C Horizon - A soil horizon normally found below the B horizon and above the R horizon. This layer is composed of weathered bedrock that has not yet been significantly influenced by the pedogenic processes. 

Caatinga - A thorny woodland with many xerophytic species that is found in northeastern Brazil.

Caballing - This term refers to the mixing of two water masses with the same density but different salinity and temperature. The result of this mixture is a denser blend than its original parts, and it sinks.

Calcareous - Something containing calcium carbonate.

Calcareous Ooze - An ocean floor deposit consisting of at least 30% calcareous plankton shells. This unconsolidated, soft ocean floor deposit accumulates in areas where seawater is saturated with dissolved calcium. Covers about 48% of the ocean floor. Compare with diatom ooze.

Calcareous Soils - A soil that is rich in calcium carbonate.

Calcicole - A plant species that thrives best in a soil rich in the nutrient calcium.

Calcification - A dry environment soil-forming process that results in the accumulation of calcium carbonate in surface soil layers.

Calcifuge - A plant species that thrives best in soils with a low pH (acidic).

Calcite - Mineral formed from calcium carbonate. A common mineral found in limestone.

Calcium Carbonate - A compound consisting of calcium (Ca²⁺)  and carbonate (CO₃^2-). Calcium carbonate has the following chemical formula: CaCO₃.

Calcrete - A layer found in soils or sediments that is concentrated with calcium carbonate. This layer is often compact, quite dense, hard, and sometimes impervious. These layers are common in arid and semiarid climates, where the movement of soil water transports and deposits calcium carbonate in a specific zone of the soil profile, where soil particles become cemented together. Also called a hardpan.

Caldera - A large circular depression in a volcano.

Caldera Volcano - An explosive type of volcano that leaves a large circular depression. Some of these depressions can be as large as 40 kilometers (25 miles) in diameter. These volcanoes form when wet granitic magma rises rapidly to Earth's surface.

Calf - A relatively small piece of free-floating ice in a water body that has broken off a much larger piece of sea ice or glacier ice at lands edge.

Caliche - An accumulation of calcium carbonate at or near the soil surface.

California Current - A major, relatively cold boundary ocean current found along the western margin of North America in the Pacific Ocean.

Calms - Wind with a speed of less than one knot (1.85 kilometers per hour or 1.11 miles per hour).

Calorie (cal) - A measured quantity of energy. Equals the amount of heat required to raise 1 gram of pure water from 14.5 to 15.5°C at standard atmospheric pressure.

Calving - The loss of glacier mass when ice breaks off into a large water body like an ocean or a lake.

Cambrian - The first of six geologic periods that occurred from 542 to 488 million years ago during the Paleozoic Era. During this time, seas were relatively warm, polar ice was absent, invertebrates became common in the oceans, and the Burgess Shale fossils were laid down. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Cambrian Explosion - Great diversification of multicellular life forms in the Earth's oceans that started during the Cambrian about 542 million years ago.

Campbell-Stokes Recorder - An instrument used to measure the amount of bright sunshine during a day. Consists of a glass sphere that focuses the Sun's light so it burns a record onto a piece of specially designed indexed paper.

Canadian High - High pressure system that develops in winter over north-central North America.

Canadian Shield - Very old igneous and metamorphic shield rock that covers much of northern Canada. Created more than two to three billion years ago.

Canadian System of Soil Classification - A hierarchical system that is used in Canada to classify soils. This system has five levels: order, great group, subgroup, family, and series. At the order level, 10 soil orders are recognized: Brunisol, Chernozem, Cryosol, Gleysol, Luvisol, Organic, Podzol, Regosol, Vertisol, and Solonetzic. For more information on this soil classification system, see the textbook Canadian System of Soil Classification, 3rd Edition, available online - http://sis.agr.gc.ca/cansis/taxa/cssc3/index.html.

Canal - An artificial channel made to supply water for crop irrigation or to transport boats and ships.

Canary Current - A major, relatively cold boundary ocean current found along the western margin of North Africa in the Atlantic Ocean. See the map associated with the definition of global ocean circulation for location information.

Canopy - The uppermost layer of leaves, branches, and stems of trees in a forest ecosystem. This layer plays an important role in intercepting sunlight and precipitation, and in providing habitat for many different kinds of animal and plant species.

Canopy Drip - Redirection of a proportion of the rain or snow falling on a plant to the edge of its canopy.

Canyon - A steep-sided valley where depth is considerably greater than width. These features are normally the result of stream erosion.

Cape - A large headland protruding from the coastline into the ocean.

Capillary Action - The movement of water along microscopic channels found within a soil. This movement is the result of two forces: the adhesion and absorption of water to the walls of the channels, and the cohesion of water molecules to each other.

Capillary Water - Water that moves horizontally and vertically in soils by the process of capillary action. This water is available for plant use.

Caprock - A layer of more resistant rock that lies on top of a less resistant rock and protects this layer from erosion. Sandstone and ultramafic igneous rock are common caprocks.

Capture - The diversion of part of a stream drainage network into another drainage system. Several mechanisms can cause this, including tectonic activity, glaciation, karst processes, and headward erosion.

Carapace - (1) The surface exposure of a calcrete or other type of hardpan layer found in a soil. (2) The upper limb of a recumbent fold having an orientation that is very close to being horizontal.

Carbohydrate - Is an organic compound composed of carbon, oxygen, and hydrogen atoms. Examples include sugars, starch, and cellulose.

Carbon Cycle - Chemical model showing the storage and cyclic movement of organic and inorganic forms of carbon between the biosphere, lithosphere, hydrosphere, and atmosphere.

Carbon Dating - See radiocarbon dating.

Carbon Dioxide - A common gas found in the atmosphere. Carbon dioxide can selectively absorb radiation in the longwave band. This absorption causes the greenhouse effect. The concentration of this gas has been steadily increasing in the atmosphere over the last three centuries due to the burning of fossil fuels, deforestation, and land-use change. Most climate scientists believe that higher concentrations of carbon dioxide and other greenhouse gases will enhance the greenhouse effect and accelerate global warming. The chemical formula for carbon dioxide is CO₂.

Carbon Monoxide - A colorless, odorless, and tasteless gas that is produced by the incomplete burning of fossil fuels. The chemical formula for carbon monoxide is CO. 

Carbonate - A compound consisting of a single atom of carbon and three atoms of oxygen. Carbonate has the following chemical formula CO₃.

Carbonation - Is a form of chemical weathering where carbonate and bicarbonate ions react with minerals that contain calcium, magnesium, potassium, and sodium.

Carboniferous - One of six geologic periods that occurred from 359 to 299 million years ago during the Paleozoic Era. During this time, vast regions of forest were found on Earth's land surface, amphibians were the dominant land vertebrates, and arthropods were very common. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale. 

Carcinogenic - A substance or thing that has the potential to cause cancer in an organism.

Cardinal Direction - The four main navigational directions (North, East, South, and West) found on a compass or a map. For navigational purposes, these directional names are assigned the following values relative to a circle: North = 0 ° or 360°, East = 90°, South = 180°, and West = 270°. In between the cardinal points are four intercardinal directions: Northeast (45°), Southeast (135°), Southwest (225°), and Northwest (315°). 

Cardinal Temperature - The lowest and highest air temperatures that determine the range of temperatures at which the growth and development of an organism can occur, with a positive effect on fitness. Within this range, there is an optimum temperature that produces the highest growth rates. Cardinal temperatures can vary with the stage of organism development.

Carnivore - A heterotrophic organism that consumes living animals or the parts of living animals for food. Examples of carnivores include lions, cheetahs, leopards, frogs, snakes, hawks, and spiders. A carnivore is also called a secondary or tertiary consumer. Also see herbivore, detritivore, scavenger, and omnivore.

Carr -A past bog and/or marsh habitat that has been reclaimed by humans through drainage.

Carrying Capacity (K) - The maximum size of the population of a single species that a certain habitat can support.

Cartographer - An individual who studies and practices the field of knowledge known as cartography.

Cartography - (1) A field of knowledge that studies the construction of maps. (2) The act of creating a map.

Cascade - A series of small waterfalls one after another.

Cascading System - A system where we are primarily interested in the flow of energy and/or matter from one system element to another, and understand the processes that cause this movement. In a cascading system, we do not fully understand the quantitative relationships among elements involved in the transfer of energy and/or matter.

Case Hardening - Is the process of forming a hard and resistant crust on the surface of boulders or outcrops composed of soft, porous rock. This crust contains naturally occurring cementing agents such as iron oxide, manganese oxide, silica, and/or calcium carbonate.

Cataclinal - A geologic feature oriented in the same direction as the dip of the surrounding rock strata.

Cataclysm - A sudden, extreme natural event that has a profound effect on the environment.

