Chapter 6: Igneous Rocks and Silicate Minerals

Question 1

magma

Answer 1

molten rock that originates beneath Earth’s surface

Question 2

magma chambers

Question 3

what features contain the most magma at or near Earth’s surface?

Answer 3

mid-ocean ridges and subduction zones; also found at continental rifts and hotspots

Question 4

why does magma move to the surface?

Answer 4

it is buoyant and less dense than the solid upper mantle and crust

Question 5

intrusive igneous rock

Answer 5

formed as magma crystallizes before it reaches the surface

Question 6

pluton

Answer 6

a general term given to any intrusive igneous rock; plutonic and intrusive can be used interchangeably

Question 7

most common intrusive rock bodies

Answer 7

plutons, dikes, and sills

Question 8

extrusive igneous rock

Answer 8

formed from flows or pyroclastic material that lithifies on the surface; generally called volcanic rocks although a volcano may not be involved

Question 9

pyroclastic material

Answer 9

ash and other debris of variable size

Question 10

granite

Answer 10

common plutonic rock containing quartz and K-spar easily seen with the naked eye

Question 11

rhyolite

Answer 11

common volcanic rock containing quartz and K-spar with microscopic crystals; occur in same settings as basalt and andesite but only common in continental areas, especially abundant at hot spots

Question 12

obsidian

Answer 12

a volcanic rock composed of volcanic glass, formed when lava cools so quickly that no crystals form

Question 13

porphyry

Answer 13

a rock in which coarse crystals (phenocrysts) are within a matrix of fine-grained crystals (groundmass)

Question 14

phenocrysts

Question 15

groundmass

Question 16

aphanitic

Answer 16

very fined grained, difficult to see crystals with the naked eye

Question 17

phaneritic

Answer 17

very coarse grained

Question 18

porphyritic

Answer 18

containing combinations of large and small crystals

Question 19

silica content of basalt and gabbro

Answer 19

45-55 wt%

Question 20

silica content of andesite and diorite

Answer 20

55-65 wt%

Question 21

silica content of rhyolite and granite

Answer 21

> 65 wt%

Question 22

basalt

Answer 22

the most abundant igneous rock in Earth’s crust, containing plagioclase, pyroxene, and often volcanic glass; forms abundantly at mid-ocean ridges

Question 23

pillow basalt

Answer 23

forms as basalt is extruded into cold ocean water

Question 24

andesite

Answer 24

the most common volcanic rock after basalt, always rich in plagioclase and may contain biotite, pyroxene, or hornblende (quartz and olivine are rare); common in island arcs and at continental margin subduction zones

Question 25

granitoids

Answer 25

granites, granodiorites, and related rocks; major components of the continental crust

Question 26

batholith

Answer 26

mass of intrusive rock (mostly granite and diorite) composed of multiple plutons; form the core of many mountain ranges

Question 27

gabbro

Answer 27

plutonic rock equivalent in composition to basalt; make up the lower part of oceanic crust

Question 28

silicic

Answer 28

Si-rich

Question 29

felsic

Answer 29

contraction of feldspar and silica; used to refer to any light-colored igneous rock

Question 30

mafic

Answer 30

contraction of the words magnesium and ferric

Question 31

ultramafic

Question 32

intermediate

Answer 32

rocks with compositions between mafic and silicic

Question 33

alkalic

Answer 33

rocks with high (K2O + Na2O):CaO ratios