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Geo 121
Chapter 3
40
History
Not Applicable
04/09/2008

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Term
Andesitic Composition
Definition
Igneous rocks having a mineral makeup between that of granite and basalt, after the common volcanic rock andesite.
Term
Aphanitic Texture
Definition
A texture of igneous rocks in which the crystals are too small for individual minerals to be distinguished with ther unaided eye.
Term
Assimilation
Definition
As magma migrates upward, it may incorporate some of the surrounding host rock.
Term

Basaltic Composition

Definition
Term used to describe igneous rocks that contain abundant dark minerals and about 50 percent silica.
Term
Bowen's Reaction Series
Definition
A concept proposed by N.L. Bowen that illustrates the relationship between magma and the minerals crystallizing from it during the formation of igneous rock.
Term
Crystallization
Definition
The formation and growth of a crystalline solid from a liquid or gas.
Term
Crystal Settling
Definition
This process occurs when the earlier-formed minerals are denser that the liquid portion and sink toward the bottom of the magma chamber.
Term
Decompression Melting
Definition

Melting that occurs as rock ascends due to a drop in confining pressure.

Term
Extrusive
Definition
Igneous activity that occurs at Earth's surface.
Term
Felsic
Definition
A term derived from feldspar and silica. It is a term used to describe granitic igneous rocks.
Term
Fragmental/Pyroclastic Texture
Definition
An igneous rock texture resulting from the consolidation of individual rock fragments that are ejected during a violent eruption.
Term
Geothermal Gradient
Definition
The gradual increase in temperature with depth in the crust. The average is 30 degrees celcius per kilometer in the upper crust.
Term
Glass
Definition
Natural glass produced when molten lava cools too rapidly to permit crystalization. Volcanic glass is a solid composed of unordered atoms.
Term
Glassy Texture
Definition
A term used to describe the texture of certain igneous rocks, such as obsidian, that contain no crystals.
Term
Groundmass
Definition
The matrix of smaller crystals within an igneous rock that has porphyritic texture.
Term
Granitic Composition
Definition
Igneous rocks composed mainly of light-colored silicates are said to have this composition.
Term
Hydrothermal Solutions
Definition
Chemical alterations that occur as hot, ion-rich water circulates through fractures in rock.
Term
Igneous Rocks
Definition
Rocks formed by the crystallization of molten magma.
Term
Intermediate/Andesitic Composition
Definition
Rocks with a composition between granitic and basaltic rocks.
Term
Intrusive Rock
Definition
Igneous rock that formed below Earth's surface.
Term
Lava
Definition

Magma that reaches Earth's surface.

Term
Mafic
Definition
Because basaltic rocks contain a high percentage of ferromagnesian minerals, they are also called mafic.
Term
Magma
Definition
A body of molten rock found at depth, including any dissolved gases and crystals.
Term
Magma Mixing
Definition
One magma body intrudes another.
Term
Magmatic Differentiation
Definition
The formation of one or more secondary magmas from a single parent magma.
Term
Melt
Definition
The liquid portion of magma excluding the solid crystals.
Term
Partial Melting
Definition
The process by which most igneous rocks melt. Because individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occured, a melt with a higher silica content results.
Term
Pegmatite
Definition
A very coarse-grained igneous rock commonly found as a dike associated with a large mass of plutonic rock that has smaller crystals. Crystallization in a water-rich environment is believed to be responsible for the very large crystals.
Term
Pegmatitic Texture
Definition
A texture of igneous rocks in which the interlocking crystals are all larger than one centimeter in diameter.
Term
Phaneritic Texture
Definition
An igneous rock texture in which the crystals are roughly equal in size and large enough so that individual minerals can be identified with the unaided eye.
Term
Phenocryst
Definition
Conspicuously large crystals in a porphyry that are imbedded in a matrix of finer-grained crystals.
Term
Intrusive/Plutonic
Definition
Igneous rocks that form at depth are termed intrusive or plutonic.
Term
Porphyritic Texture
Definition
An igneous rock texture characterized by two distinctively different crystal sizes. The larger crystals are called phenocrysts, and the matrix of smaller crystals is termed the groundmass.
Term
Porphyry
Definition
An igneous rock with a porphyritic texture.
Term
Pyroclastic Texture
Definition
An igneous rock texture resulting from the consolidation of individual rock fragments that are ejected during a violent eruption.
Term
Texture
Definition
The size, shape, and distribution of the particles that collectively constitute a rock.
Term
Ultramafic
Definition
Compositional category for igneous rocks made up almost entirely of ferromagnesian minerals.
Term
Vesicular Texture
Definition
A term applied to aphanitic igneous rocks that contain many small cavities, called vesicles.
Term
Volatiles
Definition
Gaseous components of magma dissolved in the melt. Volatiles will readily vaporize at surface pressures.
Term
Volcanic
Definition
Pertaining to the activities, structures, or rock types of a volcano.
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