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ice age
A period during which the Earth is substantially cooler than usual and a significant portion of its land surface is covered by glaciers. Ice ages generally last tens of millions of years.
ice cap
An alpine glacier that covers the peak of a mountain.
igneous rock
A rock made from molten (melted) or partly molten material that has cooled and solidified.
index fossil
The fossil of an organism known to have existed for a relatively short period of time, used to date the rock in which it is found.
inorganic chemical sediment
Sedimentary rocks - such as limestone, chert, and evaporites - that forms when the dissolved products of chemical weathering precipitate from solution. The ornate columns of travertine in caves is one example of an inorganic chemical sedimentar rock.
inselberg
A steep ridge or hill left when a mountain has eroded and found in an otherwise flat, typically desert plain.
intermolecular bonding
The act or process by which two or more groups of atoms or molecules combine due to weak positive or negative charges that develop at various points within each group of atoms due to uneven distribution of their electrons. The side of the molecule where electrons are more likely to be found will have a slight negative charge, and the side where they are less likely to be found will have a slight positive charge. Such charged regions attract oppositely charged regions of nearby molecules, forming relatively weak bonds.
internal deformation
The rearrangement of the planes within ice crystals, due to pressure from overlying ice and snow, that causes the downward or outward flow of a glacier.
intrusive rock
An igneous rock formed by the entrance of magma into preexisting rock.
ion
An atom that has lost or gained one or more electrons, thereby becoming electrically charged.
ionic bond
The combination of an atom that has a strong tendency to lose electrons with an atom that has a strong tendency to gain electrons, such that the former transfers one or more electrons to the latter and each achieves chemical stability under the octet rule. The atom that loses electrons acquires a positive electric charge and the atom that gains electrons acquires a negative electric charge, so that the resulting compound is electrically neutral.
ionic bonding
The act or process of forming of an ionic bond.
ionic substitution
The replacement of one type of ion in a mineral by another that is similar to the first in size and charge.
isostasy
The equilibrium maintained between the gravity tending to depress and the buoyancy tending to raise a given segment of the lithosphere as it floats above the asthenosphere.
isotope
One of two or more forms of a single element; the atoms of each isotope have the same number of protons but different numbers of neutrons in their nuclei. Thus isotopes have the same atomic number but differ in atomic mass.
isotope dating
The process of using relative proportions of parent to daughter isotopes in radioactive decay to determine the age of a given rock or rock stratum.

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