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Geology > Igneous Rocks 2 > Flashcards

Igneous Rocks 2 Flashcards

1
Q

What are the 4 mafic minerals?

From higher to lower crystallisation temperature
From higher to lower density
From higher to lower FeMg content

A

Biotite
Hornblende
Augite
Olivine

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2
Q

What are the 4 silicic minerals?

From lower to higher crystallisation temperature
From lower to higher density
From higher to lower SiO content

A

Quartz
Muscovite
Orthoclase
Plagioclase

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3
Q

When do rocks melt?

A

Rocks melt when the temperature exceed the melting temperature of the lowest mineral with the lowest melting temperature

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4
Q

What is magma?

A

Molten rock that forms with variable amounts of solid crystals and dissolved gas

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5
Q

Why does magma melt?

A

Increase in temperature or decrease in pressure

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6
Q

What controls what temperature if required to melt rock?

A

Mineral content of parent rock
Pressure
Effect of water

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7
Q

Mineral content: Order of Bowen’s reaction series from crystallises first to melts first

A

Olivine Plagioclase F
Pyroxene
Amphibole P F
Biotite KF Quartz Muscovite

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8
Q

Mineral content: Crystallisation temperatures for minerals

A

Olivine- 1240 to 1400

Amphibole- 940 to 1170

Plagioclase- 1060 to 1360

Biotite- 790 to 1010

Quartz- 760 to 960

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9
Q

Pressure: how does pressure affect temperatures?

A

As pressure increases the temperature needed to melt also increases

Only applies to dry rock

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10
Q

Effect of water: how does temperature affect melting?

A

The presence of water vapour in rock lowers the temperature needed to melt the rocks

At higher pressure more water can dissolve into magma temperatures will decrease with more pressure in wet rocks. Opposite to dry rocks

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11
Q

What is magmatic differentiation by partial melting?

A

Partial melting generates magmas that are more acidic than the parent rock.

When the volume of melt reaches 10-15% it separates from the parent rock and rises

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12
Q

What happens to the content of the magma when rocks melt?

A

The minerals with the lowest melting temperatures melt first and so the initial melt is enriched with them minerals so it is chemically different to the parent rock

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13
Q

melting of basic magma

A

Formed from mantle peridotite

Dry partial melting

Constructive plate margin

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14
Q

Melting of intermediate magma

A

Formed from mantle peridotite

Wet partial melting

Destructive subductive plate margin

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15
Q

Melting of acid magma

A

Forms from continental crust

Wet partial melting

Divergent convergent plate margin

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16
Q

What rock is formed from basic magma and what is important about the minerals?

A

Basalt

Contains olivine, pyroxene and plagioclase which do not contain water

17
Q

Why does dry partial melting occur?

A

Due to decompression melting when there is a decrease in pressure on materials that are close to their melting temperature

18
Q

How is decompression melting triggered?

A

1) extension and thinning of the lithospheric above reduces overburden pressure
2) mantle material begins to rise, such as within a convection current

19
Q

What are the layers of oceanic crust?

A

Basalt with pillowed structures

Dolerite with a sheeted dyke complex

Gabbro

Layered peridotite

20
Q

What happens within the oceanic rocks as they are being formed?

A

Gravity settling

Olivines sink and form a crystal mush at the bottom

21
Q

What are hotspots?

A

Areas of prolonged volcanic activity

22
Q

Where is andesitic magma found?

A

In volcanoes along destructive plate boundaries

23
Q

How is intermediate magma formed?

A

By wet partial melting of mantle peridotite

24
Q

Where does the water for wet partial melting come from?

A

It is carried by the oceanic crust in the subduction zone then given off by metamorphic dewatering

25
Q

What is significant about the fact that the composition of andesitic magma is the same as continental crust?

A

Both continental crust and Andesite are formed by wet partial melting

Continental crusts are formed at island arc volcanoes

26
Q

How is acid magma formed?

A

Partial melting of rocks in the lower continental crust

27
Q

What are the three pieces of evidence that support how acid magma is formed?

A

1) acid volcanoes are confined to continental areas
2) acid magma contains huge amounts of water which formed from the melting of minerals found in continental crust
3) lab experiments

28
Q

What 3 methods cause continental crust to melt?

A

Conduction

Slab detachment

Delamination

29
Q

What temperature does continental crust begin to melt?

A

780 degrees

30
Q

Why does the solidus for continental crust decrease with depth?

A

Cc Must contain water

31
Q

What is the process called where magma cools and is richer in the first minerals?

A

Magmatic differentiation by fractional crystallisation

32
Q

What are the two ways in which crystal-melt can separate?

A

1) can be squeezed out of the mix by weight of overlying crystals (filter pressing)
2) denser crystals will sink (gravity settling)

33
Q

What is the case study for magmatic differentiation by fractional crystallisation?

A

Palisades sill

300m thick

Sandstone 
Metaquartzite baked margin 
Basalt chilled margin 
Layered dunite with no olivine 
Layered dunite with increasing olivine 
Layered dunite with rich olivine 
Basalt chilled margin 
Metaquartzite baked margin
34
Q

What is contamination of magma?

A

Process where the composition of magma is changed as when the magma melts it reacts with or mechanically incorporates material from the surrounding rock /xenoliths

35
Q

What 2 ways can contamination occur?

A

1) melting of the xenolith due to the conduction of heat from the magma which then mixes
2) a chemical reaction not involving melting

36
Q

What is the process of contamination by xenoliths called?

A

Stoping

37
Q

What is a continuous reaction series?

A

When the chemical composition of a mineral changes but it retains its original atomic structure and remains in the same mineral group

38
Q

What is a discontinuous reaction series?

A

When the chemical composition and atomic structure of the crystal change resulting in the formation of a new mineral

Geology (14 decks)

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