Early Atmosphere - A stage in the development of the Earth's atmosphere that existed between 4.4 and 4.0 billion years ago. During this stage, the atmosphere was mainly composed of the gases water vapor (H₂O), carbon dioxide (CO₂), carbon monoxide (CO), ammonia (NH₃), nitrogen (N₂), methane (CH₄), and sulfur (SO₂ and S₂). Compare with the secondary atmosphere and the living atmosphere.

Earth Albedo - Is the reflectivity of the Earth's atmosphere and surface combined. Measurements indicate that the average Earth albedo is approximately 30%.

Earth Hummocks - A series of small, rounded mounds forming a distinct pattern on the ground surface. Thought to be of cryogenic origin, where the ground surface is subject to repeated freeze-thaw cycles. 

Earth Pillar - A column of loose, fine-textured sediment that is capped by a large rock or boulder. The caprock keeps the column of soft sediment from eroding away due to the impact of rainfall.

Earth Revolution - Refers to the orbit of the Earth around the Sun. This celestial motion takes 365.25 days to complete one cycle. Further, the Earth's orbit around the Sun is not circular, but elliptical.

Earth Rotation - A term that refers to the spinning of the Earth on its polar axis.

Earth Science - An all-encompassing term for the various fields of science that study the natural phenomena associated with our planet. Some of the science disciplines considered part of Earth Science include Biogeography, Climatology, Ecology, Environmental Science, Forestry, Geochemistry, Geology, Geophysics, Hydrology, Meteorology, Oceanography, and Physical Geography.

Earth Science Tradition - An academic tradition in modern Geography that investigates natural phenomena from a spatial perspective.

Earthflow - A rapid type of downslope mass movement that involves soil and other loose sediments. Usually triggered by water saturation from rainfall.

Earthquake - Is a sudden motion or trembling within and at the surface of the Earth. This motion is caused by the rapid release of slowly accumulated energy in the form of seismic waves. Most earthquakes are produced along faults, tectonic plate boundaries, or along the mid-oceanic ridges.

Earthquake Focus - The point of stress release in an earthquake.

Earthslide - The sudden downslope mass movement of consolidated sediment or soil along a failure plane. Earthslides are often triggered when water along the failure plane reduces friction. 

Easterly Wave - An atmospheric trough in the tropical trade winds. Occasionally, these systems intensify into tropical storms and hurricanes.

Eastern Boundary Current - A boundary ocean current found along the western margin of Earth's major continental masses. Subtropical eastern boundary currents flow from the mid-latitudes to the equator and transport relatively cold seawater. Examples of such currents include the Canary Current and the California Current. Polar eastern boundary currents flow from the mid-latitudes to one of the poles and transport relatively warm seawater. Compare with the western boundary current.

Easting - First measurement of a grid reference used to specify the location of a point on a rectangular coordinate system. The distance measured eastward from the origin of a rectangular coordinate system. Also see northing.

Ebb Current - The flow of seawater from a tidal current that is moving from shore back to the sea.  When its flow finishes, a low tide will occur. Compare with a flood current.

Ebb Tide - The time during the tidal period when the tide is falling. Compare with a flood tide.

Eccentricity - A term that describes the geometric shape of the Earth's orbit. This shape varies from being elliptical to almost circular.

Echo Dunes - A type of sand dune, long and linear in shape, that forms upwind and parallel to a vertical obstacle like a cliff.

Echo Sounder - An instrument that measures the depth of the water with sonar sound pulses. The time interval between the emission and reception of the returning pulse is used to determine depth.

Ecliptic Plane - A hypothetical two-dimensional surface on which the Earth's orbit around the Sun occurs.

Ecoclimate - The climate associated with a community of flora and fauna.

Ecological Diversity - See ecosystem diversity.

Ecological Explosion - A substantial increase in the population size of a species or group of organisms over a relatively short time. This increase occurs when factors moderating population growth relax or when factors that feed population growth become available.

