O Horizon - The topmost layer of most soils. This soil horizon is composed mainly of plant litter and humus. Typically found located above the A horizon.

Oasis - An isolated region of lush vegetation growth in a surrounding desert landscape. They are usually associated with increased water availability for plant growth due to the presence of a lake or spring. A human-made well can create an oasis. The locations of natural oases were well known by nomadic people who regularly traveled across desert regions. 

Oblate - Refers to the flattening of a rotating sphere, like a planet, at the poles and bulging at the object’s equator. Caused by rotational forces pushing the mass of the sphere outward at the equatorial regions.

Oblique Aerial Photograph - A photograph taken from a non-perpendicular angle from a platform in the atmosphere.

Obliquity - The tilt of the Earth's polar axis as measured from the perpendicular to the plane of the Earth's orbit around the Sun. One cycle of obliquity takes on average 41,040 years.  Over the last 5 million years, the angle of the Earth's tilt has varied from 22.0 to 24.5°. The current obliquity is 23.4°. However, a value of 23.5 is commonly used for simplicity.

Obsequent Stream - A stream whose course is in a direction that is opposite the slope of the land surface.

Obsidian - A naturally occurring glassy dark colored volcanic rock formed when lava extruded from a volcano cools rapidly with little crystal growth. Obsidian is 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. Cloud development and precipitation usually occur above the occluded front. Compare with a cold front and a warm front. Associated with mid-latitude cyclones.

Occult Deposition - A type of acid deposition where fog and clouds loaded with acid pollutants interact directly with receiving surfaces. Concentrations of acid compounds wet deposited on contacted surfaces can be 20 times greater than what occurs with acid precipitation.

Ocean - A body of saline water found occupying all or part of the Earth's ocean basins. There are five recognized oceans: the Atlantic Ocean, the Southern Ocean, the Pacific Ocean, the Indian Ocean, and the Arctic Ocean.

Ocean Acidification - Recent trend of decreasing seawater pH levels in the Earth's oceans caused by absorption of carbon dioxide (CO₂) from the atmosphere. Chemically, this carbon dioxide reacts with seawater to become carbonic acid. The carbonic acid then reacts with seawater to produce bicarbonate ions (HCO₃^-) and hydrogen ions (H⁺). From 1751 to 2025, the average pH of seawater dropped from 8.25 to 8.04 due to an increase in the concentration of hydrogen ions in the oceans. This acidification trend coincides with the increase in atmospheric carbon dioxide concentrations driven by fossil fuel combustion and other human activities.

Ocean Basin - Part of the Earth's outer surface composed of the ocean floor, mid-oceanic ridge, continental rise, and continental slope. The ocean basins are filled with saline water that makes up the oceans.

Ocean Current - A persistent horizontal flow of ocean water that is driven by atmospheric circulation, wave action, or the cyclical rise and fall of tides.

Ocean Floor - A flat plain found at the bottom of the oceans. The ocean floor represents the surface of the oceanic crust. The ocean floor lies between the mid-oceanic ridges and the trenches, usually at depths of 5,000 to 7,000 meters (16,400 to 22,966 feet) below the ocean surface. Also called the abyssal plain, seabed, and seafloor.

Ocean Trench - A deep linear depression found at the edge of the ocean floor. Represents an area of tectonic plate subduction.

Oceania - One of eight biogeographic realms found on Earth, where many species have a unique evolutionary history because of geographic isolation. Geographically, this realm encompasses most of the Pacific Ocean's islands, except New Zealand. The climate of this realm is mainly tropical to subtropical.

Oceanic Crust - Basaltic portion of the Earth's crust that makes up the ocean basins. Approximately 5 to 10 kilometers (3.1 to 6.2 miles) thick. See the sima layer.

Oceanic Islands - Islands that are structurally part of oceanic crust. Compare with continental islands.

Oceanic Plate - A rigid, independent segment of the lithosphere composed mainly of basalt that floats on the viscous plastic asthenosphere and moves over the Earth's surface. The Earth's oceanic plates are an average of 75 kilometers (47 miles) thick and were formed less than several hundred million years ago at one of the Earth's mid-oceanic ridges. Also see continental plate.

Oceanicity - Refers to the degree to which the climate of a land location is influenced by its proximity to an ocean. 

Oceanography - The scientific study of phenomena found in the world's oceans. Oceanography is multidisciplinary, incorporating knowledge from Astronomy, Biology, Chemistry, Earth Science, Ecology, Environmental Science, Meteorology, Physical Geography, and Physics. Also known as oceanology or marine science. 

