E and F Index Glossary geology Science
E and F Index Glossary geology Science
Ebb Tide
A tidal current that generally moves seaward and occurs during the part of the tide cycle when sea level is falling. (see also flood tide)
Effluent Stream
A stream that gains water from ground water flow. These streams are typical of humid climates where water tables are high. The discharge of an effluent stream can be sustained by ground water flow for long periods of time between runoff-producing rainfall or snowmelt. Effluent streams generally increase in discharge downstream and contain water throughout the year. The opposite is an influent stream.
Elastic Limit
The maximum stress that can be applied to a body without resulting in permanent deformation - the rock reverts to its original shape after the stress is removed. In the case of a fault or a fold the elastic limit is exceeded and the deformation becomes a permanent structure of the rock.
Elastic Rebound Theory
A theory that explains the earthquake process. In this theory, slowly accumulating elastic strain builds within a rock mass over an extended length of time. This strain is suddenly released through fault movement, producing an earthquake.
Elevation
The vertical distance between mean sea level and a point or object on, above or below Earth's surface.
Eolian
A term used in reference to the wind. Eolian materials or structures are deposited by or created by the wind.
Eon
The major divisions of the geologic time scale. Eons are divided into intervals know as "eras". Two eons of the geologic time scale are the Phanerozoic (570 million years ago to present) and the Cryptozoic (4,600 million years ago until 570 million years ago).
Epoch
A subdivision of geologic time that is longer than an age but shorter than a period. The Tertiary Period is divided into five epochs. From most recent to oldest they are Pliocene, Miocene, Oligocene, Eocene and Paleocene.
Era
A subdivision of geologic time that is longer than a period but shorter than an eon. Precambrian, Paleozoic, Mesozoic, and Cenozoic are the eras of the time scale from oldest to youngest.
Eustatic Sea Level Change
A rise or fall in sea level that affects the entire earth. Thought to be caused by an increase/decrease in the amount of available water or a change in the capacity of ocean basins.
effluent
water that flows from a sewage treatment plant after it has been treated.
erosion
The process by which particles of rock and soil are loosened, as by weathering, and then transported elsewhere, as by wind, water, ice, or gravity.
Evaporation
The process of liquid water becoming water vapor. Includes vaporization from water surfaces, land surfaces and snow/ice surfaces.
Evaporite
A chemical sediment or sedimentary rock that has formed by precipitation from evaporating waters. Gypsum, salt, nitrates and borates are examples of evaporite minerals.
Evapotranspiration
the sum of evaporation and transpiration .All methods of water moving from a liquid to water vapor in nature. Includes both evaporation and transpiration.
Exfoliation
A physical weathering process in which concentric layers of rock are removed from an outcrop.
Expansive Clay
(Expansive Soil) A clay soil that expands when water is added and contracts when it dries out. This volume change when in contact with buildings, roadways, or underground utilities can cause severe damage.
Economies of Agglomeration
The economic advantages that accrue to an activity by locating close to other activities; benefits that follow from complementarity or shared public services.
Emergent Coastline
A shoreline resulting from a rise in land surface elevation relative to sea level.
Enclave
A tract or territory enclosed within another state or country.
Equator
An imaginary circle around the Earth halfway between the North Pole and the South Pole; the largest circumference of the Earth.
Echo-sounder
An oceanographic instrument that emits sound pulses into the water and measures its depth by the time elapsed before they return.
Ecliptic
The plane that contains the Earth's orbit around the Sun.
Eclogite
An extremely high-pressure metamorphic rock containing garnet and pyroxene.
Ecology
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 forms upon the environment.
Elliptical orbit
An orbit with the shape of a geometrical ellipse. All orbits are elliptical or hyperbolic, with the Sun occupying one focus.
Eugeosyncline
The seaward part of a geosyncline; characterized by clastic sediments and volcanism.
Eustatic change
Sea level changes that affect the whole Earth.
