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Geological Terms Beginning With "I"



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Igneous Rock:

A rock formed by the crystallization of magma or lava. Pictures of Igneous Rocks

Ignimbrite:

An igneous rock formed by the lithification of ash flow or pyroclastic flow deposits.

Impermeable Layer:

A layer of rock, sediment or soil that does not allow water to pass through. This could be caused by a lack of pore space or pore spaces that are so small that water molecules have difficulty passing through.

Infiltration:

The movement of surface water into porous soil.

Injection Well:

A well that is used to force a fluid into the ground. The injection could be done for disposal or to place the fluid (such as natural gas) into a subsurface reservoir.

Interior Drainage:

A system of streams that flow into a landlocked basin and evaporate.

Intermediate Rock:

An igneous rock that has an intermediate silica content. Examples are syenite and diorite. Also see entries for acidic, basic and ultrabasic rocks.

Intermittent Stream:

A stream that goes dry at certain times of the year. Intermittent streams flow during seasons of the year when runoff and/or ground water contributions sustain the flow of the stream. They stop flowing during dry seasons when precipitation is low and the water table drops below the bed of the stream.

Intrusion:

A igneous rock body that formed from magma that forced its way into, through or between subsurface rock units.

Intrusive:

Igneous rocks that crystallize below Earth's surface.

Ion:

An atom or group of atoms that have gained or lost one or more electron and as a result has an electrical charge.

Ionic Bond:

A chemical bond formed by the electrostatic attraction between oppositely charged ions.

Iron Formation:

A layered deposit of chemical sedimentary rocks containing at least 15 percent (by weight) iron in the form of sulfide, oxide, hydroxide, or carbonate minerals.

Iron Ore:

A chemical sedimentary rock that forms when iron and oxygen (and sometimes other substances) combine in solution and deposit as a sediment. Hematite is the most common sedimentary iron ore mineral. Picture of Iron Ore.

Isograd:

A line on a map that represents a specific degree of metamorphism. Rocks on one side of the line have been subjected to a greater level of metamorphism and on the other side of the line a lower level of metamorphism.

Isostasy:

A condition of gravitational balance (similar to floating) in which a mass of lighter crustal rocks are buoyantly supported from below by denser mantle rocks. The crustal rocks above subside into the mantle until they have displaced an adequate amount of mantle material to support them.

Isotope:

One of several forms of an element. These different forms have the same number of protons but varying numbers of neutrons.

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