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A-Level Geology (2nd Year) > Igneous Petrology > Flashcards

Igneous Petrology Flashcards

This assumes you already know 1st Year Igneous Rocks

1
Q

What are the technical names for the two branches in Bowen’s reaction series?

A

Continuous series (right)
Discontinuous series (left)

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2
Q

What does the continuous series represent (Bowen’s)?

A

The crystallisation of plagioclase feldspar from Ca-rich to Na-rich when temperatures decrease

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3
Q

When do the continuous and discontinuous series occur (in relation to each other)?

A

They both happen at the same time

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4
Q

What minerals are present on the discontinuous series of Bowen’s reaction series?

A

Olivine, Pyroxene, Amphibole, Biotite

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5
Q

What is the mnemonic to remember the discontinuous series?

A

Old People Are Boring

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6
Q

In the discontinuous series, what type of pyroxene is typically crystallised?

A

Augite

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7
Q

In the discontinuous series, what type of Amphibole is typically crystallised?

A

Hornblende

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8
Q

What does the discontinuous series represent? (Bowen’s)

A

The crystallisation if minerals rich in iron & magnesium (mafic).

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9
Q

What temperatures does olivine crystallise?

A

> 1500C

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10
Q

Why does the discontinuous series happen during cooling?

A

Slow cooling allows the minerals to react with the magma to form lower temperature minerals (e.g. olivine+magma+time=Pyroxenes)

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11
Q

How could olivine be preserved and why?

A

Fast cooling (eruption) could allow for preserve. There is not enough time for the olivine to react with the magma

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12
Q

What forms when a reaction is incomplete in the discontinuous series?

A

A reaction rim

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13
Q

What is a reaction rim?

A

One minerals surrounding another. Results from a reaction between the inner minerals with the melt, to form the outer mineral

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14
Q

What type of olivine forms first at higher temperatures?

A

Magnesium-rich (Forsterite)

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15
Q

At what temperatures does forsterite form?

A

1500C

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16
Q

What type of olivine forms last (out of the olivines) at lower temperatures?

A

Iron-rich (Fayalite)

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17
Q

What temperature does fayalite form?

A

1400C

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18
Q

Why does olivine crystallisation progress from Mg-rich to Fe-rich?

A

As the temperatures temperatures decrease, iron substitutes for magnesium

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19
Q

What type of plagioclase feldspar crystallisation first at higher temperatures?

A

Calcium-rich (Anorthite)

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20
Q

What type of plagioclase feldspar crystallisation first at lower temperatures?

A

Sodium-rich (Albite)

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21
Q

What temperatures does Anorthite to crystallise? (Bowen’s)

A

<1500C

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22
Q

What temperatures does Albite crystallise? (Bowen’s)

A

> 700C

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23
Q

In what type of rocks can you find Anorthite (Ca-rich)?

A

Ultramafic and mafic

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24
Q

In what type of rocks can you find Albite (Na-rich)?

A

Intermediate and sometimes silicic

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25
Q

How do zoned crystals form?

A

At high temperatures, crystallisation is mafic. As the magma cools, more layers are added to the crystal which may be more intermediate or silicic

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26
Q

What example of zoned crystals do we use?

A

Plagioclase feldspar. Ca-rich feldspar found in the centre and Na-rich feldspar found at the outer layers

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27
Q

When do the lower temperature minerals form in Bowen’s reaction series?

A

When the discontinuous and continuous branches merge (the bottom)

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28
Q

At what temperatures do the low temperature minerals start to form? (Bowen’s)

A

Around 700C (and lower)

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29
Q

What are the low temperature minerals? (Bowen’s)

A

Orthoclase (k-feldspar)
Muscovite
Quartz

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30
Q

What is the mnemonic used to remember the lower temperature minerals? (Bowen’s)

A

Oiled Male Queers

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31
Q

What is different about the minerals that remain below 700C? (Bowen’s)

A

They will not react with the remaining liquid

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32
Q

What type of minerals are remaining at temperatures below 700C? (Bowen’s)

A

Those rich in silica. Silicic/felsic

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33
Q

When does a eutectic system occur (melt conditions)?

A

When a melt is homogenous

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34
Q

When happens when crystals form in a homogenous melt?

A

The crystals are immiscible (don’t mix), and become non-homogenous to form two solid phases

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35
Q

How can a eutectic system be shown/illustrated?

