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Earth
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The planet upon which we live
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El Chichon
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Active volcano in Mexico (17.360ø N, 93.228ø W), elevation 2225 m (7300 ft). El Chichon is known to have erupted five times, most recently in 1983.
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El Nino
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El Nino is the warm phase of an extensive cycle of atmospheric and oceanic conditions called the El Nino/Southern Oscillation (ENSO) cycle. El Nino is marked by a warming of the upper ocean off the coast of Peru every three to seven years.
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Euler pole
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Pole passing through the center of the earth about which a plate can rotate as a rigid body from any initial to any final position on the earth's surface.
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earthflow
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A detachment of soil and broken rock and its subsequent downslope movement at slow or moderate rates in a stream- or tongue like form.
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earthquake
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The violent oscillatory motion of the ground caused by the passage of seismic wave radiating froma fault along which sudden movement has taken place.
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earthquake swarm
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A series of minor earthquake, none of which may be identified as the main shock, occurring in a limited area and time.
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ebb tide
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The part of the tide cycle during which the water level is falling.
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eccentricity
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Measures how elliptical the earth-sun orbit is, with consequent effects on solar radiation received by the earth. If e is the eccentricity then (1 + e)/(1 - e) is the ratio of the farthest and closest earth-sun distance.
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echo-sounder
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An oceanographic instrument that emits sound pulses into the water and measures its depth by the time elapesd before they return.
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ecliptic
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The plane that contains the Earth's orbit around the Sun.
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eclogite
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An extremely high pressure metamorphic rock containing garnet and pyroxene.
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eclogite facies
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A metamorphic facies developed at high pressure (>10kbar) and high temperature (600°C or more), characterized by the metamorphic rock eclogite.
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ecology
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The science of the life cycles, populations, and interactions of various biological species as controlled by their physical environment, including also the effect of life form upon the environment.
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ecosystem function
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Ecological processes such as primary production, decomposition, and nutrient cycling that occur at the scale of a system and are affected by a variety of organisms and the environment's microclimate.
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ejecta
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Material that is thrown out by a volcano, including pyroclastic material (tephra) and, from some volcanoes. lava bomb.
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ejecta blanket
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Circular apron of debris surrounding an impact crater.
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elastic limit
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The maximum stress that can be applied to a body without resulting in permanent strain.
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elastic rebound theory
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A theory of fault movement and earthquake generation that holds that faults remain locked while strain energy accumulates in the country rock, and then suddenly slip and release this energy.
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elastic wave
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A wave that is propagated by some kind of elastic deformation, that is, a change in shape that disappears when the forces are removed. A seismic wave is a type of elastic wave.
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electron
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A negatively charged particle with negligible mass orbiting around the nucleus of an atom.
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electronegativity
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Relative measure (on a scale of 4) of an atom's attraction for the electron in its outer shell.
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elevation
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The vertical height of one point on the Earth above a given datum plane, usually sea level.
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elliptical orbit
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An orbit with the shape of a geometrical ellipse. All orbits are elliptical or hyperbolic, with the Sun occupying one focus.
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end moraine
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See terminal moraine.
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enrichment factor
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Ratio of chemical concentration of an element in a weathered material to that in its fresh parent material.
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eolian
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Term describing the process of wind erosion, transport, and deposition, and wind-created deposits and structure such as sand dune.
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eolian deposits
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Sedimentary deposits consisting of windblown dust.
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eon
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"The largest division of geologic time, embracing several Eras, for example, the Phanerozoic, 600 m.y. ago to present); also any span of one billion years. "
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ephemeral stream
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A stream drainage that is usually dry and fills with water only during brief episodes of rainfall. Many desert streams ephemeral.
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epicenter
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The point on the Earth's surface directly above the focus (or hypocenter) of an earthquake.
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epidote
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Family of silicate mineral containing mostly calcium, aluminum, iron and magnesium along with water. Epidote is apple green and generally form very small, stubby, prismatic crystal. It often occurs in vein or as a green coating on fracture surfaces. Most common in metamorphic rocks, but occasionally forms in igneous pluton that crystallize very deep in the crust.
