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Earth System	The global interconnecting web of physical and biological phenomena involving the solid Earth, the hydrosphere, and the atmosphere.
earthquake	A vibration caused by the sudden breaking or frictional sliding of rock in the Earth.
earthquake belt	A relatively narrow, distinct belt of earthquakes that defines the position of a plate boundary.
earthquake engineering	The design of buildings that can withstand shaking.
earthquake zoning	The determination of where land is relatively stable and where it might collapse because of seismicity.
ebb tide	The falling tide.
eccentricity cycle	The cycle of the gradual change of the Earth’s orbit from a more circular to a more elliptical shape; the cycle takes around 100,000 years.
ecliptic	The plane defined by a planet’s orbit.
ecosystem	An environment and its inhabitants.
eddy	An isolated, ring-shaped current of water.
effusive eruption	An eruption that yields mostly lava, not ash.
Ekman spiral	The change in flow direction of water with depth, caused by the Coriolis effect.
Ekman transport	The overall movement of a mass of water, resulting from the Eckman spiral, in a direction 90° to the wind direction.
El Niño	The flow of warm water eastward from the Pacific Ocean that reverses the upwelling of cold water along the western coast of South America and causes significant global changes in weather patterns.
elastic strain	A change in shape of a material that disappears instantly when stress is removed.
electromagnet	An electrical device that produces a magnetic field.
electron microprobe	A laboratory instrument that can focus a beam of electrons on a small part of a mineral grain in order to create a signal that defines its chemical composition.
embayment	A low area of coastal land.
emergent coast	A coast where the land is rising relative to sea level or sea level is falling relative to the land.
end moraine (terminal moraine)	A low, sinuous ridge of till that develops when the toe of a glacier stalls in one position for a while.
energy	The capacity to do work.
energy resource	Something that can be used to produce work; in a geologic context, a material (such as oil, coal, wind, flowing water) that can be used to produce energy.
eon	The largest subdivision of geologic time.
epeirogenic movement	The gradual uplift or subsidence of a broad region of the Earth’s surface.
epeirogeny	An event of epeirogenic movement; the term is usually used in reference to the formation of broad mid-continent domes and basins.
ephemeral (intermittent) stream	A stream whose bed lies above the water table, so that the stream flows only when the rate at which water enters the stream from rainfall or melt­ water exceeds the rate at which water infiltrates the ground below.
epicenter	The point on the surface of the Earth directly above the focus of an earthquake.
epicontinental sea	A shallow sea overlying a continent.
epoch	An interval of geologic time representing the largest subdivision of a period.
equant	A term for a grain that has the same dimensions in all directions.
equatorial low	The area of low pressure that develops over the equator because of the intertropical convergence zone.
equinox	One of two days out of the year (September 22 and March 21) in which the Sun is directly overhead at noon at the equator.
era	An interval of geologic time representing the largest subdivision of the Phanerozoic Eon.
erg	Sand seas formed by the accumulation of dunes in a desert.
erosion	The grinding away and removal of Earth’s surface materials by moving water, air, or ice.
erosional coast	A coastline where sediment is not accumulating and wave action grinds away at the shore.
erosional landform	A landform that results from the breakdown and removal of rock or sediment.
erratic	A boulder or cobble that was picked up by a glacier and deposited hundreds of kilometers away from the outcrop from which it detached.
esker	A ridge of sorted sand and gravel that snakes across a ground moraine; the sediment of an esker was deposited in subglacial meltwater tunnels.
estuary	An inlet in which seawater and river water mix, created when a coastal valley is flooded because of either rising sea level or land subsidence.
Eubacteria	The kingdom of “true bacteria.”
euhedral crystal	A crystal whose faces are well formed and whose shape reflects crystal form.
eukaryotic cell	A cell with a complex internal structure, capable of building multicellular organisms.
eustatic sea-level change	A global rising or falling of the ocean surface.
evaporate	To change from liquid to vapor.
evapotranspiration	The sum of evaporation from bodies of water and the ground surface and transpiration from plants and animals.
exfoliation	The process by which an outcrop of rock splits apart into onion-like sheets along joints that lie parallel to the ground surface.
exhumation	The process (involving uplift and erosion) that returns deeply buried rocks to the surface.
exotic terrane	A block of land that collided with a continent along a convergent margin and attached to the continent; the term “exotic” implies that the land was not originally part of the continent to which it is now attached.
expanding Universe theory	The theory that the whole Universe must be expanding because galaxies in every direction seem to be moving away from us.
explosive eruptions	Violent volcanic eruptions that produce clouds and avalanches of pyroclastic debris.
external process	A geomorphologic process—such as downslope movement, erosion, or deposition—that is the consequence of gravity or of the interaction between the solid Earth and its fluid envelope (air and water). Energy for these processes comes from gravity and sunlight.
extinct volcano	A volcano that was active in the past but has now shut off entirely and will not erupt in the future.
extinction	The death of the last members of a species so that there are no parents to pass on their genetic traits to offspring.
extraordinary fossil	A rare fossilized relict, or trace, of the soft part of an organism.
extrusive igneous rock	Rock that forms by the freezing of lava above ground, after it flows or explodes out onto the surface and comes into contact with the atmosphere or ocean.
eye	The relative calm in the center of a hurricane.
eye wall	A rotating vertical cylinder of clouds surrounding the eye of a hurricane.