Chapter 4: Magma and Igneous Rocks

Question 1

magma

Answer 1

melt underground

Question 2

lava

Answer 2

melt that has emerged from within the earth onto the surface

Question 3

igneous rock

Answer 3

any rock that forms from the ‘freezing’ (solidifying) of melt

Question 4

extrusive igneous rock and some examples

Answer 4

igneous rock that formed from lava; rocks that formed on the surface of the Earth, some examples include cooled/solidified lava flow, pyroclastic debris (cemented together clasts of volcanic rock, or volcanic ash.

Question 5

intrusive igneous rock

Answer 5

rock formed as a result of cooled magma; igneous rocks that formed inside the earth’s crust

Question 6

causes of melt

Answer 6

• decompression: pressure, even in very hot rock prevents melt, so a decrease in pressure can trigger melt
• addition of volatiles: when volatiles (such as CO2 and H2O) mix with hot, dry rock, they react with minerals and break chemical bonds so that the rock begins to melt (flux melting)
• heat transfer: hot magma that rises from the mantle into the crust brings heat with it which may be enough to cause heat-transfer melting

Question 7

How is it that melt has a variety of possible compositions?

Answer 7

• source rock composition: different source rock result in different melt composition
• partial melting: temperatures never rise high enough to melt the entire source rock
• assimilation: when rock surrounding the melt/magma chamber melt and fall into the melt
• magma mixing: when distinct magmas mix to yield a new, different magma

Question 8

viscosity

Answer 8

the resistance to flow; speed of flow

• high (more) viscosity = slow
• low (less) viscosity = fast

Question 9

which is more viscous, mafic or felsic melt?

Answer 9

Mafic melt is less viscous than felsic melt because relatively more silcon-oxygen tetrahedra occur in felsic melt (making it more viscous - slow)

Question 10

factors that contribute to cooling of magma

Answer 10

• depth of the intrusion; deeper means hotter which means it cools slower than melt closer to the surface of the Earth
• size and shape of the magma body: large bodies cool slower than small bodies
• presence of circulating groundwater: water carries away heat so presence of water helps to cool the magma faster

Question 11

intrusive igneous shapes

Answer 11

dykes: vertical intrusions
sills: horizontal intrusions
laccoliths: when a horizontal intrusion ‘blisters up’ forming a dome shape
plutons: blob shaped, typically form when magma in a magma chamber solidifies
batholith: a vast body of plutons

Question 12

xenolith

Answer 12

a chunk of wall rock that doesn’t melt entirely and falls into the magma body

Question 13

igneous rock textures

Answer 13

• crystalline
• fragmentary
• glassy

Question 14

crystalline texture

Answer 14

interlocking crystals due to different crystal growth rates; crystalline rocks can be described as phaneritic (large crystals) or aphanitic (fine-graineg/small crystals)

Question 15

fragmental igneous rocks

Answer 15

result from pyroclastic debris and consists of chunks or shards of rock that are packed together (cemented)

Question 16

glassy rock

Answer 16

means rapid cooling, no time for crystals to grow

Question 17

types of crystalline igneous rock

Answer 17

from low to high silica content

• ultramafic
• mafic
• intermediate
• felsic

Question 18

fast cooling igneous rock from mafic to felsic

Answer 18

• basalt
• andesite
• rhyolite

Question 19

slow cooling igneous rock from mafic to felsic

Answer 19

• gabbro
• diorite
• granite

Question 20

tectonically speaking: where does igneous activity occur?

Answer 20

• mantle plume
• subduction zones (convergent plate boundary)
• mid-ocean ridges (divergent plate boundary)
• continental rifts (divergent plate boundary)