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BIOS1013 - Environmental Geoscience > Lecture 3 - Igneous Rocks > Flashcards

Lecture 3 - Igneous Rocks Flashcards

1
Q

What are igneous rocks?

A

Rocks that are formed through the cooling and solidification of magma.

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2
Q

What is magma?

A

Molten rock generated deep beneath the Earth’s surface.

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3
Q

What is lava?

A

Magma on the Earth’s surface.

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4
Q

What conditions are required for rocks to melt?

A

Depressurisation.

Adding water or other volatiles.

Heating to the point of melting.

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5
Q

What are the three common types of magma?

A

Basaltic (45-55% SiO₂)

Andesitic (55-65% SiO₂)

Rhyolitic (>65% SiO₂)

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6
Q

Which minerals decrease in concentration when the silica content increases?

A

MgO and CaO.

FeO and Fe₂O₃.

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7
Q

What percentage of the weight of magma is formed of dissolved gases?

A

0.2 - 3%

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8
Q

What is the main dissolved gas found in magma?

A

Water vapour.

Water vapour and carbon dioxide account for 98% of emissions from volcanoes.

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9
Q

What is the temperature of the different magma types?

A

Rhyolitic - 600 - 900°C

Andesitic - 800 - 1000°C

Basaltic - 1200°C

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10
Q

What does the viscosity of magma depend on?

A

The temperature and composition.

A higher temperature results in a lower viscosity.

A higher silica content results in a higher viscosity.

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11
Q

What is pahoehoe?

A

A lower viscosity lava.

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12
Q

What happens when magma cools?

A

Crystallisation occurs.

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13
Q

What can happen when minerals solidify at different temperatures?

A

The crystals can be separated from the melt.

This causes the magma to be of a different composition.

The crystals left behind form a rock with a different composition from that of the magma.

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14
Q

How can crystals and magma become separated?

A

By compression.

By sinking of dense, early crystallised minerals to the bottom of the magma chamber.

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15
Q

What is Bowen’s Reaction series?

A

He noticed that basaltic magmas are more common than rhyolitic or andesitic magmas.

This is because basaltic magmas are formed first and other magmas are derived from basaltic magmas by magnetic differentiation.

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16
Q

What is Bowen’s continuous reaction series?

A

Feldspars in basaltic magma are rich in calcium whereas feldspars in rhyolitic magma are sodium rich.

This means that the first plagioclases that form are calcium rich and as crystallisation proceeds (the ratio of crystal:melt increases), they become more sodium rich.

As crystallisation continues calcium is lost and sodium is gained.

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17
Q

What can cause zoned crystals to form?

A

Plagioclase composition changing continuously but the crystal structure remains unchanged.

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18
Q

What is Bowen’s discontinuous reaction series?

A

Olivine forms first in a cooling basaltic magma.

Crystallisation of olivine leaves the residual magma higher in Si.

The olivine reacts with the Si in the melt to form a more Si rich mineral, pyroxene.

The pyroxenes can then react to form amphiboles which can then react to form biotites.

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19
Q

What is the grain size of intrusive rocks?

A

They are coarse grained.

This is because they cool slowly.

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20
Q

What is the grain size of extrusive rocks?

A

They are fine grained.

This is because they cooled rapidly.

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21
Q

What is similar between rocks of different grain size?

A

They have a similar chemistry.

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22
Q

What are phenocrysts?

A

Isolated large grains.

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23
Q

What is an igneous rock containing phenocrysts described as being?

A

Porphyritic.

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24
Q

How do phenocrysts form?

A

The crystals form early in the melt and grow to a large size.

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25
Q

What is buoyant magma?

A

Magma that is less dense than the solid rock from which it forms.

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26
Q

How does gas dissolve in magma?

A

As magma rises, the pressure on the rock decreases.

Pressure controls the amount of gas a magma can dissolve (more at a high pressure and less at a low pressure).

Gas dissolved in an upward moving magma comes out of solution and forms bubbles.

27
Q

What are vesicles?

A

Bubbles of gas preserved in rocks as lava solidifies.

28
Q

What are amygdales?

A

Vesicles that are filled by secondary minerals.

29
Q

What type of eruption do low viscosity magmas produce?

