Lesson 5: Magma Generation and Igneous Rocks

Question 1

Igneous rocks form when:

Answer 1

a melt solidifies at or below the Earth’s surface

Question 2

The molten rock beneath the Earth’s surface is called ____.

Answer 2

Magma

Question 3

The greatest amount of magma is created at

Answer 3

Divergent plate boundaries

Question 4

How is magma generated at subduction zones?

Answer 4

Addition of volatiles

Question 5

What type of magma contains the lowest amount of silica?

a. Felsic
b. Intermediate
c. Mafic
d. All magmas contain the same amount of silica

Answer 5

c. Mafic

Question 6

_______ is a mafic extrusive igneous rock.

Answer 6

Basalt

Question 7

A _________ is a concordant igneous rock body which runs parallel to pre-existing bedrock.

Answer 7

Sill

Question 8

Magma rises toward the Earth’s surface because:

Answer 8

it is less dense than the surrounding rocks

Question 9

Why are intrusive igneous rocks from a batholith generally coarser-grained than those from volcanic vents?

Answer 9

They cooled more slowly and thus had more time to grow

Question 10

A _______ texture indicates two stages of cooling.

Answer 10

Porphyritic

Question 11

Magma

Answer 11

molten/partially molten material found beneath the surface of the Earth

Question 12

Lava

Answer 12

What we call magma that reaches the Earth’s surface.

Question 13

Where does magma form in terms of plate tectonics?

Answer 13

Mostly at divergent plate boundaries
- mid-ocean ridges
- Continental rifts
Few at convergent plate boundaries
- subduction zones

Question 14

What are the three ways to generate magma

Answer 14

1. Decompression melting
2. Addition of volatiles
3. Addition of heat

Question 15

What is decompression melting?

Answer 15

a rapid decrease in pressure that can cause the mantle to melt without an increase in temperature.

Question 16

Where do we see decompression melting in terms of plate tectonics?

Answer 16

Divergent plate boundaries

Question 17

What is addition of volatiles?

Answer 17

flux melting that occur due to chemical compounds (water) lowering the melting temperature of the rock

Question 18

Where do we see addition of volatiles in terms of plate tectonics?

Answer 18

Subduction zones

Question 19

What is addition of heat?

Answer 19

Inducing melting if temperature exceeds melting temperature

Question 20

Where do we see addition of heat in terms of plate tectonics?

Answer 20

Mantle plumes/hotspots

Question 21

How does magma evolve?

Answer 21

1. Fractionation
2. Assimilation
3. Magma mixing

Question 22

What is fractionation?

Answer 22

The composition of magmas change as the magma cools

Question 23

What is assimilation?

Answer 23

The chemical evolution of magma is influenced by its reaction with, and digestion of the wall rock that it encounters during its ascent.

Question 24

What is magma mixing?

Answer 24

during the ascent of two chemically different magma bodies, the more buoyant mass may overtake the slower-rising body.

Question 25

Compare intrusive and extrusive igneous rocks.

Answer 25

Intrusive: below the surface of the earth; slow cooling of magma Extrusive: on the surface of the earth; rapid cooling

Question 26

What are the results of intrusive enviornments? (5)

Answer 26

1. Dike 2. Sill 3. Lacccolith 4. Plutons 5. Batholiths

Question 27

What is the difference between a dike and a sill?

Answer 27

Dike is a vertical intrusion and cuts across sills Sill is a horizontal intrusion that form parallel to existing features instead of cutting through it

Question 28

Describe laccoliths.

Answer 28

A mushroom-shaped intrusive body that has intruded between layers of sedimentary rock. Rising magma forces the overlying layers to rise into a dome

Question 29

Describe plutons.

Answer 29

A blob-like igneous intrusion

Question 30

Describe batholiths.

Answer 30

A huge blob-like igneous intrusion , formed as composite of many plutons

Question 31

What are the different compositions of magma (igneous rocks)?

Answer 31

1. Felsic 2. Intermediate 3. Mafic

Question 32

Describe the composition of felsic rocks

Answer 32

High in silica Low in iron and magnesium Lighter in color

Question 33

Describe the composition of intermediate rocks

Answer 33

Lighter than felsics Average silica, iron, and magnesium

Question 34

Describe the composition of mafic rocks

Answer 34

Low in silica High in iron and magnesium Darker in color than intermediate and felsic rocks.

Question 35

What are the textures of igneous rocks?

Answer 35

1. Aphantic - fine-grained 2. Phaneritic- course-grained 3. Porphyritic - 2 sizes of crystals 4. Glassy 5. Pyroclastic - various fragments 6. Pegmatitic - exceptionally coarse-grained 7. Vesicular - bubbles

Question 36

How are aphanitic textures characterized? (3)

Answer 36

No visible grains to the naked eye Rapid cooling of lava/magma May contain vesicles

Question 37

How are phaneritic textures characterized? (3)

Answer 37

Crystals large enough to see with the naked eye Slow cooling of magma Intrusive origin

Question 38

How are porphyritic textures characterized? (3)

Answer 38

Two distinct sizes of crystals (large and small) Minerals form at different temperatures and rates Usually indicates two stages of cooling Larger crystals are phenocrysts Finer crystals are ground-pass

Question 39

How are glassy textures characterized?

Answer 39

Very rapid cooling of molten rock Extrusive origin

Question 40

How are pyroclastic textures characterized? (2)

Answer 40

various fragments Extrusive origin Tuff if the largest fragments are less than 2.5 inches Volcanic breccia if fragments are larger than 2.5 inches

Question 41

How are pegmatitic textures characterized?

Answer 41

Exceptionally coarse-grained Forms in the late stages of crystallization of granitic magmas

Question 42

How is vesicular texture characterized?

Answer 42

Gas bubbles trapped in colling lava Extrusive origin