A

On a eutectic phase diagram

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36
Q

What does the liquidus line tell us?

A

Everything above the line is a melt. Everything below the line is solid or mush

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37
Q

What is a mush?

A

Solid+melt. or crystals+melt

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38
Q

What is the liquidus?

A

A phase boundary showing the temperature that the last solid particle melt

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39
Q

What is the solidus?

A

A phase boundary showing the temperature a rock first begins to melt when heated

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40
Q

What is the eutectic point?

A

The point where all three phases could exist.
Solid, mush, or melt

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41
Q

What is the end composition like in a eutectic phase diagram?

A

The same as the original composition of the melt

42
Q

What is the technical name for a ‘two-blob’ diagram?

A

Binary phase diagram

43
Q

What is the technical name for a ‘one-blob’ diagram?

A

Solid solution phase diagram

44
Q

Where do solid solution phase diagrams get their nickname from?

A

One-blob diagrams.
There is only one region for crystals+mush because there is only one type of mineral

45
Q

How do you record the original composition using the new composition in a solid solution phase diagram?

A

Move down from the new composition until you reach the mush section.
Move horizontally to the other end of this section.
Then continue to drop down and record.

46
Q

What is worth remembering about the X-axis in eutectic diagrams?

A

It measures from B
This is for solid solution and binary phase diagrams

47
Q

What are batholiths?

A

Batholiths are bigger than plutons.
Batholiths are aggregates of plutons

48
Q

What is a large igneous province?

A

A large accumulation of igneous rock, e.g. Batholith

49
Q

What does LIP stand for?

A

Large igneous province

50
Q

What reasons may a large igneous intrusion display different rock types?

A

Non-homogenous magma.
Different cooling rates.
Magma differentiation.
Intrusions of new magma.

51
Q

Why would a non-homogenous magma result in a large igneous intrusion displaying different rock types?

A

There are different compositions within the magmas which crystallise into different rock types

52
Q

Magmas are usually immiscible, so in what situations may they mix?

A

If there is a strong mixing mechanism. For example if there are strong convection currents

53
Q

How does magmatic contamination occur?

A

Stoping and assimilation. Rocks entering magma through stoping are xenoliths until they are assimilated to then change the bulk composition

54
Q

What are the three types of magmatic differentiation?

A

Fractional crystallisation, gravity settling, filter pressing

55
Q

What is magma differentiation?

A

When there are different compositions and rock types found across one magma

56
Q

What is fractional crystallisation?

A

When the composition changes over time. Different minerals form at different temperatures.
For example, Olivine and pyroxene crystallisation takes the mafic material out of the magma to form more silicic rocks.

57
Q

What is gravity settling?

A

The solid version of any substance is denser than its liquid form. The denser material sinks to the bottom off the . chamber (gravity). It forms cumulate layers with the densest material at the bottom

58
Q

What is filter pressing?

A

When the mass of overlying crystals squeezes the liquid out and forming a layer above. The more mass on top, the more pressure (less pore space), the more liquid layers

59
Q

What is the composition of the liquids in filter pressing?

A

Silicic/felsic

60
Q

What are the two real world examples of magmatic differentiation we study?

A

The Palisades sill.
The Skaergaard intrusion

61
Q

Where is the palisades sill?

A

New Jersey, NE USA

62
Q

What is different about the palisades sill from other sills?

A

Sills are usually known as small igneous intrusions, but the Palisades sill is 300m deep

63
Q

How is gravity settling present in the Palisades sill?

A

There is an olivine-rich layer at the bottom

64
Q

What rock is the chilled margin of the Palisades sill?

A

A fine basalt. It is the same composition as the original intrusion

65
Q

Why does the basalt in the Palisades sill represent the composition of the original intrusion?

A

Because the crystals cooled too quickly for magma differentiation to occur here

66
Q

What was the country rock around the Palisades sill?

A

Sandstone

67
Q

What rock type is the baked margin of the Palisades sill?

A

Metaquartzite

68
Q

How is fractional crystallisation present in the Palisades sill?

A

The olivine cooled first, which removed the ultramafic material from the magma, allowing for mafic minerals to form instead

69
Q

What rocks are found in the middle of the Palisades sill?

