Igneous Rock Study Guide

Question 1

Aphanitic

Answer 1

microscopic crystals

Question 2

Phaneritic

Answer 2

visible crystals

Question 3

Glassy

Answer 3

no crystal structure due to rapid cooling

Question 4

Porphyritic

Answer 4

visible crystals in aphaneritic groundmass

Question 5

Pegmatitic

Answer 5

very large crystals visible

Question 6

Felsic

Answer 6

rocks with high amounts of light-colored minerals, such as quartz and feldspar. Silica rich (65%)

Question 7

Mafic

Answer 7

rocks contain dark-colored ferromagnesian minerals, such as pyroxene and amphibole. Iron rich (50% silica).

Question 8

Intermediate

Answer 8

rocks with a middle composition between the felsic and mafic.

Question 9

Ultramafic

Answer 9

rocks contain high amount of olivine and pyroxene

Question 10

How does an igneous rock form?

Answer 10

When magma cools

Question 11

Intrusive

Answer 11

• magma solidifies underground
• slow cooling
• large mineral crystals

Question 12

Extrusive

Answer 12

• lava solifies at the surface
• fast cooling
• small mineral crystals

Question 13

Is phaneritic texture associated with intrusive or extrusive?

Question 14

vesicular

Answer 14

rock contains gas bubbles (vesicles)

Question 15

pryoclastic

Answer 15

fragmental texture, large clasts in a matrix of volcanic ash.