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GEOL-1403 > Lesson 5: Magma Generation and Igneous Rocks > Flashcards

Lesson 5: Magma Generation and Igneous Rocks Flashcards

1
Q

Igneous rocks form when:

A

a melt solidifies at or below the Earth’s surface

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2
Q

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

A

Magma

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3
Q

The greatest amount of magma is created at

A

Divergent plate boundaries

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4
Q

How is magma generated at subduction zones?

A

Addition of volatiles

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5
Q

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

A

c. Mafic

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6
Q

_______ is a mafic extrusive igneous rock.

A

Basalt

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7
Q

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

A

Sill

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8
Q

Magma rises toward the Earth’s surface because:

A

it is less dense than the surrounding rocks

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9
Q

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

A

They cooled more slowly and thus had more time to grow

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10
Q

A _______ texture indicates two stages of cooling.

A

Porphyritic

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11
Q

Magma

A

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

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12
Q

Lava

A

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

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13
Q

Where does magma form in terms of plate tectonics?

A

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

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14
Q

What are the three ways to generate magma

A
  1. Decompression melting
  2. Addition of volatiles
  3. Addition of heat
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15
Q

What is decompression melting?

A

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

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16
Q

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

A

Divergent plate boundaries

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17
Q

What is addition of volatiles?

A

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

18
Q

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

A

Subduction zones

19
Q

What is addition of heat?

A

Inducing melting if temperature exceeds melting temperature

20
Q

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

A

Mantle plumes/hotspots

21
Q

How does magma evolve?

A
  1. Fractionation
  2. Assimilation
  3. Magma mixing
22
Q

What is fractionation?

A

The composition of magmas change as the magma cools

23
Q

What is assimilation?

A

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

24
Q

What is magma mixing?

A

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

25
Q

Compare intrusive and extrusive igneous rocks.

A

Intrusive: below the surface of the earth; slow cooling of magma

Extrusive: on the surface of the earth; rapid cooling

26
Q

What are the results of intrusive enviornments? (5)

A
  1. Dike
  2. Sill
  3. Lacccolith
  4. Plutons
  5. Batholiths
27
Q

What is the difference between a dike and a sill?

A

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

28
Q

Describe laccoliths.

A

A mushroom-shaped intrusive body that has intruded between layers of sedimentary rock.

Rising magma forces the overlying layers to rise into a dome

29
Q

Describe plutons.

A

A blob-like igneous intrusion

30
Q

Describe batholiths.

A

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

31
Q

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

A
  1. Felsic
  2. Intermediate
  3. Mafic
32
Q

Describe the composition of felsic rocks

A

High in silica
Low in iron and magnesium
Lighter in color

33
Q

Describe the composition of intermediate rocks

A

Lighter than felsics
Average silica, iron, and magnesium

34
Q

Describe the composition of mafic rocks

A

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

35
Q

What are the textures of igneous rocks?

A
  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
36
Q

How are aphanitic textures characterized? (3)

A

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

37
Q

How are phaneritic textures characterized? (3)

A

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

38
Q

How are porphyritic textures characterized? (3)

A

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

39
Q

How are glassy textures characterized?

A

Very rapid cooling of molten rock
Extrusive origin

40
Q

How are pyroclastic textures characterized? (2)

A

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

41
Q

How are pegmatitic textures characterized?

A

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

42
Q

How is vesicular texture characterized?

A

Gas bubbles trapped in colling lava
Extrusive origin

GEOL-1403 (11 decks)

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