Ecological Niche - Is all the physical, chemical, and biological conditions required by a species for survival, growth, and reproduction. This concept is fundamental to explaining the spatial patterns of species presence and abundance across ecological communities. Two further abstractions of this concept are the fundamental niche and the realized niche.

Ecological Succession - The gradual change in the species found in a community of flora and fauna driven by an abiotic and/or biotic cause. Compare with plant succession.

Ecology - The scientific study of the abiotic and biotic factors that influence the distribution and abundance of species. Ecology is highly interdisciplinary, drawing on information from Biology, Chemistry, Physics, and Earth Science.

Economic Basement - Zone of rocks that are too deep to mine for their minerals economically for profit. For the mineral resource oil, this depth is around 6 to 7 kilometers (3.7 to 4.3 miles).

Economic Resource - A commodity, service, or other resource that is used to produce goods and services that meet human needs and wants. 

Ecosphere - See the biosphere.

Ecosystem - An ecosystem is a system in which populations of various species group into communities and interact with each other and the abiotic environment.

Ecosystem Diversity - The variety of unique ecosystems found on the Earth. One of the three components of biodiversity. Also see genetic diversity and species diversity.

Ecosystem Services - This term refers to the four types of benefits humans receive from ecosystems as described in the United Nations' Millennium Ecosystem Assessment.  Also see Cultural, Provisioning, Regulating, and Supporting Services.

Ecotone - The boundary zone between two different community types.

Ecotope - (1) The abiotic environment of a community of organisms. (2) The smallest discrete ecological unit that exists spatially in a landscape mapping system.

Ecotoxicology - The often multidisciplinary scientific study of how toxic substances influence the health, survival, and fitness of living organisms. Such studies can be done at the individual, population, community, or ecosystem level. The general public first became aware of ecotoxicology with the publication of the famous book Silent Spring by Rachel Carson in 1962.

Ecotype - A genetically distinct population of organisms from a single species that occupies a unique habitat type. This population often shows differences in morphological and physiological adaptations compared to the rest of its kind. Despite being genetically different, this population can still interbreed with other populations and produce fertile offspring.

Ectothermic - An animal that relies on external heat sources to regulate its body temperature. Animals that have this strategy include reptiles, amphibians, most fish, and many invertebrates. Compare with endothermic.

Ectotherms - Are animals that produce little or no internal heat to maintain their body temperature. These animals use the environment to supply the body heat required for their survival. Animals that have this strategy include reptiles, amphibians, most fish, and many invertebrates. Compare with endotherms.

Edaphic - Something that is related to soil and/or its formation.

Eddy - A localized chaotic movement of a gas or liquid in a generally uniform larger flow of the same substance.

Eddy Diffusion - The turbulent mixing of substances into gases and liquids by eddies. Large amounts of water vapor can enter the atmosphere from Earth's surface through this process.

Edge Wave - A wave of water that moves parallel to the shore. This wave is usually a secondary wave of a much larger wave system.

Effect - The linked outcome produced by a cause.

Effective Porosity - A measure of the ability of the porosity of a sediment or rock to contribute to the flow of a liquid or gas. This measure is important for determining the suitability of sediment or rock for underground storage of water, oil, or natural gas.

Effective Precipitation - (1) The amount of precipitation that actually enters the soil layer and is stored for use by plants. (2) The amount of precipitation that becomes part of runoff and subsequently enters stream channels.

Effective Rainfall - The amount or proportion of precipitation that is available for use in infiltration, runoff, groundwater, etc.

Effluent - (1) A flow of sewage or some other liquid substance released into a stream, lake, or ocean. (2) A flow of lava from a volcanic fissure.

Effluent Stream - Are streams that receive their discharge mainly from groundwater, progressively gaining stream flow volume downstream. Effluent streams are often found in temperate and tropical climates, examples include the Amazon, Saint Lawrence, Mississippi, and Congo rivers. Also called a gaining stream. Compare with influent stream.

Effusive Eruption - A volcanic eruption characterized by the release of low-viscosity basaltic magma. This type of volcanic eruption is not explosive and tends to form shield volcanoes.