Offshore - (1) The direction from the shore out to sea. (2) Zone extending seaward from the shoreline to the edge of the continental shelf. (3) Zone extending seaward from the edge of the nearshore to a water depth where waves no longer cause sediment transport. Compare with nearshore.

Offshore Bar - Refers to a submerged or partially exposed coastal mound of sand that is parallel to the shoreline but some distance away from it.

Offshore Zone - Is the part of the marine environment that lies beyond the coastline and extends seaward to the outer border of the continental shelf. 

Ogive - A sequence of light and dark bands of ice that occur on the surface of some glaciers. These bands often form after an ice fall. Dark bands form when ice flowing over the ice fall partially melts because of warm summer temperatures. Light bands are produced when ice flows over the ice fall during the winter season and incorporates fresh snow. One sequence of light and a dark colored bands represents the distance the glacier ice moved in one year. 

Ohm - A unit of measurement for electrical resistance (symbol: Ω)

Oil - (1) A liquid composed of hydrocarbons commonly found in the pores of sedimentary rocks of marine origin. (2) A substance rich in carbon and hydrogen that is liquid at most temperatures, often flammable, and a source of heat energy, hydrophobic, and lipophilic. Most oils originally come from animal or plant tissues.

Oil Pool - This term describes the underground accumulation of petroleum (crude oil) found within a porous and permeable sedimentary rock formation. Also known as an oil reservoir or oil field.

Oil Sand -  A naturally occurring mixture of sand, clay, water, and bitumen - a heavy, viscous form of crude oil. Also known as tar sands or bituminous sands. Oil sand deposits are found in large reserves in Canada and Venezuela.

Oil Shale - A type of organic-rich sedimentary shale that contains significant quantities of kerogen. Heating kerogen can cause the release of crude oil (50-150°C/120-300°F) and natural gas (150-200°C/300-390°F). There are considerable deposits of oil shale found all over the world, and it is estimated that in total they contain 4.8 to 5.0 trillion barrels of crude oil. Also called kerogen shale.

Old Growth Forest - A type of forest dominated by late successional species of trees that are hundreds to thousands of years old. Examples include virgin uncut forests of Douglas Fir (Pseudotsuga menziesii), Western Hemlock (Tsuga heterophylla), Giant Sequoia (Sequoiadendron giganteum), and Coastal Redwoods (Sequoia sempervirens) found in western North America. Also see second-growth forest.

Oligocene - The Oligocene is dated from 33.9 ± 0.1 to 23.03 ± 0.05 million years ago. The rapid cooling of global temperatures at the beginning of this epoch led to the quick expansion of the Antarctic Ice Cap, the latitudinal retreat of broadleaf tropical forests to a zone around the equator, and the expansion of deciduous forests in the mid-latitudes. Drier conditions drive the expansion of grasslands and savannas. Running mammals like horses, entelodonts, rhinoceroses, oreodonts, and camels became well established in grasslands and savannas. One of three epochs that occurred during the Paleogene Period. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Oligotrophic - A water body with a relatively low supply of nutrients. Oligotrophic water bodies have little algae and few aquatic plants, and their water is generally clear. Compare with eutrophic and mesotrophic.

Oligotrophic Lake - A lake with a low supply of nutrients in its waters. These lakes have little algae and few aquatic plants, and their water is generally clear.  Also see eutrophic lake and mesotrophic lake.

Olivine - A common silicate mineral found in rocks formed from mafic magma. Olivine's chemical composition varies between Mg₂SiO₄ and Fe₂SiO₄.

Omnivore - A heterotrophic organism that consumes both plants and other animals as a source of food. Examples of omnivores 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 the rejection of the null hypothesis are located entirely on one side of the center of an associated probability distribution. 

Onion Weathering - See exfoliation.

Onshore-Offshore Transport - The repetitive up and down movement of sediment roughly perpendicular to a shoreline because of wave action.

Oolite - Small spherical-shaped sedimentary rocks that are between 0.25 and 2.0 millimeters in diameter and usually composed of calcium carbonate. They form when a small fragment of rock acts as a seed for the deposition of concentric layers of chemical precipitates. Oolite formation usually occurs in shallow marine or lake environments where the water is supersaturated with calcium. Some oolites are composed of silica, magnesium, iron, and phosphate precipitates. Also called egg stone.

