2.14 Classification of igneous rocks

Question 1

What is meant by the term felsic?

Answer 1

felsic minerals are quartz and feldspar - minerals that are rich in silica

felsic is a contraction of feldspar and silica

Question 2

What is meant by the term mafic when applied to minerals?

Answer 2

mafic minerals are rich in magnesium and iron (they are sometimes also called ferromagnesian minerals) - it includes micas (biotite and muscovite), amphiboles, pyroxenes and olivine

mafic is a contraction of magnesium and ferric

Question 3

What is the silica content of an ultramafic rock?

Answer 3

<45% silica

Question 4

What is the silica content of a mafic rock?

Answer 4

45%-52%

Question 5

What is the silica content of an intermediate rock?

Answer 5

52-66%

Question 6

What is the silica content of a silicic rock?

Answer 6

> 66% silica

Question 7

What is the crystal size for coarse grained igneous rocks?

Answer 7

> 5mm

Question 8

What is the crystal size for a medium grained igneous rock?

Answer 8

1mm - 5mm

Question 9

What is the crystal size for a fine-grained igneous rock?

Answer 9

<1mm

Question 10

What is the name of a fine grained mafic rock?

Answer 10

basalt

Question 11

What is the name of a medium grained mafic rock?

Answer 11

dolerite

Question 12

What is the name of a coarse grained mafic rock?

Answer 12

gabbro

Question 13

What is the name of a fine grained intermediate rock?

Answer 13

andesite

Question 14

What is the name of a medium grained intermediate rock?

Answer 14

microdiorite

Question 15

What is the name of a coarse grained intermediate rock?

Answer 15

diorite

Question 16

What is the name of a coarse grained intermediate rock?

Answer 16

diorite

Question 17

What is the name of a fine grained silicic rock?

Answer 17

rhyolite

Question 18

What is the name of a medium grained silicic rock?

Answer 18

microgranite

Question 19

What is the name of a coarse grained silicic rock?

Answer 19

microgranite

Question 20

What is the name for an ultramafic rock?

Answer 20

peridotite

Question 21

How could you identify the approximate silica content of a rock in the field (and classify the rock)?

Answer 21

the colour - felsic minerals are light in colour while mafic minerals are darker

Question 22

Why are rhyolites relatively rare?

Answer 22

silica rich magma is viscous making it difficult for it to rise to the surface and create a fine-grained rock

Question 23

What is flow banding?

Answer 23

a banded, streaky appearance created in rhyolites due to the friction within the viscous magma as it flows

Question 24

What is obsidian?

Answer 24

a volcanic glass (high silica content)

Question 25

What is pumice?

Answer 25

a silica rich igneous rock with many vesicles (gas bubbles) and glassy texture, which is produced by the explosive eruption of the viscous magma

Question 26

What is an amygdale (resulting in amygdaloidal texture)?

Answer 26

vesicles (gas bubbles) can be filled in by minerals creating amygdales

Question 27

How can quartz be recognised in igneous rocks?

Answer 27

• crystals are grey in colour with the appearance of broken bottle glass
• the crystals lack cleavage
• the crystals have an amorphous shape as they crystallised in an enclosed space

Question 28

How can feldspars be recognised in igneous rocks?

Answer 28

• K feldspar is pink in colour
• plagioclase is usually grey or white feldspar
• if you look at it with a hand lens you will see the glassy appearance and smooth, straight cleavage surfaces

Question 29

How can biotite and muscovite mica be recognised in igneous rocks?

Answer 29

• biotite crystals are shiny and black
• muscovite crystals are silvery flakes

Question 30

Why is silica percentage not the same as the percentage of quartz?

Answer 30

quartz is made entirely of silica but the silica percentage includes all the siica in all of the rock-forming minerals - a rock with no quartz will still contain silica in silicate minerals such as olivine

silica content cannot be seen in the field - colour can be an indicator