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igneous rock

Rock formed when molten rock (magma) that has cooled and solidified (crystallized). See intrusive (plutonic) and extrusive (volcanic) igneous rock.

illite

A potassium-rich clay mineral

impermeable

Rock or sediment that does not allow passage of water.

incised

A term used to describe down-cutting (downward erosion) by a stream. Incision deepens and often steepens the stream channel.

inner core

The innermost layer of Earth. Consists of solid iron and nickel.

intensity

One way to measure the strength of an earthquake. Intensity measures of the effects of an earthquake on buildings and the reactions of people. Compare with magnitude.

internal drainage

An area in which surface water cannot reach the ocean. Any water that falls into an area with internal drainage as rain or snow does not escape out of it; not one of the streams that originate within these basins ever find an outlet to the ocean. (see drainage basin)

intrusion

Emplacement of magma (molten rock) into preexisting rock. Dikes, sills, and batholiths are intrusions.

intrusive rock

Igneous rock that cools and solidifies beneath the Earth's surface. (= plutonic rock)

island arc

An arc-shaped chain of volcanic islands produced where an oceanic plate is sinking (subducting) beneath another.

isotope

Different forms of a single element that have the same number of protons but different numbers of neutrons in their nuclei. Some radioactive isotopes are unstable and shed nuclear particles over time until they become stable. For instance, unstable isotopes of uranium break down to become lead.