Catalytic Converter - A device found in most vehicle exhaust systems that uses a chemical catalyst to convert harmful air pollutants from fossil fuel combustion into less harmful substances.

Catastrophism - General theory that suggests that certain phenomena on the Earth are the result of catastrophic events. For example, the Biblical Flood is responsible for sedimentary rock formations and the extinction of the dinosaurs.

Catena - Describes a sequence of soils that developed from the same parent material and general climate but have locations along a topographical (slope) gradient. As a result, relief is the dominant factor influencing pedogenic processes and profile development in these soils. A soil found on the slope would have better drainage, a lower humus content, and a relatively thin profile due to erosion of the surface soil. Soil at the slope bottom would have poor drainage, high humus content, be gleyed, and have a relatively thick profile because of sediment deposition.

Cation - A chemical ion carrying a positive atomic charge. Examples of cations include calcium (Ca₂⁺), magnesium (Mg₂⁺), sodium (Na⁺), and potassium (K⁺).

Cation Exchange - Chemical trading of cations between the soil minerals and organic matter with the soil solution and plant roots.

Cation Exchange Capacity - The capacity of a soil to exchange cations with the soil solution. Often used as a measure of soil fertility potential.

Causality - (1) The idea that all phenomena in the Universe operate under some type of cause-and-effect process. (2) The relationship between effects (events) and their causes (other events).

Cause - A process that results in the creation of an effect.

Cave - A natural cavity or recess that is roughly positioned horizontally to the surface of the Earth.

Cavitation - Process of intense erosion due to the surface collapse of air bubbles found in constricted, rapid flows of water. Cavitation causes material to detach from a surface.

Ceiling - The base altitude of a layer of clouds in the atmosphere.

Celestial Body - Is any natural aggregation of matter found in the Universe that is surrounded by a layer of space. Examples include asteroids, moons, planets, nebulae, and galaxies.  

Celestial Sphere - An abstract concentric sphere viewed upward from the round surface of the Earth that is embedded with the various celestial objects seen in the day or night sky. The temporal location of specific stars visible at night on the celestial sphere has been used for navigation by travellers for thousands of years.

Cell - A cell is the smallest self-functioning unit found in living organisms. Each cell is enclosed by an outer membrane or wall and contains genetic material (DNA) and other components that enable it to carry out its metabolic functions. Some organisms, such as bacteria, consist of only one cell, but most organisms on Earth are composed of many cells.

Cellular - (1) Composed of cells. (2) A process occurring between or within cells.

Cellular Respiration - Is the process where the mitochondria of cells of an organism release chemical energy from sugar and other organic molecules through chemical oxidation. This process occurs in both plants and animals. In most organisms, respiration releases the energy required for all metabolic processes. This chemical reaction can be described by the following simple equation: 

C₆H₁₂O₆ + 6O₂ ➜ 6CO₂ + 6H₂O + released energy

Cellulose - A type of carbohydrate. The primary organic substance used in the construction of plant cell walls.

Celsius Scale - Common scale used in the measurement of temperature. In this scale, water boils at 100° and freezes at 0°. This scale is used in most countries. One notable exception is the United States, where the Fahrenheit scale is used for weather forecasting and other non-scientific purposes. Compare with the Fahrenheit and Kelvin temperature scales.

Cement - (1) A natural substance that binds together loose mineral particles to produce a clastic sedimentary rock. Some common substances that do this include silica, iron, and calcium. (2) Human-made industrial binding material used in the construction of things. 

Cenozoic - One of three geologic eras that occurred during the Phanerozoic Eon from 65.5 million years ago to today. The Cenozoic is subdivided into three periods: Paleogene, Neogene, and Quaternary; and seven epochs: Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene. At the beginning of the Cenozoic, mammals, birds, reptiles, and amphibians were relatively small. However, with the demise of large dinosaurs at the end of the Cretaceous, evolutionary forces led to significant increases in body size across all these organism types. We also see a significant diversification of mammals as they filled niches once occupied by dinosaurs and other extinct species. In terms of climate, the Earth became cooler and drier during this era. Also called the Cænozoic and Cainozoic. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Central Eruption - A volcanic eruption that involves only a single volcanic vent or a group of vents close to each other.

Central Meridian - A particular line of longitude used as a starting point for map construction and the projection of spatial information.

Central Vent - The main passage in a volcano that allows magma to travel to the Earth's surface. 

Centripetal Force - Force required to keep an object moving in a circular pattern around a center of rotation. This force is directed towards the center of rotation. Common in meteorological phenomena like tornadoes and hurricanes.

Cerrado - A name given to describe a type of savanna ecosystem found in Brazil.

Chain - (1) A linear spatial pattern of natural phenomena that may be connected or associated with each other. (2) A unit of length measurement equal to 66 feet.

Chain Reaction (Nuclear) - A large number of nuclear fissions, taking place within a certain mass of a fissionable isotope. This process releases a great quantity of energy in a short time.

Chalk - A form of limestone. This sedimentary rock is composed of the shells and skeletons of marine microorganisms.

Channel Capacity - Refers to the cross-sectional area of some slice through a stream channel along its course. The height of this cross-section extends from the channel bottom up to the point where the stream's flow is just below the channel banks. This measurement is used to determine the stream's maximum flow volume.

Channel Cross-Sectional Area (A) - The area on a plane cutting perpendicular (90°) through a point on the length of a stream channel that is occupied with water flow. Mathematically, if the channel had a rectangular shape, its channel cross-sectional area (A) could be determined by multiplying channel depth (D) by channel width (W) [A = D x W]. Other formulas exist for determining channel cross-sectional area for particular channel shapes, including circular, triangular, and trapezoidal.

Channel Depth (D) - The height of the stream flow from the stream bed to the water surface. Usually measured as an average across the width of the stream channel. To get this average value, a number of measurements equal distance apart are made across the channel and then averaged.

Channel Geometry - Refers to various quantitative characteristics of a stream channel, including channel depth, channel width, channel cross-sectional area, wetted perimeter, hydraulic radius, bed roughness, and channel slope gradient.

Channel Resistance - Is the frictional roughness experienced by water flowing in a stream channel. This roughness is influenced by many factors, including stream bed sediment size, channel obstacles, and channel form.

Channel Slope Gradient - The change in the elevation of the water surface for two points along the length of the stream channel. This value is then divided by the distance between these points.

Channel Storage - The maximum volume of water that can be held in a particular length of stream channel.

Channel Width (W) - The distance across the stream channel from one bank to the opposite bank along the water surface.

Channelization - A human-caused modification of a stream channel with the aim of flood control, enhanced fish production, better land drainage, improved navigation, and erosion control.

Chaos Theory - Is the mathematical study of nonlinear dynamics, where random events and unpredictable phenomena can arise from the sensitivity of simple deterministic equations to initial value input. This area of knowledge has been applied to problems in meteorology and hydrology to explain phenomena. One of the pioneers of chaos theory was meteorologist Edward N. Lorenz. In the 1960s, Lorenz developed a computerized global circulation model for weather forecasting and discovered that small changes in model inputs could produce drastically different weather forecasts. This fact was called the butterfly effect because Lorenz suggested that, in theory, a hurricane's formation could be dependent on whether or not a butterfly in some other part of the world had flapped its wings several weeks in the past.

Chapada - Refers to a ridge or elevated plateau with savanna vegetation found in  Brazil and other areas of South America.

Chaparral - A type of plant community common to areas of the world that have a Mediterranean climate (for example, California and Italy). It is characterized by shrubs, shrubby thickets, and small trees adapted to seasonally dry conditions. Also called Mediterranean Scrubland.

Chattermarks (Chatter Marks) - (1) Are a series of crescent-shaped groves associated with glacier movement found on bedrock surfaces. These groves are oriented at right angles to the direction of ice flow. They form when rock debris at the base of the glacier repeatedly chisels into the bedrock. (2) Small crescent-shaped marks on beach pebbles. These marks form when pebbles bash into each other because of wave action.

Chelate - Organic substances that cause the chemical process of chelation.

Chelation - A chemical weathering process that involves the extraction of metallic cations (such as Fe, Al, and Mg) from rocks and minerals by chelates.

Cheluviation - The downward leaching or eluviation of soluble complexes or chelates - the combination of organic acids with metallic cations such as Fe, Al, and Mg.

Chemical - One of the millions of different elements and compounds found naturally and synthesized by humans.

Chemical Autotroph - An organism that uses the external energy found in chemical compounds to produce food molecules. The process by which these organisms produce food is known as chemosynthesis.

Chemical Energy - (1) Energy consumed or produced in chemical reactions. (2) Energy stored in chemical structures.

Chemical Reaction - Reaction between chemicals where there is a change in the chemical composition of the elements or compounds involved.

