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Ice Age - 

Period of time when glaciers dominate the landscape of the Earth. The last major Ice Age was during the Pleistocene epoch.

Ice Age - 

Period of time from about 2 million years ago to 10,000 years ago. During this period areas of land at higher and middle latitudes where covered with glacial ice.

Ice Cap - 

Large dome-shaped glacier found covering a large expanse of land. Smaller than an ice sheet.

Ice Fall - 

An area of crevassed ice on a glacier. Caused when the base of the glacier flows over steep topography.

Ice Field - 

Large level area of glacial ice found covering a large expanse of land. Similar in size to an ice cap but does not have a dome-shape.

Ice Fog - 

A fog that is composed of small suspended ice crystals. Common in Arctic locations when temperatures are below -30° Celsius and a abundant supply of water vapor exists.

Ice Sheet - 

A dome-shaped glacier covering an area greater than 50,000 square kilometers. Greenland and Antarctica are considered ice sheets. During the glacial advances of the Pleistocene ice sheets covered large areas of North America, Europe, and Asia. Larger than an ice cap.

Ice Shelf - 

Large flat layer of ice that extends from the edge of the Antarctic ice cap into the Antarctic Ocean. Source of icebergs.

Iceberg - 

A mass of ice found floating in the ocean or a lake. Often icebergs form when ice calves from land-based glaciers into the water body. Icebergs can be dangerous to shipping in high and mid-latitude regions of the ocean because 90 percent of their mass lies below the ocean surface.

Ideal Gas Law - 

This law describes the physical relationships that exist between pressure, temperature, volume, and density for gases. Two mathematical equations are commonly used to describe this law:
Pressure x Volume = Constant x Temperature
Pressure = Density x Constant x Temperature

Igneous Rock - 

Rocks formed by solidification of molten magma either beneath (intrusive igneous rock) or at (extrusive igneous rocks) the Earth's surface.

Illuviation - 

Deposition of humus, chemical substances, and fine mineral particles in the lower layers of a soil from upper layers because of the downward movement of water through the soil profile. Compare with eluviation.

Immigration - 

Migration of an organism into an area for the purpose of changing its residence permanently. Compare with emigration.

Index Contour - 

Contour line that is accentuated in thickness and is often labeled with the appropriate measure of elevation. Index contours occur every four or fifth contour interval and help the map user read elevations on a map.

Industrial Smog - 

Form of air pollution that develops in urban areas. This type of air pollution consists of a combination of sulfur dioxide, suspended droplets of sulfuric acid, and a variety of suspended solid particles. Also see photochemical smog.

Infiltration - 

The absorption and downward movement of water into the soil layer.

Infiltration Capacity - 

The ability of a soil to absorb surface water.

Infiltration Rate - 

Rate of absorption and downward movement of water into the soil layer.

Inner Core - 

Inner region of the Earth's core. It is thought to be solid iron and nickel with a density of about 13 grams per cubic centimeter. It also has a diameter of about 1220 kilometres.

Inorganic - 

Non-living thing. Usually refers to the physical and chemical components of an organism's environment. Some times called abiotic.

Inselberg - 

A German term used to describe a steep-sided hill composed of rock that rises from a pediplain.

Insolation - 

Direct or diffused shortwave solar radiation that is received in the Earth's atmosphere or at its surface.

Insolation Weathering - 

Form of physical weathering. Involves the physical breakdown of minerals and rock due to thermal expansion and contraction.

Instability - 

Atmospheric condition where a parcel of air is warmer that the surrounding air in the immediate environment. This condition causes the parcel to rise in the atmosphere. Also see unstable atmosphere.

Interception - 

Is the capture of precipitation by the plant canopy and its subsequent return to the atmosphere through evaporation or sublimation. The amount of precipitation intercepted by plants varies with leaf type, canopy architecture, wind speed, available radiation, temperature, and the humidity of the atmosphere.

Interglacial - 

Period of time during an ice age when glaciers retreated because of milder temperatures.

Intermittent Stream - 

A stream that flows only for short periods over a year. Flow events are usually initiated by rainfall.

International Date Line - 

A line drawn almost parallel to the 180 degree longitude meridian that marks the location where each day officially begins. The location of the International Date Line was decided upon by international agreement.

Intertropical Convergence Zone - 

Zone of low atmospheric pressure and ascending air located at or near the equator. Rising air currents are due to global wind convergence and convection from thermal heating. Location of the thermal equator.

Intrusive Igneous Rock - 

A mass of igneous rock that forms when magma from the mantle migrates upward and cools and crystallizes near, but not at, the Earth's surface. Also called plutonic igneous rock. Also see dyke, sill, and batholith.

Inversion - 

Situation where a layer of warmer air exists above the Earth's surface in a normal atmosphere where air temperature decreases with altitude. In the warmer layer of air, temperature increases with altitude. Also see temperature inversion.

Ionosphere - 

A region in the atmosphere above 50 kilometers from the surface where relatively large concentrations of ions and free electrons exist. The ionosphere is important for human communications because it re-directs AM radio transmissions. This process extends the distance that radio transmissions can travel.

Island Arc - 

A line of volcanic islands found of the ocean that have been created by the convergence of two tectonic plates and the subsequent subduction of one of the plates beneath the other. Subduction cause magma plumes to rise to the Earth's surface creating the volcanic islands.

Isobar - 

Lines on a map joining points of equal atmospheric pressure.

Isoline - 

Lines on a map joining points of equal value.

Isostacy - 

The buoyant condition of the Earth's crust floating in the asthenosphere. The greater the weight of the crust the deeper it floats into the asthenosphere. When weight is removed the crust rises higher.

Isostatic Depression - 

Large scale sinking of the crust into the asthenosphere because of an increase in weight on the crustal surface. Common in areas of continental glaciation where the crust was depressed by the weight of the ice.

Isostatic Rebound - 

The upward movement of the Earth's crust following isostatic depression.

Isotherm - 

Lines on a map joining points of equal temperature.

Isothermal Layer - 

Vertical layer in the atmosphere where temperature remains unchanged. In the Earth's atmosphere, three isothermal layers are found in the lower regions of the stratosphere, mesosphere, and the thermosphere.

Isotopic Dating - 

Dating technique used to determine the age of rock and mineral through the decay of radioactive elements.


Zone of low atmospheric pressure and ascending air located at or near the equator. Rising air currents are due to global wind convergence and convection from thermal heating. Location of the thermal equator.