Ekman Spiral - Describes the directional displacement of wind-generated ocean currents with depth that creates a twisting motion. The twisting motion is caused by the Coriolis effect, which gradually deflects the moving seawater with depth. The Ekman spiral begins with wind blowing over the ocean surface producing a force on the topmost layer of seawater in the same direction. Also influencing the movement of this water is a force created by the Coriolis effect acting 90° to the right (90° to the left in the Southern Hemisphere). The action of these two forces produces the realized direction of the surface ocean current in between the two forces. Beneath the surface layer, seawater also moves due to molecular-level contact friction. Simply, layers of seawater molecules are moved by the frictional drag caused by the layer directly above it. However, the amount of force available to do this declines with depth, until the movement stops at about 100 meters (330 feet) below sea level. The other interesting thing that happens in this process is that each successive layer is deflected a bit more to the right with depth. This creates the spiralling motion. At a certain depth, the seawater flow will be in the opposite direction of the surface flow. Ekman spirals also operate in the atmosphere. Named after Vagn Walfrid Ekman (1874-1954) a Swedish scientist who first proposed it in 1902.

Ekman Transport - The resulting directional movement of seawater because of the turbulent frictional drag associated with wind blowing above the surface of this fluid and the deflective properties of the Coriolis effect.

El Niño - Name given to the occasional development of warm ocean surface waters along the coast of Ecuador and Peru. When this warming occurs, the tropical Pacific trade winds weaken, and the usual upwelling of cold, nutrient-rich deep ocean water off the coast of Ecuador and Peru is reduced. Strong El Niño events can cause an accumulation of warm surface seawater that can extend across almost the entire Pacific Ocean near the equator. El Niño normally occurs around Christmas and lasts for a few weeks to a few months. Sometimes an extremely strong event can develop that lasts for as long as two years. Compare with La Niña. Also see Southern Oscillation.

Elastic Deformation - The change in the shape of a substance as the result of the force of compression or expansion. Upon the release of the force, the material returns to its original shape. Also called plastic deformation.

Elastic Limit - The maximum level of elastic deformation a substance can withstand without fracture.

Elastic Rebound Theory - Theory that describes how earthquakes can develop from the horizontal movement of adjacent tectonic plates along a linear strike-slip fault. This theory suggests that the two plates moving in opposite directions become locked together for a time due to friction. However, the accumulating stress overcomes the friction and causes the plate to suddenly move over a short time, which generates an earthquake.

Elastic Wave - An energy wave that causes elastic deformation in a material without deforming its structure and shape.

Electric Charge - A fundamental property of matter that occurs with molecules, atoms, and subatomic particles. Electric charges are of two types: negative and positive. At the subatomic level, electrons have a negative charge, protons have a positive charge, and neutrons have no charge. Atoms that have the same number of protons as electrons orbiting the nuclei are electrically neutral. Electric attraction occurs when one object has an excess of positive charges and another object has an excess of negative charges. If two objects have an excess of the same charge, electric repulsion will take place. Electric charge has an important role in forming molecules. Electric charge also gives matter the ability to hold itself together.

Electrical Energy - Energy produced by moving electricity between two objects having the physical property of electric charge.

Electromagnetic Energy - Energy stored in electromagnetic waves or radiation. Energy is released when the waves are absorbed by a surface. Any object with a temperature above absolute zero (-273.15°C) emits this type of energy. The intensity of energy released is a function of the temperature of the radiating surface. The higher the temperature, the greater the quantity of energy released.

Electromagnetic Radiation (Waves) - Emission of energy as electromagnetic waves. All objects above absolute zero (-273.15°C) radiate energy into their surroundings. The amount of electromagnetic radiation emitted by a body is proportionally related to its temperature.

Electromagnetic Spectrum - See spectrum.

Electron - A type of subatomic particle of an atom that has a negative electromagnetic charge. Electrons orbit outside the dense positively charged nucleus. The mass of an electron is 1/1836 of a proton.  Compare with proton and neutron.