Chemical Weathering - The breakdown of rocks and minerals into small-sized particles through chemical decomposition.

Chemosphere - A layer in the atmosphere between 30 and 80 kilometers (19 and 50 miles) where photochemical processes are very active.

Chemosynthesis - Process where specific autotrophic organisms extract inorganic compounds from their environment and convert them into organic nutrient compounds without the use of sunlight. Also see photosynthesis.

Chenier Ridge - A long beach ridge associated with a stream delta that is made of sand and/or shells and surrounded by low-lying wetland deposits. These ridges can be 1 to 6 meters (3 to 18 feet) in height, several hundred meters (300 to 600 feet) wide, and up to 50 kilometers (30 miles) long. They often support woody vegetation. Chenier ridges are found off the Mississippi Delta.

Chernozem Soil - (1) Soil order (type) of the Canadian System of Soil Classification. These soils are rich in humus, black in color, and have a well-developed A horizon. This soil is common in the Canadian Prairies. 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. (2) Type of soil found in grassland environments. These common grassland soils are generally colored black and have a pronounced A horizon rich in humus. 

Chert - A microcrystalline silica-rich (quartz) sedimentary rock that may contain tiny fossils. This rock varies greatly in color: white to black, brown, greyish brown, light green, to rusty red. The color is associated with trace elements present in the rock.

Chevron Crevasse - A crevasse that has a V-shape because the moving ice experiences frictional resistance along the sides of the glacier with the alpine valley wall.

Chimney - (1) A tall rock pillar created by erosion. (2) A volcanic vent. (3) A vertical tunnel that connects to a below-ground cave.

Chinook Wind - The name of a North American regional wind system that occurs on the leeward side of mountains. This wind is warm, has low humidity, and tends to form mainly in winter. 

Chloride - A compound composed of a negatively charged chlorine anion and one positively charged cation element.

Chlorofluorocarbons (CFCs) - A human-created gas that has become concentrated in the Earth's atmosphere. This very strong greenhouse gas is released from aerosol sprays, refrigerants, and the production of foams. Chlorofluorocarbons play a role in the seasonal development of ozone holes at the Earth's polar regions. The basic chemical formula for chlorofluorocarbons is CF_XCl_X.

Chlorophyll - A natural green pigment found in plants and some bacteria used to capture the energy contained in light through photosynthesis.

Chloroplast - Organelle in a cell that contains chlorophyll and produces organic energy through photosynthesis.

Chott - A seasonal lake of North Africa that becomes flooded during the winter months because of precipitation associated with mid-latitude cyclones. These lakes are often saline.

Chromosome - An organic structure that carries an organism's genetic code (DNA).

Chronosequence - Is a sequence of soils of differing ages that have been influenced by climate, parent material, relief, and organisms (soil formation factors) and should be approximately the same in terms of pedogenesis. As a result, any differences in the soils are primarily due to the time available for soil formation.

Chute - A  constricted channel with relatively fast flow. This term is applied to both streams and a stretch of water associated with a lake or ocean located between the mainland and surrounding islands.

Cinder Cone Volcano - A small volcano, between 100 and 400 meters (330 to 1,310 feet) tall, made up of exploded rock blasted out of a central vent at a high velocity. These volcanoes develop from magma of basaltic to intermediate composition.

Circadian Rhythm - Any biological process that shows an oscillation or cycle that is roughly 24-hours long. Often, these rhythms play an important role in maintaining an organism's physiology and metabolic activity.

Circle of Illumination - A line that bisects areas on the Earth receiving sunlight and those areas in darkness. Cuts the spherical Earth into halves, day and night.

Circum-Pacific Belt - A zone circling the edge of the Pacific Ocean basin where subduction due to plate tectonics causes the formation of volcanoes and ocean trenches. Also called the Ring of Fire.

Cirque - A small glacially eroded rock basin found on mountains. Most alpine glaciers originate from a cirque.

Cirque Glacier - A small glacier that just occupies a cirque. Compare with an alpine glacier, a piedmont glacier,  and a continental glacier.

Cirque Lake - A permanent water body that occupies a cirque.

Cirrocumulus Cloud - Patchy white high altitude cloud composed of ice crystals. Found in an altitude range from 5,000 to 18,000 meters (16,400 to 59,050 feet).

Cirrostratus Cloud - High altitude sheet-like cloud composed of ice crystals. These thin clouds often cover an extensive area of the sky. Found in an altitude range from 5,000 to 18,000 meters (16,400 to 59,050 feet).

Cirrus Cloud - High altitude cloud composed of ice crystals. The appearance of these clouds is white, feather-like patches, filaments or thin bands. Found in an altitude range between 5,000 and 18,000 meters (16,400 and 59,050 feet).

Cladistics - A field of biology that analyzes the traits or adaptations seen within and between groups of organisms to construct hierarchical diagrams (cladograms) that illustrate evolutionary associations. From this analysis, organisms can then be classified into species.

Class - A group or category used in the taxonomic and/or phylogenetic classification of organisms. Found between a phylum and an order, and made up of more than one class.

Classification - The procedure of grouping things into categories based on some observable differences in properties.

Clast - (1) A single fragment of mineral or rock that is part of a sedimentary rock or a deposit of sediment. Often, these fragments are created by the weathering of some larger piece. (2) A fragment of rock ejected by a volcanic eruption.

Clastic - Sedimentary rock or sediment deposit composed of clasts that have been physically transported and deposited.

Clastic Sedimentary Rock - Sedimentary rocks that are formed by the lithification of weathered mineral or rock debris that has been physically transported and deposited.

Clathrate - A substance where an element or compound (guest) is found within the crystalline lattice of another element or compound (host).  See methane clathrate.

Clay - A mineral particle with a size less than 0.002 millimeters (0.00008 inches) in diameter (less than 0.004 millimeters or 0.00016 inches in the USA, Wentworth Scale less than 0.0039 millimeters). Compare with silt, sand, gravel, cobble, and boulder.

Clay Dunes - Dunes composed of about 30 percent clay-sized grains relative to silt and sand-sized particles. These features form from finer deposits, like dried lake and lagoon sediments. These dunes can be found in southern Australia, North Africa, the coastal plain of Argentina, the Kalahari Desert, and northern and western Texas.

Clay Loam - A type of soil defined by its relative proportions of clay, silt, and sand particles. A clay loam soil has between 27-40% clay and 20-45% sand. 

Clay Mineral - A variety of different chemical forms of hydrous aluminum phyllosilicate minerals that produce clay particles. Some common types of clay minerals include kaolinite, dickite, halloysite, nacrite, montmorillonite, nontronite, illite, bentonite, and sepiolite.

Clay-Humus Complex - Is the mixture of clay particles and humus commonly found in the A horizon of many soils. This mixture is an important source of nutrient cations that plants use for growth.

Claypan - Is a dense and compact clay-rich layer in a soil. These layers are often semi-permeable, reducing the downward movement of water in soil. Not considered a hardpan.

Clean Air Act (Canada) - Federal laws created to stop, reduce, and regulate air pollution at a national level in Canada. This Act was passed in 2006. Environment Canada is the main agency responsible for enforcing these laws.

Clean Air Act (United States) - Federal laws created to stop, reduce, and regulate air pollution at a national level in the United States. This Act was first passed in 1963 and expanded in 1967. Major amendments to the Clean Air Act occurred in 1970, 1977, and 1990. The Environmental Protection Agency (EPA) is the main agency that enforces these laws.

Cleavage - The tendency of some minerals or rocks to break along planes of weakness. This weakness arises from the nature of the bonds between mineral grains or sedimentary layers.

Cliff - A common phrase used to describe a tall, steep rock face.

CLIMAP Project - A multi-university research project that reconstructed the Earth's climate for the last million years by examining proxy data from ocean sediment cores. This research project took place during the 1970s and early 1980s.

Climate - General pattern of weather conditions for a region over an extended period (at least 30 years).

Climate Change - A measurable change in the long-term statistics of climate variables such as surface air temperature, precipitation, atmospheric pressure, wind speed, or wind direction that continues over several decades or longer.

Climate Classification - The grouping of the climates of locations and regions based on a quantitative analysis of long-term statistics of climate variables such as surface air temperature and precipitation. The most commonly used climate classification is the Köppen Climate Classification system.

Climate Modeling - The application of quantitative methods to simulate the Earth's weather and climate system. This numerical analysis involves dynamic interactions among numerous components in the atmosphere, on Earth's land surface, and within the ocean system. The primary input for this model is the Sun's emission of shortwave radiation, which can be influenced by several extraterrestrial factors. These models are used to make short-term weather forecasts and to determine Earth's climate in the past and future under various input scenarios.

