Igneous Rocks

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intermediate/andesitic

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a compositional group of igneous rocks, indicating that the rock contains at least 25% dark silicate minerals. The other dominant mineral is plagioclase feldspar

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aphanitic texture

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a texture of igneous rocks in which the crystals are too small for individual minerals to be distinguished without the aid of a microscope

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assmilation

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in igneous activity, the process of incorporating country rock into a magma body

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mafic/basaltic

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a compositional group of igneous rocks indicating that the rock contains substantial dark silicate minerals and calcium-rich plagioclase feldspar

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Bowen's reaction series

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A concept produced by N.L.Bowen that illustrates the relationships between magma and the minerals crystallizing from it during the formation of igneous rocks

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crystallization

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the formation and growth of a crystalline solid from a liquid or gas

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crystal settling

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during the crystallization of magma, the earlier-formed minerals are denser than the liquid portion and settle to the bottom of the magma chamber

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decompression melting

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

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extrusive

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igneous activity that occurs at Earth's surface

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felsic/granitic

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a compositional group of igneous rocks indicating the rock is composed almost entirely of light-colored silicates

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fragmental texture

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an igneous rock texture resulting from the consolidation of individual rock fragments that are ejected during a violent volcanic eruption

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geothermal gradient

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the fradual increase in tempature with depth in the crust

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glass

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naturally, produced when molten lava cools too rapidly to permit recrystallization. The volcanic form is a solid composed of unordered atoms

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glassy texture

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a term used to describe the texture or certain igneous rocks, such as obsidian, that contain no crystals

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groundmass

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the matrix of smaller crystals within an igneous rock that has porphyritic texture

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igneous rocks

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rock formed from the crystallization of magma

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intrusive

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igneous rock that formed below Earth's surface

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lava

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magma that reaches Earth's surface

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magma

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a body of molton rock found at depth, including any dissolved gases and crystals

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magma mixing

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the process of altering the composition of a magma through the mixing of material from another magma body

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magmatic differentiation

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the process of generating more than one rock type from a single magma

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melt

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the liquid portion of magma excluding the solid crystals

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partial melting

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the process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeesed out after some melting has occured, a melt with a higher silica content results

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pegmatite

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a very course-grained igneous rock (typically granite) 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.

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pegmatitic texture

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a texture of igneous rocks in which the interlocking crystals are all larger than one centimeter in diameter

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phaneritic texture

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an igneous rock texture in which the crystals are roughly equal in size and large enough so the individual minerals can be identified without the aid of a microscope

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phenocryst

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conspicously large crystal embedded in a matrix of finer-grained crystals

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plutonic

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igneous rocks that form at depth

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porphyritic texture

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an igneous rock texture charactorized by two distinctly different crystal sizes

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porphyry

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an igneous rock with a porphyritic texture

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pyroclastic texture

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an igneous rock texture resulting from the consolidation of individual rock fragments that are ejected during a violent volcanic eruption

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texture

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the size, shape, and distribution of the particles that collectively constitute a rock

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ultramafic

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a compositional group of igneous rocks containing mostly olivine and pyroxene

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vesicular texture

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a term applied to aphanitic igneous rocks that contain many small cavaties

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volatiles

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gaseous components of magma dissolved in the melt; readily vaporize at surface pressures

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volcanic

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pertaining to the activities, structures, or rock types of a volcano