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ice age
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A glacial epoch or time of extensive glacial activity. Also, as Ice Age, which refers to the latest glacial epoch, the Pleistocene Epoch. Periods characterized by very low temperature worldwide and advancing glacier.
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ice cap
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An ice cap is a dome-shaped cover of perennial snow and ice. Ice caps are actually small ice sheet, generally less than 50,000 square kilometres in size. Today, they are found in polar regions (e.g. Baffin Island). Like ice sheets, these ice caps are not constrained by the existing relief and exert a modifying effect on the climate. The cooling effect of an ice cap, however, is considerably less than an ice sheet and the effects are more localized.
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ice core
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Deep drill into the earth's permanent glacier revealing the history of the atmospheric gas and dust content, enabling deduction of former atmospheric temperatures.
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ice sheet
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A glacier of considerable thickness and more than 50,000 sq. km in area. It form a continuous cover of ice and snow over a land surface. An ice sheet is not confined by the underlying topography but spread outward in all directions. During the Pleistocene Epoch ice sheets covered large parts of North America and northern Europe but they are now confined to polar regions (e.g., Greenland and Antarctica). Also called continental glacier.
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ice shelf
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"A sheet of very thick ice with a level or gently undulating surface. It is attached to the land on one side, but most of it is floating. On the seaward side it is bounded by a steep cliff (ice front) two to 50 m or more above sea level. Ice shelves have formed along polar coast (e.g., Antarctica and Greenland); they are very wide, with some extending several hundreds of kilometers toward the sea from the coastline. They increase in size from annual snow accumulation and seaward extension of land glacier. They decrease in size from warming, melting and calving."
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ice stream
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Large, fast glacier embedded in slow-moving ice. Flow velocities are a few hundred meters to kilometers per year.
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iceberg
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A floating ice mass up to 100 km long and 200-300 m thick that has broken off (calved) from ice shelves, glacier or coastal ice cliffs into the ocean.
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igneous
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Rock formed when molten rock (magma) that has cooled and solidified (crystallized). See intrusive (plutonic) and extrusive (volcanic) igneous rock.
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ignimbrite
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An igneous rock formed by the lithification of volcanic ash and volcanic breccia.
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illite
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A potassium-rich clay mineral
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impact crater
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Circular, rimmed depression produced by the impact of a solid body traveling at more than a few kilometers per second.
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impermeable
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Rock or sediment that does not allow passage of water.
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incised
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A term used to describe down-cutting (downward erosion) by a stream. Incision deepens and often steepens the stream channel.
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inclination
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" The angle between a line in the Earth's magnetic field and the horizontal plane; also a synonym for dip. "
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index of refraction
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" The ratio of the speed of light in a vacuum to the speed in a material; this ratio determines the amount that light is refracted as it passes into a crystal. "
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induction
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Process by which a body becomes magnetized or electrified by merely placing it in a magnetic or electric field. Also refers to the process under which electric currents are initiated in a conductor by merely placing it in an electromagnetic field.
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infiltration
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The movement of groundwater or hydrothermal water into rock or soil through joint and pores.
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infrared radiation
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"Electromagnetic radiation lying in the wavelength interval from 0.7 microns to 1000 microns. Its lower limit is bounded by visible radiation and its upper limit is bounded by microwave radiation. Most of the energy emitted by the Earth and its atmosphere is at infrared wavelengths. Infrared radiation is generated almost entirely by large-scale intramolecular processes. The tri-atomic gases, such as water vapor, carbon dioxide, and ozone absorb infrared radiation and play important roles in the propagation of infrared radiation in the atmosphere. Abbreviated IR; also called longwave radiation."
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inorganic
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Obtained from mineral, not organic.
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integrated time scale (Eocene)
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"The Eocene rock record initially was and still is recognized by its characteristic fossil; fossils are cross-checked against the geomagnetic reversal scale. But rates of change and other quantifications demand numerical calibration -- time in millions of years -- which is founded ultimately in isotopic decay in the potassium-argon and other decay series. The geomagnetic scale consists of chrons, of which Chron C24 and Chron C19 are important here. Some fossil succession have their own names; thus, e.g., the Wasatchian North American land mammal age."
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intensity
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A measure of the effects of an earthquake at a particular place on humans, structure and (or) the land itself. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the point and the local geology at that point.
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interfacial angle
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The angle between two crystal faces of a crystal, characteristic of a mineral's symmetry.
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intermontane basin
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A basin between mountain ranges, often formed over a graben.
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internal drainage
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"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 basin ever find an outlet to the ocean. (see drainage basin)"
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intrusion
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Emplacement of magma (molten rock) into preexisting rock. Dikes, sill, and batholith are intrusion.
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intrusive rock
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Igneous rock that is interpreted as a former intrusion from its cross-cutting contacts, chilled margins, or other field relations.
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ion
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An atom or group of atoms that has gained or lost electron and so has a net electric charge.
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ionic bond
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A bond formed between atom by electrostatic attraction between oppositely charged ions.
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iron formation
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"A sedimentary rock containing much iron, usually more than 15 percent as sulfide, oxide, hydroxide, or carbonate; a low-grade ore of iron. "
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island
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A body of land surrounded on all sides by water.
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island arc
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An arc-shaped chain of volcanic island produced where an oceanic plate is sinking (subducting) beneath another
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island arc
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A group of island having a curving, arclike pattern. Most island arc lie near the continental masses, but inasmuch as they rise from the deep ocean floor, they are not a part of the continents proper
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isograd
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A line or curved surface connecting rocks that have undergone an equivalent degree of metamorphism.
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isoseismal line
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A line connecting points on the Earth's surface at which earthquake intensity is the same. It is usually a closed curve around the epicenter.
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isostasy
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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 denset mantle. The theory that continent and mountain are supported by low-density crustal roots.
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isostatic adjustment
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The process whereby lateral transport at the Earth's surface from erosion or deposition is compensated for by movements in a subcrustal layer to maintain equilibrium among units of varying masses and densities. Also called isostatic compensation.
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isotherm
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A line on a chart that connects all points of equal or constant temperature.
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isotope
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One of two or more atom that have the same atomic number (i.e., the same number of protons in their nuclei) but have different mass numbers. Forms of the same element that have the same number of protons but differing numbers of neutron. This mass difference gives rise to slightly differing chemical properties for each isotope. Of interest here are the stble isotopes of carbon: carbon-12 (or 12C) and carbon-13 (or 13C), which are stable over time and carbon-14, which is a radioisotope and decays with time to an isotope of nitrogen.
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isotope geology
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The study of the relative abundance of isotope in rocks to determine their age (see geochronology) or conditions of formation.
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isotopes in paleoceanography
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"Stable isotope of some light elements occuring in nature in given ratios fractionated by the biosphere and thereby providing informative signals about the latter. Most useful ratios are 16O/18O; 12C/13C. Marine organisms biomineralize using oxygen in the water."
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isotopic age
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Age of a rock or mineral that is determined by isotopic methods in which the initial isotopic ratio of the sample assumed.
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isotropic substance
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One in which the magnitude of a physical property, such as transmission of light is independent of crystallographic direction.
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