Ooze -  A fine-textured marine sediment found on the ocean floor, consisting of clay, silt, and calcium carbonate. Marine oozes can vary in composition depending on factors such as water depth, temperature, and the presence of marine organism remains.

Open Sea - That part of the ocean that extends from the continental shelf. Compare with a coastal zone.

Open System - Is a system that transfers both matter and energy can cross its boundary to the surrounding environment. Most ecosystems are examples of open systems.

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 most significant positive effect on its physiological or reproductive fitness.

Opisometer - A mechanical device for measuring non-linear distances on maps. This device uses a small rotating wheel to measure distance traveled.

Optical Density - Is a measure of how much light is absorbed or transmitted by a substance. This term is used in various fields such as optics, spectroscopy, and biochemistry to quantify opacity or transparency.

Orbit - Refers to the circular or near-circular path an object takes around another object because of gravitational attraction. 

Order - (1) The fourth category in the classification of organisms. Classification level above the family category. It consists of families with similar morphological and physiological characteristics and related genetics. Similar orders are grouped into a class. (2) A term used in geomorphology to characterize the hierarchy of streams in a drainage network (see stream order).

Ordovician - One of six geologic periods that occurred from 485.4 to 443.8 million years ago during the Paleozoic Era. During this period, the first fish develop in the oceans and fungi species appear on land. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Ore - Is a rock that contains minerals that are of economic importance. For example, mineral metals like copper, silver, and gold.

Organ - A group of cells and tissues that have a particular function for an organism.

Organelle - Is a specialized structure found in individual cells that is designed to carry out distinct cellular functions.

Organic - (1) Relating to an organism. (2) Something derived from an organism.

Organic Acids - A class of organic compounds that contain one or more carboxyl (-COOH) functional groups. These substances are characterized by their ability to donate a hydrogen ion when dissolved in water, making them acidic in nature. Organic acids are found in various natural sources, including fruits, vegetables, and other organic materials.

Organic Matter - A mass of matter that contains living organisms or nonliving material derived from organisms. Sometimes refers to the organic constituents of soil. Also see soil organic matter. 

Organic Matter Decomposition -  The breakdown of organic matter into smaller parts or inorganic constituents by decomposing organisms or inorganic chemical processes. In terrestrial environments, much of the organic matter decomposition occurs on top or within the topmost layer of soil and sediment. In aquatic habitats, this type of decomposition occurs in the near-surface layer of sediment at the bottom of water bodies.

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. For more information on this soil type, see the textbook Canadian System of Soil Classification, 3rd Edition, available online - https://sis.agr.gc.ca/cansis/publications/manuals/1998-cssc-ed3/index.html. 

Organic Weathering - The breakdown of rocks and minerals into smaller-sized particles by organisms and processes associated with organisms.

Organism - Any form of life usually described at the individual or species level.

Oriented Lakes - A type of lake formation associated with glacial activity. Oriented lakes are often elongated and aligned in the direction of the moving glacier that formed them.

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 mountain range found on the continents.

Orogeny - The various processes associated with the deformation of the Earth's lithosphere because of the interaction of tectonic plates. Results in orogenesis and volcanism.

Orographic Lifting - See orographic uplift.

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 dry adiabatic expansion at a rate of approximately 9.8°C per 1,000 meters (5.4°F per 1,000 feet) until saturation. The large amounts of precipitation along the west coast of Canada and the United States are due mainly to this process.

Orographic Uplift - The forced uplift of an air mass because of the presence of a topographic obstruction. This uplift also cools the air mass. If sufficient cooling occurs, condensation can occur and cause orographic precipitation. Also called orographic lifting.

Orthographic Map Projection - A map projection system that presents the Earth's surface in two dimensions as if it were observed from a great distance in space. Distortion of areas and angles becomes greater as one moves from the center of the projection to its edges.

Oscillating Universe Theory - A theory that suggests the Universe undergoes an endless series of oscillations, each beginning with a Big Bang and ending with a Big Crunch.  After each Big Bang, the Universe expands for some time until the gravitational attraction of matter causes it to collapse into a Big Crunch. 

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. The outer core is believed to be liquid nickel and iron and has a density of about 10.0 to 12.2 g/cm³ -  grams per cubic centimeter (0.37 to 0.44 pounds per cubic inch). It surrounds the inner core and has an average thickness of about 2,250 kilometers (1,400 miles). 

Outer Space - The region beyond the outer limits of the Earth's atmosphere that extends to the edge of the Universe.