Climate Normal - A term used in climatology and meteorology for a period of usually 30 years, from which averages, standard deviations, and extreme values for a climate variable are calculated. Sometimes referred to as a normal.

Climatic Geomorphology - The geomorphological study of weather and climate as agents in landscape processes and forms.

Climatic Optimum - Warmest period during the Holocene Epoch. This period spans about 7,000 to 3,000 BCE. During this time, average global temperatures were 1 to 3°C (2 to 5°F) warmer than they are today. Also called Altithermal,  Holocene Megathermal, Holocene Optimum, Holocene Thermal Maximum, and Hypsithermal.

Climatic Region - An area found on the Earth that has climate characteristics that sets it apart from adjacent areas.

Climatology - Scientific study of the Earth's climate over long time spans (greater than several days). It may also involve investigating the influence of climate on phenomena in the biotic and abiotic environments.

Climatotherapy - The treatment of a medical condition or disease by placing the afflicted individual into a beneficial climate. For example, doctors sometimes recommend that people suffering from asthma should not live in locations where the climate favors high atmospheric humidity.

Climax Community - A plant community that no longer undergoes changes in species composition due to the process of plant succession.

Climbing Dune - A stationary accumulation of well-sorted sand that forms up against the windward side of a large vertical obstruction in the landscape, like a hill, butte, mesa, or mountain.  For this dune to form, there must be a source of sand and consistent winds from mainly one direction. Climbing dunes can be steeper than sand dunes. Compare with a falling dune.

Climograph - A two-dimensional graph that plots a location's surface air temperature and precipitation on time scales that range from 24 hours to a year.

Cline - The gradual change in physiology and genetic characteristics (adaptations) that occur with a species over a geographical gradient because of adaptations to different environmental conditions.

Clinometer - An instrument that is used to measure the slope or angle of an object. One can combine this slope calculation with a distance measurement to determine an elevation change or an object's height. Also called an inclinometer.

Clinosequence - A contiguous series of related soils across space that differ in characteristics because of local differences in micro-climate caused by topography or relief. In these soils, the influence of other soil formation factors is assumed to be approximately the same.

Clone - (1) A group of genetically similar plants that have originated by vegetative asexual reproduction from a single parent. (2) The replication of an individual who is genetically identical to a parent.

Closed System - A system that transfers energy, but not matter, across its boundary to the surrounding environment. Our planet is often viewed as a closed system.

Closed Talik - A form of localized unfrozen ground (talik) in an area of permafrost. These formations are completely enclosed by permafrost in all directions.

Cloud - A collection of tiny particles of water and/or ice occurring above the Earth's surface. Clouds are classified according to their height of occurrence and shape. The major types of clouds include cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus, nimbostratus, stratocumulus, stratus, cumulus, and cumulonimbus.

Cloud Forest - A forest that is frequently immersed in clouds. These forests are often found in mountainous regions worldwide, and clouds are an important source of water for the trees. 

Cloud Seeding - An artificial technique of weather modification used to enhance the amount of precipitation falling from clouds. Cloud seeding is done by adding minute particles of silver iodide and potassium iodide to clouds. These particles act as condensation nuclei and deposition nuclei, increasing the number of water droplets and ice crystals in the cloud and improving the chance of precipitation. 

Cloud Streets - A formation where cumulus or stratocumulus clouds arrange themselves in numerous long rows. These rows are oriented along the direction of the dominant wind flow. 

Cloudburst - An occurrence of a large amount of rain and/or hail released from a cumulonimbus cloud over a short period, usually a few minutes. These events can cause a flash flood.  

Coal - Sedimentary rock composed of the compacted, lithified, and altered remains of plants. Coal is a solid, combustible mixture of organic compounds, hydrocarbons, with 30 to 98 percent carbon by weight, mixed with various amounts of water and small amounts of sulfur and nitrogen compounds. It forms in several stages as plant remains are subjected to heat and pressure over millions of years. 

Coalescence - Process where two or more falling raindrops join into a single larger drop because of a mid-air collision.

Coastal Cliff - A tall, steep rock face found along the coast.

Coastal Dune - A type of sand dune that forms in coastal areas. The sand for its formation is supplied from a beach.

Coastal Landform - A landform that is located at or near the coast and owes its formation to coastal geomorphic processes.

Coastal Wetland - A wetland habitat found along a coastline and is covered with ocean seawater for all or part of the year. Examples of this habitat type include tidal marshes, bays, lagoons, tidal flats, and mangrove swamps.

Coastal Zone - A relatively nutrient-rich, shallow part of the ocean that extends from the high tide mark on land to the edge of the continental shelf.

Coastline - Specific term used to describe the shoreline of the Earth's coasts. The line that separates a land surface from an ocean or sea. Note that this line varies in elevation over time because of changes in sea level. These changes in sea level can be caused by tides, atmospheric pressure, inflowing water, winds, and climate change.

Cobble - A term used to describe unconsolidated sediments composed of rock fragments. These rock fragments have a size between 63 millimeters (2.48 inches) and 200 millimeters (7.87 inches) (Wentworth Scale 64 to 256 millimeters). Compare with clay, silt, sand, gravel, and boulder.

Coefficient of Determination - A statistic that measures the proportion of the variation in the dependent variable that is associated with the statistical regression of an independent variable. Calculated by taking the square of the correlation coefficient. 

Coevolution - The coordinated evolution of two or more species that interact and exert selective pressures on each other, causing each species to develop associated adaptations. Also see evolution and natural selection. 

Cohesion -  (1) The intermolecular attraction that occurs between molecules of a similar substance. (2) The property of clay particles in a soil being attracted to each other because of electrostatic interaction.

Col - Saddle-like depression found between two mountain peaks. Form when two opposing cirque glaciers back erode an arête. 

Cold Desert - A type of desert found in the high latitudes or at high altitudes where precipitation is low, and surface air temperatures are generally cold. 

Cold Front - A transition zone in the atmosphere where an advancing cold air mass displaces a warm air mass. A shallow zone of cloud development and heavy precipitation usually occurs behind the front. Compare with the occluded front and warm front.  Normally associated with mid-latitude cyclones.

Cold Glacier - A glacier in which the ice found from its surface to base has a temperature as cold as -30°C (-22°F) throughout the year. This is well below the pressure melting point. Pressure melting can cause ice at the base of these glaciers to melt. One of the three types of glaciers: cold glacier, temperate glacier, and subpolar glacier. 

Cold Low - A cyclonic low pressure system that has a central pool of cold air that extends upward from the ground surface.  In these storm systems, wind speed generally increases with altitude. Most mid-latitude cyclones are cold lows. Also called an upper-level low, cold cyclone, cold-core low, or a cold-core cyclone. Compare with warm low.

Cold Wall - (1) A narrow zone of cold seawater that separates the Gulf Stream from the North American coastline. (2) Zone separating two masses of seawater with very different temperatures.

Cold Wave - Term used by the United States National Weather Service that describes a forecasted sudden drop in surface air temperature within a period of 24-hours. The exact definition of this weather phenomenon depends on the rate of temperature decline, the minimum temperature reached, the time of year, and the geographical region where it occurs. Forecasting a cold wave provides a warning to various human activities about the possible hazards and damage this event could cause.

Collision - The formation of rain often involves air turbulence and the collision of small water droplets, which then coalesce into larger droplets.

Colloid - Is a substance mixed throughout another substance at the microscopic scale.  

Colluvium - An unconsolidated deposit of soil, sediment, and rock debris that has amassed at the base of a cliff or slope because of gravity. Compare with alluvium.

Colonization - The movement of individuals or propagules of a species to a new geographical location.

Colony - A group of individuals, usually of the same species, living in close association with each other.

Columnar Jointing - A common structural pattern seen in igneous rocks of basaltic origin. Columnar jointing occurs because of the contraction and solidification of basaltic lava when cooled. An example of columnar jointing is the rock found at the Giant's Causeway, northeast coast of Northern Ireland. Also called columnar structure. 

Comet - A large mass of ice and dust found in outer space that has an orbit around a star. 

Comfort Zone - Meteorological term used to describe the weather or climate conditions that the majority of the human population finds pleasant.

Commensalism - Biological interaction between a pair of species where one species benefits in fitness while the other species experiences no effect on its fitness. 

Commercial Hunting - Is a type of hunting where humans kill animals to make a financial gain. This type of hunting is normally done legally. Compare with sport hunting, subsistence hunting, and poaching.

Comminution - The process of reducing the size of rock fragments naturally by erosion and weathering or artificially by pulverizing the material using mechanical techniques.

Community - Refers to all the populations of interacting species found in a specific area or region at a certain time. 

Community Boundary - The spatial edge of a unique biological community. 

Community Ecology - A subfield of ecology that studies the relationships and interactions between species in a community at various scales of time and space. Also called synecology.