Eutrophication
A superabundance of algal life in a body of water; caused by an unusual influx of nitrate, phosphate, or other nutrients.
Exobiology
The study of life outside the Earth.
Extinction angle
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.
Erratic
A boulder that has been carried from its source by a glacier and deposited as the glacier melted. Thus, the boulder is often of a different rock type from surrounding types.
Escarpment
A long cliff or steep slope separating two comparatively level or more gently sloping surfaces and resulting from erosion or faulting.
earthflow
1. The flow of a dry, highly viscous mass of clay-like or silty regolith, typically moving at a rate of one or two meters per hour. 2. The material that descends in such a flow.
earthquake
A movement within the Earth's crust or mantle, caused by the sudden rupture or repositioning of underground rocks as they release stress.
echo-sounding sonar
The mapping of ocean topography based on the time required for sound waves to reach the sea floor and return to the research ship that emits them.
elastic deformation
A temporary stress-induced change in the shape or volume of a rock, after which the rock returns to its original shape and volume.
electron
A negatively charged particle that orbits rapidly around the nucleus of an atom. See also proton.
element
A form of matter that cannot be broken down into a chemically simpler form by heating, cooling, or chemical reactions. There are 106 known elements, 92 of them natural and 14 synthetic. Elements are represented by one- or two-letter abbreviations. See also atom, atomic number.
energy level
The path of a given electron's orbit around a nucleus, marked by a constant distance from the nucleus.
epicenter
The point on the Earth's surface that is located directly above the focus of an earthquake.
equilibrium line
The point in a glacier where overall gain in volume equals overall loss, so that the net volume remains stable. The equilibrium line marks the border between the zone of accumulation and the zone of ablation.
esker
A ridge of sediment that forms under a glacier's zone of ablation, made up of sand and gravel deposited by meltwater. An esker may be less than 100 meters or more than 500 kilometers long, and may be anywhere from 3 to over 300 meters high.
extrusive rock
An igneous rock formed from lava that has flowed out onto the Earth's surface, characterized by rapid solidification and grains that are so small as to be barely visible to the naked eye.
estuary
a place where fresh and salt water mix, such as a bay, salt marsh, or where a river enters an ocean.
- The broad lower course of a river that is encroached on by the sea and affected by the tides.
Exotic Stream
A stream found in an area that is too dry to have spawned such a flow. The flow originates in some moister section.
Extended Family
A family that includes three or more generations. Normally, that would include grandparents, their sons or daughters, and their children, as opposed to a "nuclear family," which is only a married couple and their offspring.
Exurb
A region or district that lies outside a city and usually beyond its suburbs.
Extrusive
Igneous rocks that crystallize at Earth's surface.
Facies
The characteristics of a rock mass that reflect its depositional environment. These characteristics enable the rock mass to be distinguished from rocks deposited in adjacent environments.
fall
The fastest form of mass movement, occurring when rock or sediment breaks off from a steep or vertical slope and descends at a rate of 9.8 meters per second. A fall can be extremely dangerous.
fault block
A section of rock separated from other rock by one or more faults.
fault metamorphism
The metamorphism that acts on rocks grinding past one another along a fault and is caused by directed pressure and frictional heat.
feldspar
Any of a group of light-colored, silicate, rock-forming minerals most often found in plutonic igneous rocks and metamorphic rocks and often containing potassium, sodium, or calcium. Feldspar constitutes 60% of the Earth's crust
firn
Firmly packed snow that has survived a summer melting sea son. Firn has a density of about 0.4 grams per cubic centimeter. Ultimately, firn turns into glacial ice.
fission
The division of the nucleus
fission-track dating
A form of absolute dating that relies on the constant rate of fission to determine the age of a crystal, by counting the fission tracks left in a given area of the crystal.
fission tracks
Marks left in the latticework of a mineral crystal by subatomic particles released during the fission
fluorescence
Emission of visible light by a substance, such as a mineral, that is currently exposed to ultraviolet light and absorbs radiation from it. The light appears in the form of glowing, distinctive colors. The emission ends when the exposure to ultraviolet light ends.