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epoch
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One subdivision of a geologic period, often chosen to correspond to a stratigraphic series. Also used fo a division of time corresponding to a paleomagnetic interval.
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equigranular
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A rock texture in which all the mineral grain have similar size.
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equilibrium line
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The level on a glacier where accumulation equals ablation and the net balance equals zero.
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era
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A time period including several periods, but smaller than an eon. Commonly recognized eras are Precambrian, Paleozoic, Mesozoic, and Cenozoic.
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erosion
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The set of all processes by which soil and rock are loosened and moved downhill or downwind.
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eruption
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Occurs when solid, liquid, or gaseous volcanic materials are ejected into the Earth’s atmosphere or surface by volcanic activity. Eruptions may occur as quiet lava flows or violent explosive events.
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eruption column
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A mixture of hot volcanic ash, gases and entrained atmospheric air that rises vertically above an erupting volcano to heights of several tens of kilometers in the earth's atmosphere. The rise of the column is in part due to kinetic energy of the jet of gases and ash issuing from the volcano, but primarily due to the buoyant rise of the low-density mixture in the cooler atmosphere.
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eruption plume
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Dispersla of an eruption column downwind from the volcano, due to the effects of prevailing atmospheric circulation in the atratosphere and troposphere.
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escarpment
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A more or less continous cliff or relatively steep slope seperating two level or gently sloping surfaces produced by erosion or faulting.
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eskar (spell)
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A glacial deposit in the form of a continuous, winding ridge, formed from the deposits of a stream flowing beneath the ice.
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eugeosyncline
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" The seaward part of a geosyncline; characterized by clastic sediment and volcanism. "
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eustatic change
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Sea level changes that affect the whole Earth.
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eutrophication
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"A superabundance of algal life in a body of water; caused by an unusual influx of nitrate, phosphate, or other nutrients. "
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evaporation
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The change from a liquid into the form of a vapor or gas.
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evaporite
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A chemical sedimentary rock consisting of mineral precipitated by evaporating waters, especially sal and gypsum.
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evapotranspiration
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Water used by plants and animals and subsequently returned directly to the atmosphere.
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evergreen
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Coniferous, needle-leaved or sclerophyllous plant that does not echibit a marked seasonal leaf fall.
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exfoliation
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A physical weathering process in which sheets of rock are fractured and detached from an outcrop.
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exogenic cycle
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Portion of the earth's system involved in the movement and mass transfer of substances through the cycle of weathering, erosion, transportation, deposition, burial, diagenesis and uplift. Reservoirs in the cycle generally include the atmosphere, ocean biota, sediment and shallow crust.
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exogenic system
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Earth's interacting envelopes: the hydrosphere (mostly the ocean), atmosphere, biosphere, reactive lithosphere (that part of the rocky crust within reach of the processes of weathering). The processes of deep seawater circulation at spreading ridges overlap with endogenic (within the earth) processes. Also relevant are extraterrestrial processes. Refractory problems such as mass extinction usually become arenas for competing theories drawn from the three sources.
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exotic terrane
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See terrane.
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explosion earthquake
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Events with ground wave (including P-waves and S-waves) and often air waves, which travel through the air and are transmitted back into the ground in the vicinity of the seismometer.
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exposure
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A place where rocks can be seen in their natural position and not covered by debre, vegitation, or man-made objects.
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extension
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In geology, the process of stretching the Earth’s crust. Usually cracks (faults) form, and some blocks sink, forming sedimentary basin.
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extension
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In geology, the process of stretching the Earth’s crust. Usually cracks (faults) form, and some blocks sink, forming sedimentary basin.
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extinct volcano
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A volcano that is not presently erupting and is not likely to do so for a very long time in the future.
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extinction angle
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The angle between a crystallographic direction, such as a face or cleavage plane, and the direction in which all light is blocked by a pair of crossed polarizers.
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extract assemblage
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Assemblage of accumulative mineral extracted from a parent magma duting fractional crystallization to produce a daughter magma.
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extrusive
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Igneous rocks that cool and solidify rapidly at or very near the Earth’s surface. Also known as volcanic rock.
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