Compaction - The loss of porosity in a deposit of sediment because of the overhead compressing effects of new deposits because of their weight. 

Compass - A navigation instrument that uses the Earth's magnetic field to find direction.   

Compensation Flow - A rate of stream flow that is established legally for a stream to manage this water for resource use or conservation purposes.       

Competence - This term refers to the maximum size of particle that flowing water can transport in a stream as bed load. The size of the transported particle increases as streamflow velocity increases. 

Competition - Biological interaction where two or more organisms (of the same or different species) in close proximity to each other require the same resource (e.g., nutrients, food, water, nesting space, and ground space), which is in limited supply.  The outcome of this interaction is that all of the organisms involved have their fitness reduced, as the amount of the limiting resource they receive is less than needed for optimal health and survival. This reduction in fitness can lead to the death of one or more of the competing individuals, freeing up resources to the remaining competitors, and increasing their fitness. Competition may also result from two processes: exploitation or interference. Competition can occur as an interspecific or intraspecific interaction.

Competitive Exclusion - A situation where no two competitively interacting species can occupy exactly the same fundamental niche indefinitely because of resource limitations. The outcome of this process is the local extinction of the species that is a poorer competitor. 

Competitive Species - According to J.P. Grime's theory of plant strategies, these plant species have the highest fitness in habitats with low stress and disturbance intensity. These plants can outcompete others by having adaptations that enable them to efficiently acquire the resources required for growth (e.g., light, soil nutrients, and soil water). Competitive plant species also have adaptations for rapid growth, high biological productivity, and the ability to respond to environmental change through significant morphological and physiological adjustments. Compare with ruderal and stress-tolerant species.

Compiled Map - A map, usually of a reduced scale, that is generated from data found on another map. A compiled map is not drawn from original real-world measurements.

Composite Coast - A coastline that has been created by alternating tectonic forces producing bays due to subsidence and headlands because of tectonic uplift.

Composite Map - A map that is produced to display compiled information from several other maps.

Composite Volcano - A type of volcano created from alternating layers of lava flows and exploded volcanic rock. These volcanoes range in height from 100 to 3,500 meters (330 to 11,480 feet). The chemistry of the magma that produces composite volcanoes varies from basalt to granite. Mount Fuji is considered a composite volcano. Also called a stratovolcano. 

Composites - The various plant species that belong to the Compositae family (also called Asteraceae). Common examples of these flowering plants include thistles, dandelions, and sunflowers. 

Compound - A compound is a mass of matter created by the atoms of different elements joined together. 

Compressibility - The ability of a substance to change its volume and density when subject to compressional forces.

Compressing Flow - Compressing flow occurs when a glacier experiences a reduction in the area over which it flows, compressing the glacier ice. For example, an alpine glacier responds to compressive flow by thickening and deepening in the valley where it flows. Compare with extending flow.

Compressional Wave -  See P-wave.

Computer Model - An abstract representation of some phenomena (model) in the form of data, a set of rules, logical decisions, and mathematical equations that are executed and calculated in a computer to create output.

Concavity - A structural property of a slope where the gradient of the slope surface becomes less steep as one moves to its bottom lower edge. Compare with convexity.

Concordant - A term used in Geomorphology and Geology to describe a situation where the surface morphology of a feature or landscape matches the underlying geological structure.

Concordant Coast - A situation where the shape of a coastline mirrors the geologic structure and local topography of the land adjacent to it. Compare with a discordant coast.

Concordant Intrusion - An igneous intrusion that forms parallel to the orientation of rock bedding adjacent to it.

Concrete Space - Actual geographic space of the Earth or some other celestial object. Geographers approximate concrete space when they try to represent it in a model or map. This estimation is referred to as abstract space. 

Concretion - A nodule of more resistant rock located inside a larger mass of less resistant rock.

Condensation - The change in the state of matter from vapor to liquid that occurs with cooling. This term is often used in meteorology when discussing the formation of liquid water from water vapor. This process releases latent heat energy to the environment. 

Condensation Nuclei - Microscopic particle of dust, smoke, or salt that allows for the condensation of water vapor into water droplets in the atmosphere. The nucleus for the formation of a raindrop. Condensation normally occurs on these particles when relative humidity becomes 100%. Some condensation nuclei, like salt, are hygroscopic, and water can condense on them at relative humidities lower than 100%. 

Conditional Instability - Situation in the atmosphere where the stability of a rising air parcel is determined by its altitude, the formation of water droplets and ice crystals, and the release of latent heat energy when water makes a phase change. In conditional instability, the air parcel undergoes adiabatic cooling with uplift at the dry adiabatic lapse rate and remains stable because the surrounding air is warmer. At some altitude above the ground, the dew point is reached, condensation and/or deposition occur, and the parcel of air continues its cooling now at the saturated adiabatic lapse rate. With further ascent, the release of latent heat will at some altitude cause the parcel of air to become warmer than the air around it. When this occurs, the air parcel becomes unstable.

Conduction - (1) The direct transfer of heat energy from atom to atom through a substance along a temperature gradient. (2) The transfer of electrical energy from atom-to-atom through a substance when a difference in electrical potential exists.

Conductivity - A method used to measure the ionic content of a solution. Most instruments used for this measurement determine electrical resistance between two electrodes placed at a fixed distance apart. As a result, the lower the resistance, the higher the conductivity. The conductivity of water is often strongly correlated with total dissolved solids.

Conduit - (1) A vertical channel in a volcano that allows for the upward flow of magma to a surface volcanic vent. (2) A below-ground channel filled with water that connects two larger subterranean cavities containing water.     

Cone - (1) A funnel-shaped mass of natural sediments in the landscape. This term is often used to describe conical-shaped features related to volcanic activity (ash cone, adventive cone, cinder cone volcano, and volcanic cone). (2) The fruiting body of coniferous vegetation.

Cone of Depression - Cone-shaped depression occurring horizontally across a water table. Caused by excessive removal of groundwater by a surface well. 

Conference of the Parties - An official meeting of all the parties (nation states) involved in the United Nations Framework Convention on Climate Change that is held annually. This event began in 1995 to ensure that the process of mitigating human-caused global climate change remained ongoing, as new information and recommendations from the Intergovernmental Panel on Climate Change Assessment Reports were considered by national governments.

Confined Aquifer - Aquifer between two layers of relatively impermeable earth materials, such as clay or shale. 

Confined Groundwater - Groundwater trapped between two impervious layers of rock. 

Confluence -  (1) A flow of air that moves toward the center and becomes less spread out with distance travelled. Compare with diffluence. (2) Describes two or more expanses of water coming together.

Congeliflucation - The downslope mass movement of soil, sediment, and rock by way of repeated freeze-thaw action.

Congelifraction - The weathering of minerals and rocks by way of repeated freeze-thaw action.

Congeliturbation - The heaving and mixing of soil and sediment at the ground surface due to repeated freeze-thaw action.

Conglomerate - Coarse-grained sedimentary rock composed of rounded rock fragments cemented in a mixture of clay and silt.

Conic Map Projection - Is a two-dimensional map projection system that creates its maps by projecting the Earth's surface onto a cone. This cone can be placed on the Earth so that its edge runs parallel to a particular line of latitude. Conic map projections are common in atlases and are often used for maps of large countries. See Albers Equal Area Map Projection.

Coniferous Forest - Cone-bearing vegetation of the middle and high latitudes that is mostly evergreen and has needle-shaped or scale-like leaves. Compare with a deciduous forest. 

Coniology - See koniology.

Conjunction - The perceived meeting or passing of two or more celestial bodies. Earth's moon is in conjunction with the Sun every 29.53 days, producing a new moon. Conjunction also occurs with the Sun, the Earth, and the various planets in our Solar System.

Connate Water - Seawater or freshwater trapped in the internal spaces found within sedimentary rock from the time the rock was originally formed. 

Connectivity - A term used in science to describe the number of connections between things in a networked system.

Consequent Stream - A stream whose course is a direct outcome of the slope of the land surface.

Conservation - Philosophical belief in the preservation, protection, and/or restoration of the Earth's natural abiotic environment, natural biomes and ecosystems, and wild species.

Conservation Biology - Multidisciplinary science that deals with the conservation of genes, species, communities, and ecosystems that make up Earth's biodiversity. This science often investigates human effects on biodiversity and tries to develop practical approaches to preserving biodiversity and ecological integrity. 

Conservation International -  Is a non-governmental organization (NGO) that focuses on protecting biodiversity on our planet. This nonprofit organization was founded in 1987 and has its headquarters in Arlington, Virginia, USA. Conservation International's website can be found at http://www.conservation.org.

Consistence - (1) A soil's resistance to deformation and fracturing. (2) The relative amount of particle cohesion in a soil.