fold
A bend that develops in an initially horizontal layer of rock, -~ usually caused by plastic deformation. Folds occur most frequently in sedimentary rocks.
fold-and-thrust mountain
A mountain consisting of folds, which developed from extremely thick layers of sediment, and thrust fault blocks, and containing both igneous and metamorphic rocks. Fold- and-thrust mountains may be several thousand kilometers high and a few hundred kilometers wide. The Alps, the Appalachians, the Carpathians, the
footwall
The section of rock that lies below the fault plane in a dip- slip fault. See also hanging wall.
forearc basin
A depression in the sea floor located between an accretionary wedge and a volcanic arc in a subduction zone, and lined with trapped sediment. See also backarc basin.
Fault plane
The plane that best approximates the fracture surface of a fault.
Fiord
A former glacial valley with steep walls and a U-shaped profile now occupied by the sea.
Flow cleavage
In a metamorphic rock, the parallel arrangement of all planar or linear crystals as a result of rock flowage during metamorphism.
Flume
A laboratory model of stream flow and sedimentation consisting of a rectangular channel filled with sediment and running water.
Forset bed
One of the inclined beds found in crossbedding; also an inclined bed deposited on the outer front of a delta.
Free oscillation
The ringing or periodic deformation of the whole Earth at characteristic low frequencies after a major earthquake.
Friction breccia
A breccia formed in a fault zone or volcanic pipe by the relative motion of two rock bodies.
foreshock
A minor, barely detectable earthquake, generally pre ceding a full-scale earthquake with approximately the same focus. Major quakes may follow a cluster of foreshocks by as little as a few seconds or as much as several weeks.
foreshore
The portion of a beach that lies nearest to the sea, extending from the low-tide line to the high-tide line.
fractional crystallization
The process by which a magma produces crystals that then separate from the original magma, so that the chemical composition of the magma changes with each generation of crystals, producing igneous rocks of different compositions. The silica content of the magma becomes proportionately higher after each crystallization.
fracture
(n) A crack or break in a rock. (v) To break in random places instead of cleaving. Said of minerals.
fringing reef
A reef that forms against or near an island or continental coast and grows seaward, sloping sharply towards the sea floor. Fringing reefs usually range from 0.5 to 1.0 or more kilometers in width.
frost wedging
A form of mechanical weathering caused by the freezing of water that has entered a pore or crack in a rock. The water expands as it freezes, widening the cracks or pores and often loosening or dislodging rock fragments. As the ice forms, it attracts more water, increasing the effects of frost wedging.
fuel
A source of energy, especially a combustible substance that can be burned for heat or power, or matter used in nuclear fission.
Fault
A fracture or fracture zone in rock along which movement has occurred.
Faunal Succession
A principle of relative dating that is based upon the observed sequence of organisms in the rock record. The relative age of two rock units can frequently be determined by matching the fossils found in those rocks to their positions in the rock record.
Felsic
A term used to describe an igneous rock that has a large percentage of light-colored minerals such as quartz, feldspar, and muscovite. Also used in reference to the magmas from which these rocks crystallize. Felsic rocks are generally rich in silicon and aluminum and contain only small amounts of magnesium and iron. Granite and rhyolite are examples of felsic rocks. (See mafic to contrast.)
Fjord
A deep, narrow, steep-walled, U-shaped valley that was carved by a glacier and is now occupied by the sea.
Fall Line
The physiographic border between the piedmont and coastal plain regions. The name derives from the river rapids and falls that occur as the water flows from hard rocks of the higher piedmont onto the softer rocks of the coastal plain.
Fallow
Agricultural land that is plowed or tilled but left unseeded during a growing season. Fallowing
is usually done to conserve moisture.
A mountain mass created either by the uplift of land between faults or the subsidence of land outside the faults.
