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Iapetus Ocean

The proto-Atlantic Ocean, located against the eastern coast of North America’s ancestral landmass before Pangaea formed.
In Greek mythology, Iapetus was the father of Atlantis.

ice age

A period of global cooling of the Earth’s surface and atmosphere, resulting in the presence or expansion of ice sheets and alpine glaciers. Throughout the Earth’s history, it has been periodically plunged into ice ages, dependent upon the climate and position of the continents. Over the past 2.6 million years, North America has experienced about 50 glacial advances and retreats. The most recent ice age ended about 12,000 years ago.

ice cap

An ice field that lies over the tops of mountains.

ice dome

The spreading center of an ice sheet. Glacial ice flows outward from the ice dome, where snow continues to accumulate, like like the pouring of pancake batter onto a griddle.
See also: glacier, ice sheet

ice field

An extensive area of interconnected glaciers spanning less than 50,000 square kilometers (19,305 square miles). Ice fields are usually constrained by an area’s topography. Ice fields that lie over the tops of mountains are called ice caps.

ice lobe

A broad, rounded section of a continental glacier that flows out near the glacier’s terminus, often through a broad trough.

ice sheet

A mass of glacial ice that covers part of a continent and has an area greater than 50,000 square kilometers (19,000 square miles).


A large chunk of ice, generally ranging in height from 1 to 75 meters (3 to 246 feet) above sea level, that has broken off of an ice shee t or glacier and floats freely in open water.


An extinct Mesozoic marine reptile that was probably similar in size and habitat to the toothed whales, dolphins, and large sharks of today.
See also: extinction, Mesozoic

igneous rocks

Rocks derived from the cooling of magma underground or molten lava on the Earth’s surface.
Igneous rocks differ not only in their cooling rates and subsequent crystal sizes, but also in their chemical compositions. Rocks found in continental crust, such as granite, have high silica content and low iron and magnesium content. They are light in color and are called felsic. Rocks found in oceanic crust, like basalt, are low in silica and high in iron and magnesium. They are dark in color and are called mafic.
Although the composition of magma can be the same as lava, the texture of the rocks will be quite different due to different rates of cooling. It is because of this difference in genesis that geologists are able to make the distinction between extrusive and intrusive igneous rocks when encountered at an outcrop at the Earth’s surface.

Illinoian glaciation

A period of glaciation that occurred during the Pleistocene, 191 to 131 thousand years ago.

Illinois Basin

An inland basin centered in the state of Illinois, which formed when Baltica approached North America in the Ordovician.
More than four billion barrels of petroleum have been extracted from the Illinois Basin.


An ore of titanium, produced for use as a white pigment in paint.

in situ

Situated in its original position.


A soil order ; these are soils that exhibit only moderate weathering and development. They are often found on steep (relatively young) topography and overlying erosion -resistant bedrock.


A fragment of older rock located within a body of igneous rock. Inclusions typically form when igneous rock intrudes into and envelopes older material.

index fossil

A fossil used to determine the relative age of sedimentary deposits. An ideal index fossil lived during a short period of time, was geographically and environmentally widespread, and is easy to identify. Some of the most useful index fossils are hard-shelled organisms that were once part of the marine plankton.

inland basin

A depression located inland from the mountains, and formed by the buckling (downwarping) of the Earth’s crust. Basins naturally preserve thick sediment layers because they accumulate eroded sediment and commonly continue to subside under the weight of the sediment.

inland sea

A shallow sea covering the central area of a continent during periods of high sea level. An inland sea is located on continental crust, while other seas are located on oceanic crust.
An inland sea may or may not be connected to the ocean. For example, Hudson Bay is on the North American plate and connects to the Atlantic and Arctic oceans, while the Caspian Sea is on the European plate but does not drain into any ocean at all.

intensity (earthquake)

A subjective measurement that classifies the amount of shaking and damage done by an earthquake in a particular area.


A period of geologic time between two successive glacial stages.


Between or among mountains.


Areas that are above water during low tide and below water during high tide.

intrusion, intrusive rock

A plutonic igneous rock formed when magma from within the Earth’s crust escapes into overlying strata. As the magma rises, pushing through overlying layers of rock, it begins to cool. The cooling magma can crystallize and harden to become intrusive igneous rock, locked within layers of older rock.


A metallic chemical element (I) that is essential in humans for thyroid hormone production. It is found in trace amounts in seawater and is assimilated by seaweeds. Iodine is a lustrous, black, crystalline solid that appears violet in gas form.


A metallic chemical element (Fe). Iron is most often found in combination with other elements, such as oxygen and sulfur, to form ores like hematite, magnetite, siderite, and pyrite.
The ready availability of iron at Earth’s surface made it one of the earliest mined mineral resources in the US.


An equilibrium between the weight of the crust and the buoyancy of the mantle.


A form of an element that contains a specific number of neutrons. For example, the isotope of carbon with six neutrons is known as carbon-12 (12C) and the isotope of carbon with eight neutrons is carbon-14 (14C).