Consociation - A plant community that is dominated by just one species of plant.

Consolidation - (1) Any process that converts loose soil or sediment into more compressed materials. (2) The reduction of the volume of a soil mass through compression.

Constructive Wave - Are shallow ocean waves with a long wavelength that wash up on the shoreline. These waves produce a strong swash that transports material up the beach, forming a berm. The wave energy dissipates over a wide area, which results in a weak backwash and a wide beach profile. Constructive waves have alow frequency, producing only about 6 to 8 waves per minute. Compare with a destructive wave.

Consumer - An organism that receives the nutrients (food) required for maintenance, growth, and reproduction from the consumption of tissues of producers and/or other consumers. Also called a heterotroph. Several different kinds of consumers have been recognized, including: carnivores, omnivores, scavengers, herbivores, detritivores, secondary consumers, and tertiary consumers. 

Contact - The physical interface between two different masses of rock that touch each other.

Contact Metamorphism - The small-scale thermal metamorphic alteration of rock because of localized heating. It is usually caused by an igneous intrusion, such as a sill or a dyke. 

Continental Antarctic Air Mass (cAA) - Air mass that forms over the Antarctic landmass in the Southern Hemisphere. These air mass systems form during the entire year. Continental Antarctic air masses are very cold, extremely dry, and very stable. On weather maps, the symbol cAA is used to identify a Continental Antarctic air mass.  Also see Continental Arctic air mass, Continental Polar air mass, Maritime Polar air mass, Maritime Tropical air mass, Continental Tropical air mass, and Maritime Equatorial air mass.

Continental Arctic Air Mass (cA) - Air mass that forms over extensive landmass areas in the high latitudes of the Northern Hemisphere. These air mass systems form only in winter over Greenland, northern Canada, northern Siberia, and the Arctic Basin. Continental Arctic air masses are very cold, extremely dry, and very stable. On weather maps, the symbol cA is used to identify a Continental Arctic air mass. Also see Continental Antarctic air mass, Continental Polar air mass, Maritime Polar air mass, Maritime Tropical air mass, Continental Tropical air mass, and Maritime Equatorial air mass.

Continental Climate - The typical climate of locations found in the interior of a continent. Seasonal variations in temperature are large due to the land's heating and cooling characteristics. In middle and high latitudes, summers tend to be warm to hot, while winters are cold to very cold compared to maritime locations at the same latitude. Continental climates generally receive less precipitation because of their distance from moisture sources, such as the oceans. See continental effect. Compare with a maritime climate.

Continental Crust - Granitic portion of the Earth's crust that makes up the continents. The thickness of the continental crust varies between 20 and 75 kilometres (12 and 47 miles). See sial layer. 

Continental Divide - The elevated area that occurs on a continent that divides continental-scale drainage basins. 

Continental Drift - A theory that suggests that the Earth's crust is composed of several continental plates that can move. First proposed by A. Snider in 1858 and further developed by F.B. Taylor (1908) and Alfred Wegener (1915). 

Continental Effect - The effect that continental surfaces have on the weather and climate of locations or regions. This effect results in a greater range in surface air temperature at both daily and annual scales. Compare with a maritime effect.

Continental Glacier - The largest type of glacier with a surface coverage of about 5 million square kilometers (2 million square miles). Compare with the piedmont glacier, alpine glacier, and cirque glacier.

Continental Ice Sheet - See continental glacier. 

Continental Islands - Islands that are found close to the coastline but have a geological origin similar to the adjacent continent. These islands were once part of the continent but became separated from the mainland by an ocean. Compare with oceanic islands.

Continental Margin - The area between a continent's coastline and the beginning of the ocean floor. It includes the continental shelf, continental rise, and continental slope. 

Continental Plate - A rigid, independent segment of the lithosphere mainly composed of granite that floats on the viscous plastic asthenosphere and moves over the surface of the Earth. The Earth's continental plates are an average 125 kilometers (78 miles) thick and were formed more than 3 billion years ago. Also see oceanic plate. 

Continental Polar Air Mass (cP) - Air mass that forms over extensive landmass areas of middle to high latitudes. In North America, these systems form over northern Canada. Continental Polar air masses are cold and very dry in winter and cool and dry in summer. These air masses are also atmospherically stable throughout the year. On weather maps, the symbol cP is used to identify a Continental Polar air mass. Also see Continental Arctic air mass, Continental Antarctic air mass, Maritime Polar air mass, Maritime Tropical air mass, Continental Tropical air mass, and Maritime Equatorial air mass.

Continental Rift Zone - The pulling apart of continental crust along a relatively linear stretch because of upwelling magma beneath the Earth's surface. This may cause volcanic activity and lava flows along this rift zone. Examples include the East African Rift, the Red Sea Rift, the Oslo Graben, the Rio Grande Rift, the Baikal Rift Zone, and the Gulf of Suez Rift.

Continental Rise - Thick layers of sediment found between the continental slope and the ocean floor. Also called a continental apron.

Continental Shelf - Shallow submerged margin of the continents that lies between the edge of the coastline and the continental slope. This nearly level area of the continental crust generally has surface layers made of sediment or sedimentary rock. 

Continental Shelf Break - A boundary zone between the continental shelf and continental slope. 

Continental Shield - See shield.

Continental Slope - Steeply sloping portion of continental crust found between the continental shelf and continental rise. 

Continental Tropical Air Mass (cT) - Air mass that forms over extensive landmasses in the low latitudes. In North America, these systems form over the southwestern United States and northern Mexico. Continental Tropical air masses are warm and dry in the winter and hot and dry in the summer. These air masses are also generally unstable in the winter but stable in the summer. On weather maps, the symbol cT is used to identify a Continental Tropical air mass. Also see Continental Arctic air mass, Continental Antarctic air mass, Continental Polar air mass, Maritime Polar air mass, Maritime Tropical air mass, and Maritime Equatorial air mass.

Continentality - The tendency of terrestrial surfaces on Earth to experience more temperature variation than ocean surfaces. The main factor responsible for this fact is that the soil, sediment, and rock that make up land have a lower specific heat than water.

Continuous Permafrost - A form of permafrost that exists across a landscape as an unbroken layer.

Contour Interval - The difference in elevation between two successive contour lines. The interval at which contours are drawn on a map depends on the amount of the relief depicted and the scale of the map.

Contour Line - Lines (isolines) on a topographic map that connect all points with the same elevation.

Contrails - Are long, thin artificial clouds composed of water droplets or ice crystals that can form behind aircraft when their engines cause water vapor to condense or freeze. Abbreviation for condensation trail. 

Contributing Area - The region within a drainage basin that provides water for runoff after precipitation falls from a storm.

Control System - A system that is intelligently controlled by the activities of humans. For example, a human-controlled dam on a stream. 

Convection - A process that involves the transfer of mass and heat energy using vertical motions through a fluid substance like air or water. Also see advection.

Convection Current - The movement of a gas or a fluid in chaotic vertical mass motions because of localized heating. 

Convectional Lifting - The vertical lifting of air parcels through convective heating of the atmosphere. This process can initiate adiabatic processes inside the air parcel. 

Convectional Precipitation - The formation of precipitation because of surface heating of the air at the ground surface. If sufficient heating occurs, the air becomes warmer and lighter than the surrounding air, initiating convection. Just like a hot air balloon, it begins to rise, expand, and cool. When sufficient cooling has taken place, saturation occurs, forming precipitation. This process is active in continental interiors and near the equator, forming cumulus clouds and possibly thunderstorms. Rain is usually the precipitation type formed, and in most cases, this moisture is delivered in large amounts over short periods in extremely localized areas. 

Convention on Biological Diversity - A legally binding international treaty dealing with regional and global biodiversity issues that was first available for signing by nations at the United Nations Earth Summit, in Rio de Janeiro, on June 5, 1992. This treaty has three primary goals: (1) to advance the conservation of biodiversity; (2) to promote the sustainable use of biodiversity; and (3) to ensure the just sharing of genetic resources between nations. The Convention on Biological Diversity website can be found at http://www.cbd.int.

Convergence - Horizontal inflow of wind into an area. Once in the area, the wind then travels upward. Associated with cyclonic airflow.

Convergence Lifting - The vertical lifting of parcels of air through the convergence of opposing air masses in the atmosphere. This process can initiate adiabatic processes inside the air parcel.

Convergence Precipitation - The formation of precipitation due to the convergence of two air masses. In most cases, the two air masses have different climatological characteristics. One is usually warm and moist, while the other is cold and dry. The leading edge of the latter air mass acts as an inclined wall or front, causing the moist warm air to be lifted. The lifting causes the warm, moist air mass to expand, leading to saturation. This precipitation type is common at the mid-latitudes, where cyclones form along the polar front. Also called frontal precipitation. 