Electron Spin Resonance - A radiometric method used for dating samples up to 300,000 years old. Based on the principle that background radiation causes electrons to move from their usual locations in atoms to become embedded in the crystalline lattice of substances like calcium carbonate. Normally used with samples of teeth, limestone, coral, shells, and fragments of eggshell.

Element - A molecule composed of one type of atom. Chemists have identified or synthesized 112 different elements. Two or more different elements form a compound. See the following link for the chemical descriptions of these elements: http://www.webelements.com.

Elevation - The height or altitude of something above a base level. The base level commonly used to describe the elevation of something on the Earth is sea level.

Eluviation - Movement of humus, chemical substances, and mineral particles from the upper layers of a soil to lower layers by the downward movement of water through the soil profile. Compare with illuviation.

Eluvium - Material that is the byproduct of in situ rock weathering.

Embayment - An indentation in a coastline that forms a bay.

Emergent Coastline - An area along a coastline that has been influenced by a fall in sea level because of isostasy or eustasy. Landscape features associated with emergent coastlines include raised beaches, wave cut platforms, and sea caves. Compare with a submergent coastline.

Emigration - The migration of an organism out of an area as a result of changing its residence permanently. Compare with immigration.

Emission - (1) The process by which an object releases energy in the form of a photon into its surrounding environment. This process produces electromagnetic radiation. (2) The creation and release of something into the surrounding environment.

Emissivity - The ratio of the total output of electromagnetic radiation from a body per unit time per unit area at a specific temperature and wavelength to that of a black body under the same environmental conditions.

Empirical - A logical conclusion based on careful observation or experimental manipulation.

Endangered Species - A term used to describe the possible extinction risk of a species. (1) A species found in nature that has so few surviving individuals that it could soon become extinct in all or most of its natural geographic range. Also see threatened species. (2) 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 a very high risk of extinction in the wild.

Endemic Species - A species that is often found living in a single spatially restricted area and nowhere else on Earth. As a result, they have a very limited geographic range. However, these species may have broad ecological niches, and their limited distribution may result from their inability to disperse. Endemic species are commonly found on islands. These species are often a conservation concern because they are easily at risk of extinction. Many of the extinctions in the last 300 years have been of endemic species. Compare with specialist and cosmopolitan species.

Endemism - A species that is quite limited in its geographical range. For example, the Nēnē (Branta sandvicensis) is endemic because it is found only on the Hawaiian Islands.

Endogenic - A term that is used to describe a system that is internal to the Earth. Compare with exogenic.

Endothermic - An animal that maintains a stable internal body temperature through internal metabolic heat. Includes birds and mammals. Compare with ectothermic.

Endothermic Reaction - A process where a system absorbs heat energy from its surrounding environment. Compare with an exothermic reaction.

Endotherms - Are organisms that produce the heat needed to maintain their body temperature through metabolism.  Includes birds and mammals. Compare with ectotherms.

End Moraine - See terminal moraine.

Energy - Is defined as the capacity for doing work. Energy can exist in the following forms: radiation, kinetic energy, potential energy, chemical energy, atomic energy, electromagnetic radiation, electrical energy, and heat energy.

Energy Crop - A crop grown to produce chemical energy in the form of ethanol or methane.

Energy Flow - This term can be broadly defined as the movement and transformation of energy through the elements of some type of system.

Energy Flux - The rate of energy flow from, into, or through a substance.

Englacial - Refers to processes and conditions occurring within the body of a glacier. Same as supraglacial. Compare with proglacial and subglacial.  

Entisols - Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. A soil type of recent development with no or poorly developed soil horizons.

Entrainment - One of the three distinct processes involved in erosion. More specifically, it is the process of particle lifting by an agent of erosion. Also see detachment and transport.

Entrenched Meander - A stream meander that has eroded to a base-level well below its original valley floor. 

Entropy - (1) In physics, entropy is the measure of the disorder or randomness of energy and matter in a system. (2) Geologists use the term entropy to describe the characteristics of a sediment. Sediments that are very uniform and consistent are said to have high entropy. Unorganized and mixed sediments are claimed to have low entropy.

