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E


earthquake
A movement within the Earth's crust or mantle, caused by the sudden rupture or repositioning of underground rocks as they release stress.
Earth systems
The Earth is made up of four basic systems: the lithosphere (the rocks of the earth); the hydrosphere (the waters of the Earth); the atmosphere (the gases that surround the Earth); and, the biosphere (the life on Earth). These systems interact to produce most of the geological processes that occur on Earth. An event involving one of these systems may affect some or all of the others.
ebb tide
A tide that lowers the water surface of an ocean and moves the shoreline farther seaward.
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.
elastic limit
See yield point.
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 115 known elements, 92 of them natural and 23 man-made. 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.
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.
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.
evaporite
An inorganic chemical sediment that precipitates when the salty water in which it had dissolved evaporates.
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.

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