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ICEBERG CALVING: The breaking off of blocks of ice from a glacier when it moves to a shoreline, forming icebergs.

IGNEOUS ROCK: A rock formed by the solidification of a magma.

INFILTRATION: The movement of groundwater or hydrothermal water into rock or soil through pores and joints.

INFLUENT STREAM: A stream or portion of a stream that recharges groundwater through the stream bottom because its elevation is above the groundwater table. (Compare Effluent stream.)

INTRUSIVE IGNEOUS ROCK: Igneous rock that forced its way in a molten state into the country rock. Also called an "intrusion."

ION: An atom or group of atoms that has gained or lost electrons and so has a net electric charge.

IONIC BOND: A bond formed between atoms by electrostatic attraction between oppositely charged ions.

IRON FORMATION: A sedimentary rock containing much iron, usually more than 15 percent, as sulfide, oxide, hydroxide, or carbonate; a low-grade ore of iron.

ISLAND ARC: A linear or arc-shaped chain of volcanic islands formed at a convergent plate boundary. The island arc is formed in the overriding plate from rising melt derived from the subducted plate and from the asthenosphere above that plate.

ISOCHRON: A line connecting points of equal age.

ISOSTASY, PRINCIPLE OF: The mechanism whereby areas of the crust rise or subside until the mass of their topography is buoyantly supported or compensated by the thickness of crust below, which "floats" on the denser mantle. The theory of isostasy holds that continents and mountains are supported by low-density crustal "roots."

ISOTOPE: One of several forms of one element, all having the same number of protons in the nucleus, but differing in their number of neutrons and thus in their atomic weight.