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Geological Terms Beginning With "E"



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Earthflow:

A detached mass of soil that moves downslope over a curved failure surface under the influence of gravity. An earthflow is more complex than a slump; it has a higher moisture content and the moving mass of soil has some internal movement or "flow". Rates of movement are typically a few inches per year but faster rates can occur.

Earthquake:

A trembling of the earth caused by a sudden release of energy stored in subsurface rock units.

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.

Electron:

A subatomic particle with a negative charge and of negligible mass that orbits the nucleus of an atom.

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).

Ephemeral Stream:

A stream that flows for a short interval of time after precipitation or snow melt in the immediate area. Ephemeral streams carry runoff water. Their channels are above the water table and they receive no contribution from ground water.

Epicenter:

The point on the Earth's surface directly above the focus of an earthquake.

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.

Erosion:

A general term applied to the wearing away and movement of earth materials by gravity, wind, water and ice.

Esker:

A long winding ridge of sorted sands and gravel. Thought to be formed from sediment deposited by a stream flowing within or beneath a glacier.

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.

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:

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.

Exploration:

The work of identifying areas that may contain viable mineral resources. This work can include surface mapping, remote sensing, exploratory drilling, geophysical testing, geochemical testing and other activities.

Exploratory Drilling:

Drilling done to locate mineral deposits in an area where little subsurface data about those minerals is available. Exploratory wells may not have the ability to produce the minerals if they are discovered.

Extractive Industries:

Industries involved in mineral resource exploration, acquisition, assessment, development or production.

Extrusive:

Igneous rocks that crystallize at Earth's surface.

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