Environment - (1) Abiotic and biotic factors that influence the life of an organism. (2) Abiotic and biotic factors that influence the function of some nonliving natural system.

Environmental Assessment - The evaluation of the quality of abiotic and/or biotic components of the environment in a quantitative way. This is done mainly for socioeconomic reasons, where human development is being planned for these components in the future.

Environmental Gradient - A spatial gradient where abiotic and biotic factors vary.

Environmental Hazards - The types of natural and human-made risks that potentially influence the survival of an organism. Also see natural hazard.

Environmental Impact Assessment - The determination of the positive and negative repercussions that a human development project may have on some components of the environment.

Environmental Lapse Rate (ELR) - The rate of air temperature increase or decrease with altitude. The average ELR in the troposphere is an air temperature decrease of 6.5°C per 1,000 meters (3.6°F per 1,000 feet) rise in elevation. Also called a normal lapse rate.

Environmental Management - The techniques used by humans to manage the use of natural resources. The goals of this management are often to encourage sustainability.

Environmental Science - A field of knowledge that studies how humans and other species interact with one another and with the nonliving environment. It is both a physical and social science that integrates knowledge from a wide range of disciplines, including Physics, Chemistry, Biology, Ecology, Geology, Geography, Economics, Political Science, Sociology, Psychology, and Philosophy.

Environmental System - A system where life interacts with the various abiotic components found in the atmosphere, hydrosphere, and lithosphere.

Enzyme - An organic substance rich in proteins that is used to facilitate and regulate chemical reactions within cells. Many different types of enzymes are found in cells.

Eocene - An epoch that occurred 55.8 ± 0.2 to 33.9 ± 0.1 million years ago. The Eocene climate began with a warming trend, with global temperatures peaking around 49 million years ago. After this peak, global temperatures cooled, and snow and ice reappeared at the poles by the end of the epoch. Modern mammal lineages developed at the beginning of this time. Most of the planet is covered by forests during this time, except the driest desert regions. Oceans are warm and rich in fish and other forms of sea life. One of three epochs during the Paleogene Period. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Eolian - A landscape process that involves wind. Alternative spelling - aeolian.

Eolian Landform - Is a landform formed from the erosion or deposition of weathered surface materials by the wind. This includes landforms with some of the following geomorphic features: sand dunes, deflation hollows, and desert pavement. Alternative spelling - aeolian landform.

Eon - The second longest unit of geologic time. Generally, a time span of half a billion years or more. Four eons exist in Earth's geologic history: Hadean, Archean, Proterozoic, and Phanerozoic. See the International Commission on Stratigraphy (stratigraphy.org) for more information on the geologic time scale.

Epeiric Sea - Is a large area of saline water or ocean water that is located within the landmass of a continent. In Earth's geologic past, epeiric seas were more common than they are today, especially when sea levels were higher. Examples of epeiric seas that exist today include the Caspian Sea, Black Sea, and the Aral Sea. Also called an inland sea and an epicontinental sea.

Epeirogeny - The warping of the Earth's continental crust or oceanic crust at a large scale without any significant folding and faulting of rock layers. The warping can cause the crust to be uplifted or depressed. Compare with orogeny.

Ephemeral - Something that lasts for a relatively brief time.

Ephemeral Plant - A species of plant that has a short life cycle. Often refers to an annual plant.

Ephemeral Stream - (1) Is a stream that has a water flow only for a short period of time, usually after a precipitation event or because of spring snowmelt. Compare with a perennial stream and an intermittent stream. (2) This term can also refer to the outermost links in a drainage network. These segments, once again, usually have intermittent stream flow when water is supplied from precipitation or snowmelt. 

Epicenter - Surface location of an earthquake focus.

Epidiorite  - A metamorphic rock derived from the thermal alteration of igneous diorite or gabbro rock. Epidiorite has a similar mineral composition to diorite. 

Epigenetic - Something that influences the function of a cell, organ, or individual without directly involving its DNA.