Outgassing - The release of gas from cooling magma or from the interior of the Earth. Much of the atmosphere's gaseous constituents, like water vapor, nitrogen, and argon, came from outgassing.

Outlet Glacier - A glacier that comes off an ice dome following the topography of the landscape.

Outlier - (1) In geology, it is an outcrop of rock that is dissimilar from the surrounding rock layers due to its composition, age, or geological history. (2) In statistics, an outlier is an observation that significantly differs from other values in a sample or distribution.  It is numerically distant from the central tendency, creating inconsistency with the overall data distribution.

Output - The movement of matter, energy, or information out of a system. Also see input.

Outwash - Glaciofluvial sediments deposited by meltwater streams found at the edge of a glacier.

Outwash Plain - A flat or gently sloping surface of glaciofluvial sediments deposited by meltwater streams at the edge of a glacier. Usually found in close spatial association with moraines.

Outwash Terrace - A terrace produced by the past deposition of glacial meltwater sediment. These terraces are often occupied by a (post-glacial) stream that has cut down into the terrace, making the outwash deposits visible. Outwash terrace sediments can range in type from clay to gravel and can be many meters (feet) thick. Often associated up valley of an outwash terrace are an outwash plain and moraines.

Over-Harvesting - Occurs when too many individuals of a particular plant or animal species are captured for the purpose of producing food or for making commercial products, and the population size of this species declines significantly in number. Over-harvesting occurs because the number of new individuals produced by reproduction is less than the number of individuals harvested over time. 

Overbank Deposit - Deposits of alluvial sediments found along the sides of a stream channel that are the result of flooding and overbank flow. Coarse sediments, such as gravel and sand, are found close to the channel edge. Sediments composed of silts and clays are found some distance from the channel. This pattern is seen because the velocity of overbank flow decreases with distance from the stream channel edge.

Overbank Flow - The movement of flood waters outside a stream channel during a period of high stream discharge.

Overcast - A condition where the sky is completely covered with clouds.

Overdeeping - An erosional process where a valley or basin is excavated or deepened by a glacier to a depth that exceeds the typical depth of a stream valley or basin in a non-glaciated area.

Overfit Stream - A type of misfit stream that is too large to be responsible for eroding the valley that surrounds it.

Overland Flow - The topographic movement of a thin film of water from precipitation to lower elevations. Over time, this water will begin to organize 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.

Overstep - Situation where there is a deposition of sediment or rock at an unconformity where the younger deposit is laid during a transgression on older strata.

Overthrust Fault - A fault produced by the fracturing of rock in a fold because of intense compression.

Overtopping - A coastal process where sediment carried by swash waves is mainly deposited on the top of barrier beaches. Compare with overwashing. 

Overturned Fold - A fold in rock layers where one limb is pushed up past the perpendicular. This results in both limbs having dips in the same direction. Compare with anticline, syncline, monocline, and recumbent fold.

Overwashing - A coastal process where sediment carried by swash waves is mainly deposited on the landward side of barrier beaches. This process occurs with more energetic wave action due to strong winds. Compare with overtopping.

Oxbow Lake - Is a portion of an abandoned stream channel filled with stagnant water and cut off from the rest of the stream. Oxbow lakes are produced 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 type 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 contain accumulations of mineral oxides and lack available base cations for exchange. 

Ozone - Triatomic oxygen (O₃) that exists in the Earth's atmosphere as a gas. Ozone is most concentrated in the stratosphere, at altitudes of 10 to 50 kilometers (6.2 to 31.1 miles) above 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 (CFCs) are primarily responsible for ozone destruction in the stratosphere. It is also abundant near Earth's surface (in the troposphere) in highly polluted urban centers. In these areas, it forms as a byproduct of photochemical smog and poses a hazard to human health.

Ozone Hole - Is a seasonal decrease in stratospheric ozone concentration that occurs over Antarctica and to a lesser extent over the Arctic during the spring season. First detected in the late 1970s, the Antarctic ozone hole continues to appear as a result of complex chemical reactions in the atmosphere that involve chlorofluorocarbons (CFCs). 

Ozone Layer - Atmospheric concentration of ozone found at an altitude of 10 to 50 kilometers (6.2 to 31.1 miles) above the Earth's surface. This layer is important to life on Earth because ozone absorbs harmful ultraviolet radiation, drastically reducing the amount of this radiation reaching Earth's surface. Also called ozonosphere.

Ozonosphere - Another name for the ozone layer found in Earth’s atmosphere.