Chap 4 Magma and Igneous Rocks

Question 1

Geothermal gradient

Answer 1

The rate at which the Earth’s temperature increase with
dept

Question 2

Formation of Magma

Answer 2

 Pressure release = decompression melting= Geothermal gradient

 Heat transfer from rising magma

 Addition of volatiles
Wet magma = up to 15% volatiles ∴ melts at lower temps
Dry Magma = no volatiles ∴ melts at higher temps

Question 3

What is magma made of?

Answer 3

1. Liquid = melt
2. Solids = silicates
3. Volatiles ( AND )

Question 4

Magma types:

Answer 4

Felsic,
Intermediate,
Mafic,
Ultramafic

Question 5

Felsic magma

Answer 5

%: 66-76%
Density: Very low
Temp: Very low
Viscosity: Very high: Explosive
Plutonic: Granite
(Obsidian
=glass)
Volcanic: Rhyolite

Question 6

Intermediate magma:

Answer 6

%: 52-66%
Density: Low
Temp: Low
Viscosity: High: Explosive
Plutonic: Diorite
Volcanic: Andesite

Question 7

Mafic magma:

Answer 7

%: 45-52%
Density: High
Temp: High
Viscosity: Low: thin,
hot, runny
Plutonic: Gabbro
Volcanic: Basalt

Question 8

Ultramafic magma:

Answer 8

%: 38-45%
Density: Very High
Temp: Very high
Viscosity: Very low
Plutonic: Peridotite
Volcanic: Komatiite

Question 9

Processes involved in magma variation

Answer 9

 Partial melting
 Assimilation
 Mixing magmas
 Fractional crystallisation

Question 10

Partial melting

Answer 10

= different types of rock melt/partially melt by different amounts
∴ partial melting of: Ultramafic –> Mafic –> Intermediate –> Felsic

Question 11

Assimilation

Answer 11

= rock surrounding magma may partially melt and mix with original magma

Question 12

Mixing magmas

Answer 12

= one magma composition can be derived from another during
crystallisation

Question 13

Fractional crystallisation

Answer 13

= magmas of different compositions can mix to produce new, inbetween compositions

Question 14

Characteristics of intrusive igneous rock

Answer 14

 Baked zone
 Chill margin
 Xenoliths

Question 15

Baked zone

Answer 15

= rim of heat
altered country rock

Question 16

Chill margin

Answer 16

= magma at
contact that cooled rapidly

Question 17

Xenoliths

Answer 17

= country rock
fragment in magma
∴ magma cooled before xenolith could
be assimilated

Question 18

Invasion in colder country rock

Answer 18

 Thermal metamorphism and melting
 Inflation of fractures pushing the rock aside
 Incorporation of country rock fragments (xenolith)
 Hydrothermal (hot water) alteration

Question 19

Intrusive rock bodies

Answer 19

 Concordant
 Discordant
 Tabular
 Plutonic

Question 20

Concordant Intrusive rock body

Answer 20

= does not cut across rock layers (sill, locolith, lopolith)

Question 21

Discordant Intrusive rock body

Answer 21

= cuts across rock layers (batholith, stock, dyke)

Question 22

Tabular Intrusive rock body

Answer 22

= flat with uniform thickness (sill, dyke)

Question 23

Plutonic Intrusive rock body

Answer 23

= body/blob of intrusive igneous rock (batholith, stock, lopolith, locolith)

Question 24

Factors affecting the cooling time of magma

Answer 24

1. Depth of intrusion
2. Shape and size
3. Presence of circulating ground water

Question 25

Types of textures

Answer 25

1. Glassy
2. Interlocking mineral crystals/ crystalline
3. Fragmental

Question 26

Glassy texture

Answer 26

= fast cooling OR highly viscous; made of solid glass or solid shards

Question 27

Interlocking mineral crystals/ crystalline texture

Answer 27

= slow cooling; minerals that fit like jigsaw pieces

Question 28

Fragmental texture

Answer 28

= pieces of pre-existing rocks

o Aphanitic = finely crystalline, only microscopically visible ∴ rapid cooling

o Phaneritic = coarsely crystalline, mm-cm

o Porphyritic = a mixture of coarse and fine grained (Indicates a 2-stage history ∴ initial slow cooling and subsequent eruption cools magma quickly)

o Pegmatitic = exceptionally coarse mineral crystals; form from water rich melts in
which atoms can move around so rapidly (not very viscous) that large crystals can
grow very quickly