Epilimnion - The topmost layer in a lake stratified according to water temperature. The surface of this layer interacts with the atmosphere. The water in this layer normally has a higher temperature, pH, and dissolved oxygen concentration than the hypolimnion layer below it. Also see thermocline layer.

Epipedon - A characteristically dark upper horizon in a soil that contains only the mineral particle byproducts of rock weathering, an enrichment of decaying organic matter, and/or signs of elluviation. This layer is not the same as an A horizon because it may include part or all of the B horizon.

Epiphyte - A type of plant that gets its physical support from the branches of other plants. Commonly found in the tropical rainforest communities.

Epithermal - Geologic term describing processes that occur at temperatures between 100 and 200°C in the lithosphere.

Epoch - A geologic time unit that is shorter than a period but longer than an age. See the International Commission on Stratigraphy (stratigraphy.org) for more information on the geologic time scale.

Equator - The area on the Earth's surface that has a latitude of 0°. The Great Circle of the Earth that corresponds with a latitude of 0°. 

Equatorial Bulge - A slight surface bulge that occurs around the equator related to the Earth's oblate spheroid shape. This roughly 20-kilometer (12.5 miles) outward extension is caused by the forces associated with axis rotation.

Equatorial Rainforest - A zone of tropical rainforest that runs approximately 5° either side of the equator. Uniform monthly rainfall and temperatures allow broadleaf evergreen species of tropical trees to dominate this biome.

Equatorial Trough - A meandering zone of surface low atmospheric pressure and ascending air that exists between the Northeast Trade Winds and Southeast Trade Winds. Many thunderstorms develop in this narrow area. This zone circles the Earth and adjusts its general position with the change in seasons. It is located mainly north of the equator during the June Solstice and south of the equator during the December Solstice. Rising air currents are driven by global wind convergence and thermal convection. The equatorial trough roughly corresponds to the thermal equator. Also called the Intertropical Convergence Zone.

Equifinality - An idea in systems theory that suggests that a particular end state can occur by way of different processes in an open system. Compare with multifinality.

Equilibrium - Equilibrium describes the average condition of a system, as measured through one of its elements or attributes, over a specific interval of time.

Equilibrium Line - A specific elevation on the glacier where ice accumulation is in balance with ablation.

Equilibrium Shoreline - A theoretical dynamic state that would occur if a balance were reached between the energy, materials, and geomorphic processes that produce a shoreline. This model is used to determine the state of an actual shoreline. Shorelines with sand beaches often resemble this ideal, while bedrock or marsh shorelines tend to be quite different.

Equinox - The two days during the year when the declination of the Sun is at the equator. The September Equinox occurs on September 22 or 23. The March Equinox occurs on March 20 or 21. On these days, all locations on our planet (except the poles) experience equal (12 hours) day and night.

Equipotential(s) - A line in two-dimensional space or a plane in three-dimensional space that has the same potential, where potential represents gravity, electric charge, groundwater flow, etc.

Era - A geologic time unit that is shorter than an eon but longer than a period. See the International Commission on Stratigraphy (stratigraphy.org) for more information on the geologic time scale.

Erg - A unit of energy and work in the centimeter-gram-second (CGS) system. 1 erg = 1.0 x 10⁻⁷ joules = 2.388459 x 10⁻⁸ calories. 

Erg Desert - A region in a desert where sand is in abundant supply.

Ergodic Hypothesis - A principle in Geomorphology that suggests that sampling in space can be equivalent to sampling through time under the right conditions. This idea is sometimes used to temporally model landform evolution by finding present-day surrogates.

Erodibility - A qualitative or quantitative measurement of how vulnerable soil, sediment, or rock is to erosion by wind, water, or ice.

Erosion - The removal of weathered sediment, soil, or rock by the forces of wind, water, and ice. 

Erosion Surface - A relatively flat landscape that was produced by erosion rather than sediment deposition.

Erosional Landform - A landform formed from the removal of weathered and eroded surface materials by wind, water, glaciers, and gravity. This includes landforms with some of the following geomorphic features: stream valleys, glacial valleys, and coastal cliffs.