Convergent Evolution  - The independent evolution of similar adaptations in organisms that are not closely genetically related because they live in similar environments. Convergent evolution occurs because natural selection picks similar adaptations that are optimal for survival in like environments.

Convexity - A structural property of a slope where its surface gradient becomes steeper as one moves to its top. Compare with concavity. 

Convolute Bed - A visually disturbed or crumpled sedimentary layer found between undisturbed strata deposits in a sedimentary rock bedding sequence. Such convoluted beds can be caused by slumping (see rotational slide) or by erosional disturbance after deposition.

Cooling Degree Day - For a single day, this climatological measurement is the number of degrees that the daily mean temperature exceeds a specified base temperature. For example, if the daily mean temperature was recorded as 30°C and the base temperature was  24°C, the cooling degree day measurement would be 6°C.  Sometimes these daily departures are determined for longer than one day. In this case, the departures are summed together to get a cumulative total of cooling degree days for that period. Compare with heating degree day.

Coordinated Universal Time (UTC) - Current official world time reference for civil and scientific purposes. Simply, it is the current time at the Prime Meridian. Coordinated Universal Time is measured from six standard atomic clocks at the International Bureau of Weights and Measures (BIPM) in Paris, France. Implemented in 1964. To find out the current Coordinated Universal Time, go to the following website: http://time.is/UTC

Coprolite - Fossilized excrement, feces, or droppings from an organism.

Coquina - A sedimentary rock rich in the mineral calcite composed mainly of fragments of marine shells that have been transported, worn down, and sorted according to size. The fragments making up this rock are normally 2 mm or greater in size. This rock can vary from being crumbly to well-cemented. 

Coral - Simple marine animals that live symbiotically with algae. In a symbiotic relationship, the algae provide the coral with nutrients, while the coral provides the algae with a structure in which to live. Coral animals secrete calcium carbonate to produce a hard external skeleton. 

Coral Bleaching - A situation where corals lose their colorful symbiotic algae. Believed to be caused by unusually warm water, changes in the salinity of ocean seawater, or excessive exposure to ultraviolet radiation. 

Coral Mud - A deposit of fine-textured coral fragments often found around coral reefs and parts of the continental slope. Compare with coral sand.

Coral Reef - A ridge of limestone found generally below the ocean surface. This marine feature is produced by numerous colonies of tiny coral animals, called polyps, that create calcium carbonate structures around themselves for protection. When corals die, their empty skeletons form layers that cause the reef to grow. Coral reefs are found in the coastal zones of warm tropical and subtropical oceans. 

Coral Sand - A deposit of coarse-textured, sand-sized, coral fragments often found around coral reefs. Compare with coral mud.

Cordillera - A term used to describe a group of mountains or a larger region of numerous mountains.

Core - The core is a layer rich in iron and nickel found in the interior of the Earth. The core is composed of two sub-layers: the inner core and outer core. The core is about 7,000 kilometers (4,350 miles) in diameter. The density of the core is estimated to vary from 10.0 to 13.0 g/cm³ -  grams per cubic centimeter (roughly 0.36 to 0.47 pounds per cubic inch). 

Core Sampling - Technique used to sample soil, peat, ice, sediments, and rock by using a drill and a hollow tube to extract a cylinder of material for investigation and scientific analysis. 

Corestone - A relatively unweathered boulder found inside weathered rock.

Coriolis Effect - An apparent force due to the Earth's rotation and the spherical shape of our planet's surface.  This apparent force deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis effect does not exist on the equator. This force is responsible for the direction of flow in meteorological phenomena such as mid-latitude cyclones, hurricanes, and anticyclones. The Coriolis effect also has a role in the formation of the Ekman spiral and Ekman transport in ocean bodies.

Cornice - A relatively large overhanging mass of snow and ice located on the ridge of a mountain or at the edge of a cliff. Cornice forms when prevailing winds blow snow over the mountain ridge or a cliff. 

Corona - A common optical phenomenon that produces a perimeter rings of bright, diffused light of various colors around the Moon and the Sun.

Corrasion - The erosion of rock surfaces by collision or grinding action of material being carried by wind, water, or ice.

Correlation - The degree of possible cause and effect influence that occurs between two phenomena and the particular attributes that can be measured from them.

Correlation Coefficient - A statistic that measures the degree of linear association between two variables. The values for this statistic vary from -1.0 to +1.0. A perfect positive linear association (the dependent variable increases with the independent variable) has a correlation coefficient of 1.0. A perfect negative linear association (the dependent variable decreases with an increase in the independent variable) has a correlation coefficient of -1.0. Absolutely no association between variables has a value of zero.

Corrosion - The combination of the effects of chemical weathering of rock surfaces and the active transport of the weathered material in solution by water. Very effective with limestone and dolomite rocks rich in carbonate.

Cosmic - A phenomenon that is external to the Earth and exists somewhere in space.

Cosmic Radiation - Refers to high-energy charged subatomic particles originating from outer space. These particles are mainly protons from hydrogen and helium nuclei and isolated electrons.

Cosmopolitan Species - Is a species that has a relatively broad ecological niche and a geographic range over a large portion of the Earth. Cosmopolitan species live in a wide range of habitat types, can live in various environmental conditions, and are adapted to consume many types of food. Compare with endemic and specialist species.

Cost-Benefit Ratio - An economic measure of the ratio of the costs of a venture or financial activity relative to its anticipated benefits, expressed in monetary terms. Used to determine whether a project is a sound investment decision.

Coulee (or Coulée) - (1) Steep-sided flow of volcanic lava that has solidified. (2) Abandoned glacial meltwater channel. (3) A term used in Canada and the United States to describe a steep-sided stream valley. 

Counter-Radiation - The redirection of the Earth's ground emitted longwave radiation back to the surface because of the greenhouse effect. 

Country Rock - Term used to describe the rock surrounding an igneous intrusion or a vein of rock that has a concentrated occurrence of particular minerals. 

Cove - A protective bay found along a coastline that has a relatively narrow entryway.

Coversand - A relatively thin layer of sand deposited by wind action across vast areas that may have been reworked by fluvial, glacial, periglacial, and other geomorphic processes. The source of the sand can be coastal dune or continental glacial outwash deposits.

Crater - Circular depression in the ground surface created by volcanic activity or an asteroid impact. 

Craton - Stable foundation core of the Earth's various plates of continental crust. Composed of the shield and platform. 

Creationism - The religious belief that the Universe, Earth, humans, and other lifeforms on our planet were brought into being by a supernatural entity or God. 

Creek - A narrow natural watercourse that connects to a much larger stream.

Creep - (1) A very slow mass movement of soil downslope (see image - Image Source: USGS). Creep occurs when the stresses on the slope material are too small to cause rapid failure of the slope materials. See soil creep. (2) Another term used to describe traction. 

Crest - (1) The topmost point on an anticline. (2) The highest point on an ocean or lake wave. (3) The highest point on a mountain ridge. (4) The highest point on a sand dune, or stream bed ripple, antidune, or wind ripple. (5) The maximum stream discharge attained for a flood as measured on a hydrograph. 

Cretaceous - Geologic period that occurred roughly 65.5 to 145.5 million years ago. During this period, the first flowering plant species appear, and dinosaurs are at their greatest diversity. Dinosaurs became extinct at the end of the Cretaceous. The Cretaceous was a time of a relatively warm global climate. Coniferous forest was the dominant plant community on the land surface. One of three geologic periods during the Mesozoic. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Cretaceous–Paleogene Extinction Event - Mass extinction event that occurred 65.5 ± 0.3 million years ago at the end of the Cretaceous. This event caused the complete loss of non-avian dinosaurs, many marine invertebrates, some mammals, and some plants. One theory suggests this mass extinction event was caused by the impact of a large asteroid with the Earth.

Crevasse - (1) Opening on a levee that allows for the drainage of water from the floodplain to the stream channel. (2) A large and deep fracture found on the brittle surface of a glacier.

Crevice - A narrow crack on the surface of something.

Critical Entrainment Velocity - Velocity required to entrain a particular-sized particle into the moving medium of air or water. Also called critical erosion velocity.

Critical Erosion Velocity - See critical entrainment velocity.

Critical Load - This is the threshold level of pollution beyond which any further increase in the presence of polluting substances will cause harmful effects on organisms in an ecosystem.

Critical Temperature - A threshold temperature value for some condition or process. For example, the growth of many bacterial species is curtailed at 0°C or lower.

Critical Velocity - (1) The threshold speed where a fluid's flow changes from smooth (laminar) to chaotic (turbulent). (2) The speed at which a falling object's gravity and air resistance are in balance.

Critically Endangered - One of the categories used by the International Union for Conservation of Nature's (IUCN) Red List to describe the status of a species. This category indicates that the species has been evaluated as having an extremely high risk of extinction in the wild.