A

Non-explosive eruptions.

30
Q

What volcanoes do basaltic eruptions produce?

A

Shield volcanoes that have broad, gently sloping, dome shaped mountains.

This is because the lavas are low viscosity and can flow for a few kilometres.

31
Q

Why do viscous magmas produce explosive eruptions?

A

Expanding gases result in explosive eruptions and produce fragments.

32
Q

What is tephra?

A

Airborne volcanic debris.

33
Q

What are the classifications of tephra?

A

Bombs: > 64 mm
Lapilli: 2 - 64 mm
Ash: < 2 mm.

34
Q

What are the features of andesitic eruptions?

A

Andesitic volcanoes emit both viscous lava flows and tephra.

The emissions tend to alternate which forms layers of lava and tephra and form a stratovolcano.

35
Q

What are pyroclastic flows?

A

A mixture of hot gases and pyroclasts of a higher density than the atmosphere that flow down the volcano.

36
Q

What do rhyolitic eruptions cause?

A

Calderas to form.

They are created by collapse of the surface rock following an eruption.

37
Q

What are the two main textural features of igneous rocks?

A

The size of the mineral grains.

How the mineral grains are packed together.

38
Q

What are phanerite?

A

Coarse grained igneous rocks.

39
Q

What are pegmatite?

A

Contains unusually large mineral grains (2cm or larger).

40
Q

What are aphanite?

A

Fine-grained igneous rocks.

41
Q

What happens when magma cools down very quickly?

A

The atoms lack the time to organise themselves into minerals.

The resulting rock is a glass (no crystals).

42
Q

What are extrusive igneous rocks that are largely or wholly glassy called?

A

Obsidian.

43
Q

What is a mineral assemblage?

A

The group of minerals that occur in a rock.

44
Q

What minerals do common igneous rocks consist mainly of?

A

Quartz

Feldspar

Mica

Amphibole

Pyroxene

Olivine

45
Q

What are examples of light coloured minerals?

A

Quartz

Feldspar

Muscovite

46
Q

What are examples of dark coloured minerals?

A

Biotite

Amphibole

Pyroxene

47
Q

What are plutons?

A

All bodies of intrusive igneous rock.

48
Q

What are examples of minor plutons?

A

Dykes

Sills

Laccoliths

Volcanic pipes

49
Q

What is a dyke?

A

Tabular, sheet-like body of igneous rock.

This cuts across the layering or fabric of the rock into which it intrudes.

50
Q

What is a sill?

A

Tabular and sheet-like.

Runs parallel to the layering or fabric of the rocks into which it intrudes.

51
Q

What is a laccolith?

A

Parallel to the layering of the rocks into which it intrudes, but forces the layers of rock above it to bend, forming a dome.

52
Q

What is a volcanic pipe?

A

A roughly cylindrical conduit that once fed magma upward to a volcanic vent.

53
Q

What are examples of major plutons?

A

Batholith

Xenoliths

Columnar Joints

Chilled Margins

54
Q

What are types of major plutons?

A

Batholith

Xenolith

Columnar Joints

Chilled Margins

55
Q

What is a batholith?

A

It is the largest type of pluton.

It is an intrusive body of rock that is an irregular shape that cuts across the layering or other fabric of the rock into which it intrudes.

56
Q

What is an example of a batholith?

A

Half Dome in Yosemite National Park

57
Q

What is a xenolith?

A

Any rock fragment still enclosed in a magmatic body when it solidifies.

58
Q

How is a xenolith formed?

A

As rising magma can dislodge fragments of overlying rock, they sink down through the magma as they are denser.

This process is called stoping.

59
Q

What are two examples of cooling features?

A

Columnar joints

Chilled margins

60
Q

What is a columnar joint?

A

Polygonal fracture pattern associated with cooling.

61
Q

What is a chilled margin?

A

Fine grained, rapidly cooled margins to a pluton.

62
Q

What are layered intrusions?

A

Layers produced by crystal settling in a magma chamber.

These are the main source of the world’s platinum group elements, Cr, Ni, V.

63
Q

What is mafic magma?

A

A basaltic magma.

64
Q

What is felsic magma?

A

Rhyolitic magma.

BIOS1013 - Environmental Geoscience (8 decks)

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