A

Gabbro in the centre. Dolerite around this. These have the same composition but cooled at different types

70
Q

When was the Skaergaard intrusion intruded?

A

The tertiary period

71
Q

Where is the Skaergaard intrusion?

A

Greenland

72
Q

What is the marginal border zone of the Skaergaard intrusion?

A

The chilled margin.
Finer grained basalt and has similar composition to the original melt.

73
Q

Why is the chilled margin of the Skaergaard intrusion not exactly the same as the original composition?

A

It has been contaminated by assimilation of the country rock and alteration of hot liquids

74
Q

What is the country rock of the Skaergaard intrusion?

A

Gneiss

75
Q

What is the bottom layer of the Skaergaard intrusion?

A

The lower zone layered series

76
Q

What is the lower zone layered series?

A

Here there are rhythmic layers of olivine underlying less dense layers of plagioclase. Cumulate layers

77
Q

What magmatic differentiation processes are involved in the lower zone layered series?

A

Gravity settling (olivine)
Filter pressing
Fractional crystallisation (olivine + pyroxenes first)

78
Q

Why are there alternating cumulate layers?

A

Density driven convection within the magma chamber

79
Q

What is the upper border series?

A

Mirrors the lower zone layered series, but thinner

80
Q

How did the upper border series form?

A

Early formed crystals are carried by convection currents and stick to the top of the chamber

81
Q

Why is there a section of silicic rock withing the upper border series?

A

After the mafic material has crystallised, only the silicic magma remains. The last material to crystallise is the most evolved, most silicic and the lowest melting point materials.

82
Q

What is special about the Bushveld igneous complex?

A

It contains some of the richest ore deposits on Earth and the largest reserves of platinum group elements

83
Q

How big is the Bushveld igneous complex?

A

400 km x 800 km with a total volume of over 1,000,000 km^3.
In some areas, it is up to 9km thick

84
Q

What is the Bushveld igneous complex?

A

A large layered igneous intrusion in South Africa

85
Q

What are the platinum group elements?

A

Platinum, palladium, osmium, iridium, ruthenium, rhodium

86
Q

What is the mnemonic for remembering the platinum group elements?

A

Rude People Really Irritate Old People

87
Q

Why are platinum group elements rare in the crust?

A

Due to their iron-loving nature, they’re usually found in the core

88
Q

What happens to platinum group elements in the absence of other minerals?

A

They react with sulphides and act like chalcophiles

89
Q

Quartz

A

Hardness of 7. Framework tetrahedra. All oxygens are bridging. Vitreous. White/grey. No streak

90
Q

Muscovite Mica

A

Light in colour. Breaks into sheets with a perfect cleavage in one directions. Sheet silica tetrahedra. 2-2.5 hardness

91
Q

Biotite mica

A

Dark in colour. Perfect cleavage in one directions. Sheet silica tetrahedra. 2-3.5 hardness

92
Q

Pyroxene

A

In igneous and metamorphic rocks.
Vitreous and dark. White streaks. 5-7 hardness. Mafic. Single chain silicate

93
Q

Augite

A

Most common type of pyroxene. Near 90 cleavage. Brittle with a 5.5-6 hardness. Dark blue or green. White streak. Vitreous. Single chain silicate

94
Q

Diopside

A

Type of pyroxene.
5-6 hardness. Green, black, yellow, brown. Two perfect cleavage. Vitreous. Single chain silicate

95
Q

Amphibole

A

Dark coloured. Colours range. Chain silicate. Hornblende is a type of amphibole

96
Q

Hornblende

A

Most common type of amphibole.
Black or dark green. Cleavage of 60 and 120. 5-6 hardness. Vitreous or dull

97
Q

Orthoclase feldspar (K-feldspar)

A

Light in colour. Green, yellow, pink. White streak. Vitreous. Hardness 6-6.5. Two cleavages at 90

98
Q

Plagioclase feldspar

A

White, grey. Hardness 6-6.5. White streak. Dominate mafic igneous rocks. Framework silicate

99
Q

Olivine

A

Include forsterite and fayalite. No cleavage. Green, pale yellow. Ultramafic

100
Q

Forsterite

A

Type of olivine. Magnesium rich. Green, yellow

101
Q

Fayalite

A

Type of olivine. Iron rich. Pale yellow, amber. Cleavage at 10 and 100

A-Level Geology (2nd Year) (7 decks)

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