geology ch 4

granitic composition

a coarse-grained, light-colored igneous rock composed mainly of feldspars and quartz with minor amounts of mica and amphibole minerals.

pegmatites

a coarsely crystalline granite or other igneous rock with crystals several centimeters to several meters in length.

intrusions/ plutons

a formation in which magma (molten rock) is trapped beneath the surface of the Earth and pushes the rock located above it into a dome shape.

laccoliths

a mass of igneous rock, typically lens-shaped, that has been intruded between rock strata causing uplift in the shape of a dome.

sills

a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock.

dikes

a type of later vertical rock between older layers of rock. Technically, it is any geologic body which cuts across: flat wall rock structures, such as bedding. massive rock formations, usually igneous in origin.

batholiths

a very large igneous intrusion extending deep in the earth's crust.

vesicular texture

a volcanic rock texture characterized by a rock being pitted with many cavities (known as vesicles) at its surface and inside. comes through and extrusive process.

felsic

a word to describe granitic composition, or being made from feldspar and silica.

bowen's reaction series

able to explain why certain types of minerals tend to be found together while others are almost never associated with one another.

melt

composed of mainly of mobile ions of the eight most common elements found in Earth's crust.

intermediate/ andesitic

contain at least 25 percent dark silicate minerals, mainly amphibole, pyroxene, and biotite mica with the other dominant mineral being plagioclase feldspar

discordant

cut across existing structures.

crystal setting

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

stocks

extremely small plutons.

concordant

form parallel to features like sedimentary strata.

intrusive igneous rocks/ platonic rocks

form when magma cools slowly below the Earth's surface. most have large, well-formed crystals. examples include granite, gabbro, diorite and dunite.

extrusive igneous rocks/ volcanic rocks

form when magma reaches the Earth's surface a volcano and cools quickly. most have small crystals. examples include basalt, rhyolite, andesite, and obsidian.

columnar joints

geological structure where sets of intersecting closely spaced fractures, referred to as joints, result in the formation of a regular array of polygonal prisms, or columns.

magma

hot fluid or semifluid material below or within the earth's crust from which lava and other igneous rock is formed by cooling.

pyroclastic/ fragmental texture

igneous rocks composed of rock fragments.

aphantic texture

igneous rocks that form at the surface, or within the upper crust where colling is relatively rapid, and have a very fine texture.

assimilation

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

porphyritic texture

indicates that the magma sat and cooled a bit below the Earth's surface, thus giving time for the large crystals to grow, before erupting onto the surface and cooling very quickly.

tabular

intrusive igneous bodies are generally classified according to their shape, or the orientation with respect to the host rock (horizontal).

decompression melting

involves the upward movement of the earth's mantle to an area of lower pressure. The reduction in overlying pressure enables the rock to melt, leading to magma formation.

phaneritic texture

means that the size of matrix grains in the rock is large enough to be distinguished with the unaided eye as opposed to aphanitic grains (which are too small to be seen with the naked eye).

lava

molten rock that reaches earth's surface.

glassy texture

occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals.

mafic

relating to, denoting, or containing a group of dark-colored, mainly ferromagnesian minerals such as pyroxene and olivine.

igneous rocks

rock formed through the cooling and solidification of magma or lava.

ultramafic

rocks composed chiefly of mafic or ferromagnetism minerals.

glass

rocks that consist of unordered tons that are frozon randomly in place

xenoliths

suspended blocks of country rock found in plutons

magmatic differentiation

the formation of one or more secondary magmas from a single parent magma.

volatiles

the group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, methane and sulfur dioxide.

partial melting

the incomplete melting of rocks.

geothermal gradient

the increase of temperature with depth.

texture

the overall appearance of a rock based on the size, shape, and arrangement of its mineral grains.

magma mixing

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