consumes both plants and other animals as a source of food. Examples of ominvores include pigs, raccoons, rats, bears, turtles, monkeys, and humans. Also see carnivore, herbivore, scavenger, and detritivore.

 

One-Tailed Statistical Test

Is an inferential statistical test where the values for which one can reject the null hypothesis are located entirely in one side of the center of the probability distribution.

 

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 System

Is a system that transfers both matter and energy can cross its boundary to the surrounding environment. Most ecosystems are an example of an open system.

 

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.

 

Optimum

The level of an abiotic factor or condition in the environment within the tolerance range at which a species or population can function most efficiently or with the greatest positive effect to its physiological or reproductive fitness.

 

Opisometer

Mechanical device for measuring non-linear distances on maps.

 

Organ

Group of cells and tissues that have a particular function for an organism.

 

Organelle

Is a specialized structure found in cells that carry out distinct cellular functions.

 

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.

 

Organic Soil

Soil order (type) of the Canadian System of Soil Classification. This soil type is common in fens and bogs. This soil is mainly composed of organic matter in various stages of decomposition.

 

Organism

Any form of life.

 

Origin

The arbitrary starting point on a graph or grid coordinate system. Defined by the intersection of the x and y-axes. Also see false origin.

 

Orogenesis

The process of mountain building through tectonic forces of compression and volcanism.

 

Orogenic Belt

A major range of mountains on the continents.

 

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.

 

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.

 

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 kilometers.

 

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.

 

Output

Movement of matter, energy, or information out of a system. Also see input.

 

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.

 

Ordovician

Geologic period that occurred roughly 438 to 505 million years ago. During this period, the first fish and fungi species appear.

 

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.

 

Oxisol

Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. These soils are found in moist tropical environments. They are extremely weathered and very old. Layers within the profile have accumulations of mineral oxides and have a lack of base cations available for exchange.

 

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.

 

 

Citation: Pidwirny, M. (2006). "Glossary of Terms: O". Fundamentals of Physical Geography, 2nd Edition. Date Viewed. http://www.physicalgeography.net/physgeoglos/o.html