Chapter 4: Igneous Rocks: Solids from Melts

Question 1

andesite

Answer 1

A volcanic rock type intermediate in composition between rhyolite and basalt.

Question 2

basalt

Answer 2

A fine-grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxene.

Question 3

batholith

Answer 3

A great irregular mass of coarse-grained igneous rock that covers at least 100 km2; the largest pluton.

Question 4

concordant intrusion

Answer 4

An intrusive igneous rock whose boundaries lie parallel to layers of preexisting bedded rock.

Question 5

country rock

Answer 5

The rock surrounding an intrusive igneous rock.

Question 6

dacite

Answer 6

A light-colored, fine-grained extrusive igneous rock with the same general composition as andesite. The extrusive equivalent of granodiorite.

Question 7

decompression melting

Answer 7

Melting that occurs when mantle material rises to an area of lower pressure at a mid-ocean ridge.

Question 8

dike

Answer 8

A tabular igneous intrusion that cuts across layers of bedding in country rock.

Question 9

diorite

Answer 9

A coarse-grained intrusive igneous rock with composition intermediate between granite and gabbro.

Question 10

discordant intrusion

Answer 10

An intrusive igneous rock that cuts across the layers of the country rock that it intrudes.

Question 11

extrusive igneous rock

Answer 11

A fine-grained or glassy igneous rock formed from a rapidly cooled magma that erupts at the surface through a volcano.

Question 12

felsic rock

Answer 12

A light-colored igneous rock that is poor in iron and magnesium and rich in high-silica minerals such as quartz, orthoclase feldspar, and plagioclase feldspar.

Question 13

fluid-induced melting

Answer 13

Melting that takes place when water-laden sediments on a subducting oceanic plate are carried downward into the subduction zone.

Question 14

fractional crystallization

Answer 14

The process by which the crystals formed in a cooling magma are segregated from the remaining liquid at progressively lower temperatures.

Question 15

gabbro

Answer 15

A dark gray, coarse-grained intrusive igneous rock with an abundance of mafic minerals, particularly pyroxene.

Question 16

granite

Answer 16

A felsic, coarse-grained, intrusive igneous rock composed of quartz, orthoclase feldspar, sodium-rich plagioclase feldspar, and micas.

Question 17

granodiorite

Answer 17

A light-colored, coarse-grained intrusive igneous rock similar to granite in having abundant quartz, but whose predominant feldspar is plagioclase, not orthoclase.

Question 18

hydrothermal vein

Answer 18

A vein filled with minerals that contain large amounts of chemically bound water and are known to crystallize from hot-water solutions.

Question 19

intermediate igneous rock

Answer 19

An igneous rock midway in composition between mafic and felsic, neither as rich in silica as the felsic rocks nor as poor in it as the mafic rocks.

Question 20

intrusive igneous rock

Answer 20

A coarse-grained igneous rock that crystallized slowly when magma intruded into country rock deep in Earth�s crust.

Question 21

mafic rock

Answer 21

A dark-colored igneous rock containing minerals (such as pyroxenes and olivines) rich in iron and magnesium and relatively poor in silica.

Question 22

magma chamber

Answer 22

A magma-filled cavity in the lithosphere that forms as ascending drops of melted rock push aside surrounding solid rock.

Question 23

magmatic differentiation

Answer 23

A process by which rocks of varying composition can arise from a uniform parent magma.

Question 24

obsidian

Answer 24

A dense, dark, glassy volcanic rock, usually of felsic composition.

Question 25

ophiolite suite

Answer 25

An unusual assemblage of rocks, characteristic of the seafloor but found on land, consisting of deep-sea sediments, submarine basaltic lavas, and mafic igneous intrusions.

Question 26

partial melting

Answer 26

The incomplete melting of a rock that occurs because the minerals that compose it melt at different temperatures.

Question 27

pegmatite

Answer 27

A vein of extremely coarse grained granite, crystallized from a water-rich magma in the late stages of solidification

Question 28

peridotite

Answer 28

A coarse-grained ultramafic intrusive igneous rock composed of olivine with small amounts of pyroxene and amphibole.

Question 29

pluton

Answer 29

A large igneous intrusion ranging in size from a cubic kilometer to hundreds of cubic kilometers, formed at depth in the crust.

Question 30

porphyry

Answer 30

A lava of mixed texture in which large crystals (phenocrysts) �float� in a predominantly fine crystalline matrix.

Question 31

pumice

Answer 31

A frothy mass of volcanic glass with a great number of holes (vesicles) that remain after trapped gas has escaped from the solidifying melt.

Question 32

pyroclast

Answer 32

A volcanic rock fragment ejected into the air during an eruption.

Question 33

rhyolite

Answer 33

A light-brown to gray, fine-grained extrusive igneous rock with a felsic composition.

Question 34

sill

Answer 34

A sheetlike concordant intrusion formed by the injection of magma between parallel layers of preexisting bedded rock.

Question 35

stock

Answer 35

An irregular mass of coarse-grained igneous rock less than 100 km2 in area.

Question 36

tuff

Answer 36

Any volcanic rock lithified from pyroclasts

Question 37

ultramafic rock

Answer 37

An igneous rock consisting primarily of mafic minerals and containing less than 10 percent feldspar.

Question 38

vein deposit (vein)

Answer 38

A sheetlike deposit of minerals precipitated in fractures or joints that are foreign to the host rock.

Question 39

viscosity

Answer 39

A measure of a fluid�s resistance to flow.

Question 40

volcanic ash

Answer 40

Extremely small fragments, usually of glass, that form when escaping gases force a fine spray of magma from a volcano.