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G1 - Rock Forming Processes MS > Igneous > Flashcards

Igneous Flashcards

1
Q

Why might granite and peridotite be wet at depths > 50km at a convergent plate boundary

A

-subduction
-introducing hydrous minerals (clays) / water

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2
Q

Why is a partial melt generated in the continental crust of a convergent plate boundary unlikely to be erupted at the surface

A
  • the rising magma falls below/crosses the melting point curve
  • it will crystallise before reaching the surface
  • ~25km from the surface
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3
Q

Why might lava at the surface have a range of composition from mafic to silicic

A
  • parent rock - partial melt of peridotite produces mafic magma
  • evolution of magma:
    =differentiation
    =gravity setting
    =contamination - Xenoliths
    = magma mixing (mafic and silicic)
    = underplating/partial melt of continental crust/overlying lithos wedge
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4
Q

What type of magma forms at convergent plate boundaries

A

Oceanic to oceanic volcano = basalt/mafic/basic
Oceanic to continental volcano = andesite/rhyolite/acidic/silicic/intermediate

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5
Q

What is the temperature and state of the mantle at 150km deep

A

1300c
Solid

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6
Q

How might mantle convection result in the generation of magma from dry peridotite beneath ocean ridges

A
  • rising convection currents
  • decompression melting/pressure reduction
  • melting point crossed/partial melting occurs
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7
Q

What is the origin and effect of water in the generation of magma from wet peridotite

A

Origin- seawater/wet sediment/release of water from hydrous sediment
- brought down by subduction

Effect - lowers melting temperature

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8
Q

What processes can change the original composition of magma as it rises through the lithosphere at convergent margins

A

Gravity differentiation
- early crystals, more dense, settle out
- leaving magma above more silicic

Contamination
- stoping intrusion
- xenoliths fall into the magma
- become assimilated
- and change the original composition

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9
Q

What is the relationship between viscosity and SiO2

A

Viscosity increases with increase in SiO2%
- positive relationship

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10
Q

What are the similarities and differences in the relationship between viscosity and temperature for granitic magma compared with basaltic magmas

A

Similarity
- both melts are less viscous at higher temperatures

Difference
- granitic magmas are always more viscous than basaltic magmas at any temperature

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11
Q

Why do basalt lava flows commonly form low angle shield cones extending many kilometres Demi the volcanic vent

A

-basalt is non viscous (runny)
-extruded hot with low SiO2 content
- gas is readily able to escape in bubbles
- flows further before it cools
- runs in tunnels (lava tubes) many km - insulated

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12
Q

Why are there seawater ions in volcanic gases collected at destructive plate margins

A
  • subduction if ocean lithosphere (basalt) at trench
  • seawater dragged down with descending plate
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13
Q

What is the effect of seawater of the regeneration and eruption of magmas at destructive plate margins

A
  • lowers the melting point
  • allowing magma to be generated at shallower depths
  • lower temperatures if magma
  • more explosive eruption
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14
Q

What type of lava is most likely to erupted at mid ocean ridges an mid plate hotspots and why

A

Basaltic (pillow lava)
- partial melting
- mantle/asthenosphere/peridotite
- pillow lava - erupted underwater

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15
Q

What processes enable solid rock at 110km at mid ocean ridges to produce lava at the surface

A
  • upward convection currents
  • decompression melting
  • as it crosses the solidus line
  • liquid magma - less dense/buoyancy
  • fluid pressure - if it exceeds confining pressure
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16
Q

Why might there be a hotspot

A
  • rising
  • mantle plume
  • hot rock (not magma)
17
Q

How might mid plate volcanism in continental areas (flood basalts) be responsible for mass extinctions in the fossil record

A

Volcanic winter - shirt term
- fires, ash, soot, dust, S02 in atmosphere
= reduces photosynthesis by sun

Global warming - long term
- increased C02 - ozone destruction
- acid rain - death if primary producers

18
Q

How does the average geothermal gradient in the mantle compare with the crust

A
  • lower in the mantle
  • gradient is steeper in crust
  • rate if temperature changes with depth decreases
19
Q

Why is it not possible for dry granite to begin melting at the base of the crust

A
  • geothermal gradient is not high enough/crust not hot enough
  • temperature at Moho/base of crust is not hot enough
  • to cross the melting point curve
  • water reduces melting point
20
Q

What is the importance of gas content in the emplacement and extrusion of magma

A

Affects
- viscosity
- buoyancy
- explosion
- density
- fluid pressure

21
Q

What is the variety if plagioclase feldspar to crystallise at the highest temperature

A

Anorthite (calcium rich)

22
Q

Why are there different compositions in a zoned crystal

A
  • Ca higher melting point/first to crystallise
  • solid solution series
  • temp drops too fast fri equilibrium to be maintained
  • crystals unable to completely react back with melt
  • before more Na rich crystals firm around older crystals
  • process continues until crystallisation complete
23
Q

Why are plagioclase ground mass crystals not zoned

A
  • fine grained ground mass (porphyritic texture)
  • crystallisation was too fast - rapid cooling
  • Ca removed
  • not enough time for reacting back with the melt - diffusion of ions
24
Q

What process during the crystallisation of an igneous rock would produce a magma with a different composition form the original melt

A

Contamination during magmatic intrusion
- xenoliths
- magmatic stoping
- e.g. peridotite in basalt, mafic rock in granite

Gravity differentiation/ cumulates/ gravity settling/ fractional crystallisation
- e.g. olivine first to crystallise
- reference to position in bowens reaction series
- olivine more dense that other feldspar/augite
- olivine sinks in liquid magma
- olivine trapped at chilled margins/unable to differentiate

25
Q

How do we know an igneous body is a sill rather than a lava flow or dyke

A

Evidence
- concordant
- upper baked margin
- dolerite

Explanation
- sill not a dyke (discordant)
- no upper baked margin in lava
- not cooled fast - not lava

26
Q

Why is there an olivine layer above the lower chilled margin in a sill

A
  • olivine crystallises first/ highest temps/ high melting point
  • olivine crystals more dense
  • gravity differentiation - settling/sink
  • above the chilled margin - already solidified
27
Q

Why might there be peridotite rock fragments in a sill

A
  • xenoliths
  • inclusions from country rocks
  • mantle rock
  • accept idea of currents ripping up cumulate layer
28
Q

Why is the composition of olivine wishing the chilled margin of a sill more Mg rich than that within the Fe rich olivine layer

A
  • early formed Mg rich olivine in chilled margin cooled fast
  • no time for early crystals to react back with melt
29
Q

Why are there olivine rich layers near the base of some intrusions

A
  • olivine is the first to crystallise/ at highest temp
  • high density minerals
  • thus gravity settling
  • cooling too quick to allow olivine to react back
30
Q

How do we know if an igneous rock had crystallised very slowly

A
  • large grain size/euhedral/well formed thus slow cooling
    -olivine would of have had enough time to complete react back/ reaching equilibrium
31
Q

Average crystal sizes in sills

A

Description - largest size near the top
- smallest at the base/top (chilled margins)

Account
- cooling rate v crystal size
- effect of gravity settling and room for crystal growth further down

32
Q

When and why might dolerite have a similar composition to the original magma

A
  • chilled margins
  • faster cooling
  • less time to differentiate
33
Q

Why is dolerite more prone to chemical weathering than granite

A
  • granite contains silicic minerals e.g. quartz/Muscovite
  • lower in bowens reaction series
  • keep in equilibrium than mafic minerals

G1 - Rock Forming Processes MS (3 decks)

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