Fault Zone
A fracture in the Earth's crust along which movement has occurred. The movement may be in any direction and involve material on either or both sides of the fracture. A "fault zone" is an area of numerous fractures.
Federation
A form of government in which powers and functions are divided between a central government and a number of political subdivisions that have a significant degree of political autonomy.
Feral Animal
A wild or untamed animal, especially one having reverted to such a state from domestication.
Fish Ladder
A series of shallow steps down which water is allowed to flow; designed to permit salmon to circumvent artificial barriers such as power dams as the salmon swim upstream to spawn.
Focality
The characteristic of a place that follows from its interconnections with more than one other place. When interaction within a region comes together at a place (i.e., when the movement focuses on that location), the place is said to possess "focality."
Functional Diversity
The characteristic of a place where a variety of different activities (economic, political, social) occur; most often associated with urban places.
Flood
An overflow of water onto lands that are normally above local water levels. Can be caused by stream discharge exceeding the capacity of the stream channel, storm winds and reduced pressure drawing water from a lake or ocean onto the coastline, dam failure, lake level increase, local drainage problems or other reasons.
Flood Basalt
A sequence of parallel to subparallel basalt flows that were formed during a geologically brief interval of time and which covered an extensive geographic area. Thought to have formed from simultaneous or successive fissure eruptions.
Flood Plain
An area of alluvium-covered, relatively level land along the banks of a stream that is covered with water when the stream leaves its channel during a time of high flow.
Flood Tide
A tidal current that generally moves landward and occurs during the part of the tide cycle when sea level is rising. (See neap tide for contrast.)
Flowing Well
A well that taps an aquifer that is under enough pressure to force water to the surface. Caused when the aquifer has a recharge area at a higher elevation.
Fluid Inclusion
A small amount of fluid (liquid and/or gas) trapped within a rock and which is thought to represent the fluid from which the rock crystallized.
Focus
A point beneath Earth's surface where the vibrations of an earthquake are thought to have originated. Also known as a hypocenter.
fault-zone metamorphism
The metamorphism that acts on rocks grinding past one another along a fault and is caused by directed pressure and frictional heat.
floodplain
The flat land that surrounds a stream and becomes submerged when the stream overflows its banks.
flow
A relatively rapid mass-movement process that involves a mixture of rock, soil, vegetation, and water moving downslope as a viscous fluid. Within a flow (such as a mudflow), each particle, regardless of its size, moves independently.
Foliation
The planar or layered characteristics of metamorphic rocks that are evidence of the pressures and/or temperatures to which the rock was exposed. These can be structural such as cleavage, textural such as mineral grain flattening or elongation, or compositional such as mineral segregation banding.
Foraminifer
A group of single-celled organisms, mostly marine, that produce a calcium carbonate shell. Their shells can make up a significant portion of the carbonate sediment in some areas.
Foraminiferal Ooze
A calcareous sea-floor sediment composed of foraminifer shells.
Forset Beds
The distinctly dipping sediment layers deposited on the front of a prograding delta or on the lee side of a sand dune.
Formation
A laterally continuous rock unit with a distinctive set of characteristics that make it possible to recognize and map from one outcrop or well to another. The basic rock unit of stratigraphy.
Fossil
Remains, imprints or traces of an ancient organism that have been preserved in the rock record. Bones, shells, casts, tracks and excrement can all become fossils.
Fossil fuel
A carbon-rich rock material or fluid, of organic origin that can be produced and burned as a fuel. Coal, oil and natural gas are examples of fossil fuels.
Fumarole
A vent that emits hot gases, usually associated with past or current magmatic activity below.
flood stage
The elevation at which overflow of the natural banks of a stream or body of water begins in the reach or area in which the elevation is measured.
flowing well spring
a well or spring that taps ground water under pressure so that water rises without pumping. If the water rises above the surface, it is known as a flowing well.
freshwater
water that contains less than 1,000 milligrams per liter (mg/L) of dissolved solids; generally, more than 500 mg/L of dissolved solids is undesirable for drinking and many industrial uses.