| Word |
Definition |
| andesitic composition |
Igneous rocks having a mineral makeup between that of granite and basalt, after the common volcanic rock andesite. |
| aphanitic texture |
A texture of igneous rocks in which the crystals are too small for individual minerals to be distinguished with the unaided eye. |
| assimilation |
In igneous activity, the process of incorporating country rock into a magma body. |
| basaltic composition |
Term used to describe igneous rocks that contain abundant dark (ferromagnesian) minerals and about 50 percent silica. |
| batholith |
A large mass of igneous rock that formed when magma was emplaced at depth, crystallized, and subsequently exposed by erosion. |
| Bowen's reaction series |
A concept that illustrates the relationship between magma and the minerals crystallizing from it during the formation of igneous rock. |
| crystal settling |
A process that occurs when the earlier-formed minerals are denser (heavier) than the liquid portion and sink toward the bottom of the magma chamber. |
| decompression melting |
Melting that occurs as rock ascends due to a drop in confining pressure. |
| dike |
A tabular-shaped intrusive igneous feature that cuts through the surrounding rock. |
| extrusive |
Igneous activity that occurs at Earth's surface. |
| felsic |
A term derived from feldspar and silica (quartz). It is a term used to describe granitic igneous rocks. |
| geothermal gradient |
The gradual increase in temperature with depth in the crust. |
| glassy texture |
A term used to describe the texture of certain igneous rocks, such as obsidian, that contains no crystals. |
| granitic composition |
A compositional group of igneous rocks indicating the rock is composed almost entirely of light-colored silicates. |
| groundmass |
The matrix of smaller crystals within an igneous rock that has porphyritic texture. |
| intrusive |
A process by which igneous rock forms below Earth's surface. |
| laccolith |
A massive igneous body intruded between existing strata. |
| lava |
Magma that reaches Earth's surface. |
| mafic |
A term derived from magnesium and ferrum (the Latin word for iron). It is used to describe igneous rocks. |
| magma |
A body of molten rock found at depth, including any dissolved gases and crystals. |
| magmatic differentiation |
The process of generating more than one rock type from a single magma. |
| obsidian |
A volcanic glass of felsic composition. |
| pegmatite |
A very coarse-grained igneous rock (typically granite) commonly found as a dike associated with a large mass of plutonic rock that has smaller crystals. |
| peridotite |
An igneous rock of ultramafic compositionthought to be abundant in the upper mantle. |
| phaneritic texture |
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. |
| phenocryst |
Conspicuously large crystals that are embedded in a matrix of finer-grained crystals. |
| pluton |
A structure that results from the emplacement and crystallization of magma beneath the surface of Earth. |
| porphyritic texture |
An igneous rock texture characterized by two distinctively different crystal sizes. |
| pumice |
A light-colored, glassy vesicular rock commonly having a granitic composition. |
| pyroclastic texture |
An igneous rock texture resulting from the consolidation of individual rock fragments that are ejected during a violent eruption. |
| sill |
A tabular igneous body that was intruded parallel to the layering of pre-existing rock. |
| ultramafic |
Refers to a compositional group of igneous rocks containing mostly olivine and pyroxene. |
| vein deposit |
A mineral that fills a fracture or fault in a host rock. Such rocks have a sheetlike or tabular form. |
| vesicular texture |
A term applied to aphanitic igneous rocks that contain many small cavities, called vesicles. |
| volatiles |
Gaseous components of magma dissolved in the melt. They readily vaporize (form a gas) at surface pressures. |