Cropland - Land used to grow cultivated plants for human or animal consumption and to produce industrial products. Estimates suggest that about 5% of our planet's terrestrial surface is continually cultivated for crop production.

Cross-Bedding - These are inclined sub-layers found within some larger sedimentary stratum or bed. These sub-layers indicate past occurrences of depositional processes that created the much larger depositional layer. Cross-bedding deposits commonly form in stream beds, tidal flats, and aeolian environments.

Cross-Lamination - Are inclined sub-layers found within some larger sedimentary stratum or bed. These layers differ from cross-beds in that they are only millimeters (fractions of an inch) thick. Each cross-laminae represents a unique sedimentation event caused by minor fluctuations in velocity and/or rates of sediment supply in the depositional environment.

Cross-Profile -  See cross-section.

Cross-Section - An illustration that represents a slice or two-dimensional plane cutting through some three-dimensional object, usually at right angles to its long axis. Cross-sections are sometimes exaggerated along the vertical axis of the diagram to make details more apparent. Also called cross-profile.

Crude Oil - A natural liquid mainly found in subsurface geologic formations that contains various hydrocarbons and other liquid organic compounds. When refined, crude oil can be used to produce various fuels, lubricants, waxes, asphalt, plastics, and pharmaceuticals.

Crumb Structure - Refers to soils where particles accumulate to form larger aggregates. Soils rich in humus and clay form these aggregates. Soils with this property are much easier to work with for agriculture.

Crushing Strength - The amount of sustained force needed to cause solid material to fracture.

Crust - The Earth's outer-most layer of solid rock. The crust is between 7 to 70 kilometers (4.4 to 43.5 miles) thick. Two types of crust have been recognized: oceanic crust and continental crust. 

Cryergic - Some feature or process that is associated with periglacial environmental conditions.

Cryophyte - A plant that has adaptations for survival in extremely cold environments.

Cryoplanation - The lowering of relief and flattening of the landscape by way of processes that involve frost action.

Cryoplankton - Species of plankton that have adaptations to survive in extremely cold environments.

Cryosol Soil - Soil order (type) of the Canadian System of Soil Classification. This soil is common to high latitude tundra environments. The main identifying feature of this soil is a layer of permafrost within one meter (3 feet) of the soil surface. 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.

Cryostatic Pressure - The pressure applied on a substance or feature by ice while it is motionless. 

Cryosphere - Components of the Earth system where frozen water is a dominant feature. The cryosphere includes seasonal snow, snowfields, river ice, lake ice, sea ice, glaciers, ice caps, ice sheets, and frozen ground.

Cryotic - Something that is in a frozen state. 

Cryoturbation - The active mixing of soil, sediment, and rock by repeated freezing and thawing. A common feature of cryosol and gelisol soils.

Cryovegetation - Plants that are adapted to live in habitats where there is permanent snow and ice. Common plant types include algae, lichens, and mosses.

Cryptovolcano - A landscape feature on Earth or other planets that resembles a volcano but which may or may not be of volcanic origin.

Cryptozoic - Later part of the Precambrian fossil record, where there are only some indications that primitive lifeforms exist. Distinct from the Phanerozoic, when fossils of lifeforms are common.

Crystallography - The scientific study of mineral crystals.

Cuesta - A hill or ridge with a gradual slope on one side (back slope) and a sharp slope (front slope) on the opposite side (see image - Image Source: Wikipedia Commons, this image is in the public domain). Cusetas often form on rock outcrops composed of sedimentary rock strata that have gentle dip angles. In these geologic formations, erosion cuts into the strata creating the front slope. The back slope of the Cuseta normally runs parallel to the top of the stratum that is harder to erode.

Culmination - (1) In Geology, this term refers to the highest point in a fold. (2) A term used in astronomy to describe the highest point achieved by the transit of a celestial body, seen from Earth, across the horizon.

Cultural Services - One of four benefits humans receive from ecosystems as described in the United Nations' Millennium Ecosystem Assessment. This service suggests that humans can benefit from ecosystems through spiritual enhancement, recreation, and aesthetic experience. Compare with provisioning services, regulating services, and supporting services. 

Cultural Vegetation - Vegetation that has been influenced by the direct or indirect activities of humans.

Cumec - Abbreviation for cubic meters per second. A measurement unit used for stream discharge.

Cumulonimbus Cloud - A well-developed vertical cloud that often displays a top shaped like an anvil. Cumulonimbus clouds are very dense with condensed water droplets and deposited ice crystals. Common weather associated with this cloud includes strong winds, hail, lightning, tornadoes, thunder, and heavy rain. When this weather occurs, these clouds are then officially called thunderstorms. These clouds can extend from near the ground surface to altitudes of more than 12,000 meters (39,400 feet). 

Cumulose Deposits - Deposits rich in organic matter located at or near the ground surface.

Cumulus Cloud - Puffy clouds with relatively flat bases. Cumulus clouds form when moist warm air bubbles vertically escape from the Earth's surface. Found in an altitude range between 300 and 2,000 meters (984 and 6,560 feet).

Cupola - A large dome-shaped mass of igneous rock that extends from the top of a batholith.

Current Bedding - The accumulated cross-beds of sediment laid down by the flow of water or air.

Current - (1) An isolated moving mass of water within the flow of a stream. (2) Vertical movement of a relatively small isolated mass of air within the atmosphere. (3) A relatively small, isolated moving mass of near-surface seawater within the ocean. (4) Movement of near-surface seawater through a channel because of tidal forces.

Current Base - The maximum depth, as measured from the surface of the water body, at which near-surface currents are unable to cause the movement of bottom sediments.

Current Meter - An instrument used to measure the flow velocity of water in freshwater and marine environments. In streams, the measurement of flow velocity combined with volumetric measurements of the cross-sectional area of the stream channel can be used to determine stream discharge.

Current Ripple - A type of ripple produced by a consistent stream flow.

Cuspate Delta - A type of delta that normally forms when a stream empties its stream flow and carries sediment into a standing water body like a large lake or ocean, producing a cuspate foreland.

Cuspate Foreland - Is a triangular accumulation of sand and/or gravel located along the coastline. This feature is formed by the joining of two spits. 

Cut-Off - A section of stream channel (usually a meander loop) that has become disconnected from the rest of the stream because of lateral erosion. The disconnected channel section becomes an oxbow lake, which will, over time, fill in with sediment. Image Source: Wikimedia Commons, U.S. Department of Agriculture.

Cyanobacteria - A phylum of bacteria that can carry out photosynthesis as part of their metabolism. Scientists hypothesized that Cyanobacteria have played an important role in adding oxygen to the Earth's ancient atmosphere. This process is believed to have begun between 2.5 and 3.5 billion years ago. 

Cycle - A repeating sequence of events.

Cycle of Erosion - Concept of landscape development first proposed by William Morris Davis in 1899. This concept, originally called the geographical cycle, suggests how a block of uplifted land over time becomes reduced to a peneplain through the processes of weathering, erosion, and deposition.

Cyclogenesis - The process of cyclone formation, maturation, and death. Associated with tropical storms, hurricanes, and mid-latitude cyclones.

Cyclone - An area of low pressure in the atmosphere that displays circular inward movement of air near the ground surface. In the Northern Hemisphere, circulation is counterclockwise, while in the Southern Hemisphere, cyclones have clockwise circulation. This term can refer to mid-latitude cyclones, tropical storms, and hurricanes. Compare with an anticyclone.

Cyclostrophic - A type of wind flow where there is a balance between the atmospheric pressure gradient force and the centripetal force. Cyclostrophic winds occur in situations where air is flowing in a curved fashion around a cyclone or anticyclone.

Cyclothem - (1) A stratigraphic unit composed of a series of rock strata deposited during one sedimentary cycle. (2) A repeating sequence of marine and non-marine strata that suggests a cyclical depositional environment.

Cylindrical Map Projection - Is a map projection system that creates two-dimensional maps by projecting the Earth's surface onto a cylinder. In this map, lines of longitude are portrayed as equally spaced apart from the equator to the poles. On the Earth's surface, lines of longitude converge on each other as one goes from the equator to the poles. Lines of latitude retain the property of being parallel to each other, but are distorted with distance. The distance between consecutive lines of latitude becomes progressively greater as one moves from the equator to the poles. See Mercator Map Projection.

Cymatogeny - The large-scale horizontal warping of the Earth's crust on a scale of 10s to 100s kilometers (6+ to 60+ miles) with vertical movement in the 1000s of meters (3,000+ feet). Produces minimal rock folding and faulting. Caused by tectonic activity. 

Cytoplasm - All the protoplasm in an organism's cell except for what is contained in the nucleus.