Chapter 4 Geology Vocab

extrusive igneous rocks

formed at the crust's surface as a result of the partial melting of rocks within the mantle and crust.

intrusive igneous rocks

formed below the Earth's surface

volatiles

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

lava

magma that reaches the Earth's surface

glass

natural glass produced when molten lava cools too rapidly to permit recrystallization volcanic glass is a solid composed of unordered atoms

partial melting

process by which most igneous rocks melt since individual minerals have different melting points. Igneous rocks melt over a temperature range of a few hundred degrees. Liquid is squeezed out after some melting and melts with a higher silica content.

magma mixing

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

magmatic differentiation

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

fragmental texture

pyroclastic texture

igneous rocks

rock formed from the crystallization of magma

concordant

A term used to describe intrusive igneous masses that form parallel to the bedding of the surround rock.

discordant

A term used to describe plutons that cut across existing rock structures such as bedding planes

Bowen's reaction series

Concept proposed by Norman L. Bowen that illustrates relationships between magma and minerals crystallizing during formation of igneous rocks

crystal settling

During crystallization of magma earlier-formed materials are denser than the liquid portion and settle to the bottom of the magma chamber

geothermal gradient

Gradual increase in temperature with depth in the crust. Average 30 degrees Celsius in the upper crust

felsic

Granitic composition

basaltic composition

Group of igneous rocks indicating rock contains substantial dark silicate minerals and calcium-rich plagioclase feldspar

porphyry

Igneous rocks with porphyritic texture

assimilation

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

andesitic composition

Intermediate composition

decompression melting

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

coarse-grained texture

Phaneritic texture

intrusions

Pluton

aphanitic texture

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

crystallization

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

Pegmatite

Very coarse-grained igneous rock like granite that is found as a dike and is associated with a large mass of plutonic rocks. There are smaller crystals. Bigger crystals crystallize in a rich-water environment

magma

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

granitic composition

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

intermediate composition

a compositional group of igneous rocks, indicating that the rock contains at least 25% of dark silicate minerals like plagioclase feldspar

stocks

a pluton similar to but smaller than a batholith

Plutons

a structure that results from the emplacement and crystallization of magma beneath the surface of the earth.

sills

a tabular igneous body that was intruded parallel to layering of preexisting rock

massive

an igneous pluton that is not tabular in shape

xenoliths

an inclusion of unmelted country rock in an igneous pluton

fine-grained texture

aphanitic texture

mafic

basaltic composition

Phenocryst

conspicuously large crystal embedded in a matrix of finer-grained crystals.

volcanic rocks

created when rocks are melted through enormous amounts of heat and pressure and liquid magma is forced through other rocks to erupt on Earth's surface as lava

tabular

describing a feature such as an igneous pluton having two dimensions that are much larger than the third

ultramafic

group of igneous rocks containing mostly pyroxene and olivine.

porphyritic texture

igneous rock texture characterized by two distinctively different crystal sizes. The larger crystals are called phenocrystals, whereas the matrix of smaller crystals is termed to groundmass.

phaneritic texture

igneous rock texture in which crystals are roughly equal in size. Large enough so individual minerals can be identified.

pegmatitic texture

igneous rocks interlocking as crystals are all larger than 1 centimeter in diameter

plutonic rocks

igneous rocks that form at depth. Pluto, the god of the lower world in classical mythology.

batholiths

large mass of igneous rock formed when magma was emplaced at depth, crystallized, and subsequently exposed by erosion.

laccoliths

massive igneous body intruded between preexisting strata

groundmass

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

columnar joints

pattern of cracks that form during cooling of molten rocks to generate columns

pyroclastic texture

rock texture resulting form consolidation of individual rock fragments ejected during volcanic eruption

texture

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

dikes

tabular-shaped intrusive igneous feature that cuts through surrounding rock

vesicular texture

term applied to aphanitic igneous rocks contain many small cavities called vesicles.

glassy texture

texture of certain igneous rocks such as obsidian, contains no crystals

melt

the liquid portion of magma excluding the solid crystals