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EOS 120 > Igneous Processes and Rocks > Flashcards

Igneous Processes and Rocks Flashcards

1
Q

what do igneous rocks crystallise from?

A

molten rock
– Magma (sub-surface)
– Lava (at the surface)

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2
Q

what does slow subsurface cooling of magma create?

A

intrusive igneous rocks = plutonic rocks

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3
Q

what does fast cooling of erupted lava create?

A

extrusive igneous rocks = volcanic rocks

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4
Q

Why are igneous rocks important?

A

Advance our knowledge of how planets work inside since deeper Earth is inaccessible
They help us understand volcanoes
They host Gold! (Plus copper, tin, platinum, silver, zinc, lead…, most metal resources are associated with magmatic processes or associated hydrothermal systems)

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5
Q

Where do igneous rocks form?

A

mid ocean ridges
volcanic arcs at subduction zones
hotspots/plumes

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6
Q

How does magma form?

A

Most magma forms by partial melting of hot subsurface rocks by:
(1)Decompression melting (->basalt magma)
OR
(2) Flux melting (add volatiles)

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7
Q

where does decompression melting occur?

A

mid ocean ridge

hotspots (shield volcano)

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8
Q

where does flux melting occur?

A

convergent plate boundaries

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9
Q

magma composition varies with …

A

source rock, temperature, pressure (depth)

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10
Q

what is the source region for magma?

A

mantle or crust

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11
Q

magma composition reflects …

A

different contributions from different minerals

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12
Q

How can magmas change their composition? (differentiate)

A

crystal fractionation
assimilation of country (host) rock
magma mixing

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13
Q

what is crystal fractionation?

A

Magma cools, begins to form mineral crystals
Crystals denser than liquid -> sink
Crystal removal ‘fractionates’ certain components (Mg,Ca,Al,..)
Change composition of remaining magma

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14
Q

how do we know about crystal fractionation?

A

deep ‘fossil’ magma chambers exposed by erosion

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15
Q

Which minerals crystallise early (higher T) or late (lower T) as magma cools?
(bowens reaction series)

A

earliest

olivine
pyroxene, calcium rich plagioclase
amphibole
biotite, sodium rich plagioclase
potassium feldspar
muscovite
quartz

latest

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16
Q

what is the mantle source rock for peridotite (ultramafic)?

A

olivine (±pyroxene)

17
Q

what are the stages in the differentiation of magma?

A

(1) Partial melting of (ultramafic) mantle rock -> Basalt magma 1 (mafic)
(2) Basalt magma 1 – [olivine] -> Basalt 2 (less Mg)
(3) Basalt 2 – [olivine, plagioclase] -> Andesite (less Ca, Al, Mg, more Si)
(4) Andesite – [plagioclase, pyroxene] -> Rhyolite (felsic) (more Si, less Mg)

Result: different magma compositions derived from one ‘parent’ magma

18
Q

why can magma differentiation proceed furthest in continental settings (continental arcs & hot spots)?

A

More crust for magma to travel through = more time for fractionation, assimilation (esp. felsic minerals

19
Q

what happens to felsic, intermediate and mafic rocks as differentiation increases?

A

mafic rocks: high %MgO, low %SiO2, low differentiation, low viscosity magma

intermediate rocks: medium %MgO, medium %SiO2, medium differentiation, medium viscosity magma

felsic rocks: low %MgO, high %SiO2, high differentiation, high viscosity magma

20
Q

what is viscosity?

A

resistance to flow (Pascal seconds, Pa.s)

21
Q

velocity increases with …

A

SiO2 content

22
Q

more viscous =

A

less runny

granite/rhyolite magma (felsic)

23
Q

less viscous =

A 
more runny
basalt magma (mafic)
24
Q

Crystal size and shape of basalt (mafic) are dictated by …?

A

time-temperature history:

Erupted quickly vs cooled slowly (at depth)

25
Q

what is an example of a fast cooled basaltic (mafic) rock?

A

Fine grained < 1 mm
Olivine, pyroxene, plagioclase
BASALT

26
Q

what is an example of a slow cooled basaltic (mafic) rock?

A

Coarser grained > 2-5 mm
Olivine, pyroxene, plagioclase
GABBRO

27
Q

which has a coarser texture? slow or fast cooled basaltic rock (mafic)?

A

slow cooled

28
Q

For granitic (felsic) magma, what is an example of a fast cooled rock? what is its texture?

A

No crystals – glass (obsidian) to fine-grained quartz, feldspar, mica:
RHYOLITE

29
Q

For granitic (felsic) magma, what is an example of a slow cooled rock? what is its texture?

A

Quartz, feldspar, mica

GRANITE

30
Q

what is an example of an intrusive, mafic rock?

A

gabbro

31
Q

what is an example of an intrusive, intermediate rock?

A

diorite

32
Q

what is an example of an intrusive, felsic rock?

A

granite

33
Q

what is an example of an extrusive, mafic rock?

A

basalt

34
Q

what is an example of an extrusive, intermediate rock?

A

andesite

35
Q

what is an example of an extrusive, felsic rock?

A

rhyolite

EOS 120 (24 decks)

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