Erosivity - A qualitative or quantitative measure of the erosion capability of a soil to a particular geomorphological factor.

Erratic - A large rock boulder that has been transported by glaciers away from its origin and deposited in a region of dissimilar rock.

Eruption - See volcanic eruption.

Escarpment - A long, almost vertical slope separating two relatively flat land areas of dissimilar elevation. These landforms often form because of faulting or erosion.

Esker - Long twisting ridges of sand and gravel found on the Earth's surface. These features were created when deposits from subsurface glacial streams were deposited on the ground after glacial melting.

Establishment - Subsequent growth and/or reproduction of a colonized species in a new permanent home.

Estimator - Is any value calculated from a data sample. For example, the sample mean is an estimator of the population mean.

Estuary - A somewhat enclosed coastal area at the mouth of a stream where nutrient-rich freshwater meets seawater.

Etchplain - An erosional landform surface located on an ancient shield formation found in warm and moist tropical climates. Chemical weathering of the shield produced sediment and soil that was removed by erosion, leaving behind scattered steep-sided hills composed of unweathered bedrock. 

Ethanol - A type of colorless alcohol that is produced by the fermentation of sugars and commonly used as a fuel. Chemical formula CH₃CH₂OH.

Eukaryota - All the organisms with an eukaryotic cell type. This group includes animals, plants, fungi, and protists.

Eukaryote - Organisms whose cells have a membrane-bound nucleus and many specialized structures located within their cell boundary. In these organisms, genetic material is organized into chromosomes that are found in the nucleus.

Eulittoral Zone - The area along a coastline that stretches from the high water mark to the point where aquatic plants fastened to the ocean floor no longer grow (a depth of about 40 to 60 meters or 120 to 180 feet).

Euphotic Zone - See photic zone.

Eustacy - Variations in sea level that are related to changes in the volume of seawater in the ocean.

Eutrophic - A water body that contains an overabundance of nutrients, usually nitrates and/or phosphates. This environmental condition typically leads to excessive growth of algae and aquatic plants. Compare with mesotrophic and oligotrophic.

Eutrophic Lake - A lake that has an excessive supply of nutrients, mostly in the form of nitrates and/or phosphates. This environmental condition usually causes the excessive growth of algae and aquatic plants in the lake. Also see mesotrophic lake and oligotrophic lake.

Eutrophication - Physical, chemical, and biological changes in a water body as a result of the input of nitrogen and phosphorus.

Evaporation - Is the process by which liquid water is converted into a gaseous state. Evaporation can only occur when water is available. It also requires that the atmosphere's humidity be lower than the evaporating surface's (at 100% relative humidity, there is no further evaporation). The evaporation process requires large amounts of energy. For example, the evaporation of 1 gram of water at 100°C requires 540 calories of heat energy (600 calories at 0°C).

Evaporation Fog - A type of fog produced from the advection of cold air over warm water or warm or moist land. This type of fog is sometimes called steam fog, sea smoke, frost smoke, or Arctic smoke.

Evaporation Pan - A meteorological instrument that is used to measure rates of evaporation from the ground surface.

Evaporimeter - See atmometer.

Evaporite - A type of sedimentary rock that is formed from the concentration of dissolved salts through evaporation.

Evapotranspiration - The combined loss of water to the atmosphere through the processes of evaporation and transpiration.

Everglades - An extensive area of flat, often inundated marshland and mangroves found in southern Florida. Seasonal flooding of the Everglades usually occurs with the cyclic migration of thunderstorms associated with the intertropical convergence zone during the summer.

Evergreen Vegetation - Vegetation that keeps most of its leaves or needles throughout the year. Also see deciduous vegetation and succulent vegetation.

Evolution - The process by which species come to possess genetic adaptations to their environment. The mechanism of evolution is natural selection. Evolution also requires genetic mutations to create new adaptations. This process was first properly conceptualized by Charles Darwin in 1859.

Evorsion - Describes the erosion of sediment and/or rock from a stream bed by eddies in the flow of water.

Ex Situ Management - A strategy of conservation that houses and breeds endangered and threatened species in human-controlled zoos, aquariums, and other facilities with the ultimate goal of releasing the organism back to the wild in the future.

Exaration - The erosive plucking of fragments of rock material from bedrock at the base of a glacier.

Exfoliation - A type of physical weathering where sheets of rock material peel off the surface of a much larger mass of rock (Image Source: Wikimedia Commons). Exfoliation is generally caused by pressure release. Also called onion weathering.

Exfoliation Dome - A physical weathering feature associated with granite that is the result of the erosion of overburden material and pressure release. With the release of pressure, layers of rock break off in sheets or shells, leaving a dome-like bedrock feature.

Exhumation - The process of erosion revealing a geologic feature originally found beneath the ground surface by removing overlying sediment and/or rock.

Exogenic - Refers to a system that is external to the Earth. Compare with endogenic.

Exoplanet - A planet found outside our Solar System orbiting another star. Thousands of exoplanets have been discovered.

Exosphere - The outermost zone in the Earth's atmosphere. This layer has an altitude greater than 480 kilometers (298 miles) and is primarily composed of hydrogen and helium gas.

Exothermic Reaction - A process where a system releases heat energy to its surrounding environment. Compare with an endothermic reaction.

Exotic Species - See invasive species. See an alien species and an introduced species.

Exotic Stream - A stream that has a course that begins in a humid climate and ends in a dry climate. Because of reduced precipitation and increased evaporation, the discharge of these streams decreases downslope. Examples of exotic streams include the Nile and Colorado Rivers. Compare with an ephemeral stream.

Expansive Soil - A soil that can expand and contract with changes in moisture content. Soil with this ability generally contains significant amounts of the clay minerals bentonite and montmorillonite.

Experiment - A controlled investigation designed to evaluate the outcomes of causal manipulations on some system of interest.

Exploitation - A type of competitive biological interaction where the indirect effects of two or more species or individuals reduce the supply of the limiting resource or resources needed for survival.

Explosive Eruption - A volcanic eruption where high-viscosity granite-rich magma causes an explosion of ash and pyroclastic material. This type of eruption is common to composite and caldera volcanoes.

Exposure - (1) The position of a surface relative to climatic elements of the Sun, precipitation, wind, etc. (2) The location selected for measuring climatic variables with instruments. (3) A location where bare bedrock is exposed at the ground surface.

Extending Flow - This occurs when a glacier increases the area over which it flows, causing the glacier ice to stretch out. For example, an alpine glacier responds to extending flow by thinning and decreasing its depth in the valley where it flows. Compare with compressing flow.

Extinct - 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 suggests the species is no longer found in the wild or in captivity.

Extinct in the Wild - 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 suggests the species is no longer found in the wild, but individuals do exist in captivity, cultivation, or as an established population or populations outside of its usual geographic range.

Extinct Volcano - A volcano that was once active but is now permanently inactive.

Extinction (adjective Extinct) - The disappearance of a species from all or part of its geographic range. Also see background extinction and mass extinction.

Extrapolation - The process of extending the linear or non-linear trend seen in a set of bivariate or multivariate data beyond its lower and/or upper distribution limits. Extrapolated data should be used with caution because it may lead to incorrect assumptions about patterns that occur outside the modeled data.

Extratropical Cyclone - Synoptic scale cyclonic storms with low atmospheric pressure that develop in the middle to high latitudes of our planet. These systems usually have fronts associated with them, and their formation is related to the polar jet stream. Also see the mid-latitude cyclone.

Extrusive Igneous Rock - Igneous rock that forms on the surface of the Earth. Also called volcanic igneous rock.

Eye - In meteorology, this term refers to the circular area at the center of a well-developed hurricane, often devoid of clouds and characterized by light winds. Hurricane eyes are about 30 to 65 kilometers (20 to 40 miles) in diameter and are bordered by a ring of thunderstorms that collectively make a feature known as the eyewall.  

Eyewall - A circular wall of thunderstorms found at the edge of a hurricane eye.

Eyot - A small island in a stream or lake.