Igneous Flashcards
Why might granite and peridotite be wet at depths > 50km at a convergent plate boundary
-subduction
-introducing hydrous minerals (clays) / water
Why is a partial melt generated in the continental crust of a convergent plate boundary unlikely to be erupted at the surface
- the rising magma falls below/crosses the melting point curve
- it will crystallise before reaching the surface
- ~25km from the surface
Why might lava at the surface have a range of composition from mafic to silicic
- 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
What type of magma forms at convergent plate boundaries
Oceanic to oceanic volcano = basalt/mafic/basic
Oceanic to continental volcano = andesite/rhyolite/acidic/silicic/intermediate
What is the temperature and state of the mantle at 150km deep
1300c
Solid
How might mantle convection result in the generation of magma from dry peridotite beneath ocean ridges
- rising convection currents
- decompression melting/pressure reduction
- melting point crossed/partial melting occurs
What is the origin and effect of water in the generation of magma from wet peridotite
Origin- seawater/wet sediment/release of water from hydrous sediment
- brought down by subduction
Effect - lowers melting temperature
What processes can change the original composition of magma as it rises through the lithosphere at convergent margins
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
What is the relationship between viscosity and SiO2
Viscosity increases with increase in SiO2%
- positive relationship
What are the similarities and differences in the relationship between viscosity and temperature for granitic magma compared with basaltic magmas
Similarity
- both melts are less viscous at higher temperatures
Difference
- granitic magmas are always more viscous than basaltic magmas at any temperature
Why do basalt lava flows commonly form low angle shield cones extending many kilometres Demi the volcanic vent
-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
Why are there seawater ions in volcanic gases collected at destructive plate margins
- subduction if ocean lithosphere (basalt) at trench
- seawater dragged down with descending plate
What is the effect of seawater of the regeneration and eruption of magmas at destructive plate margins
- lowers the melting point
- allowing magma to be generated at shallower depths
- lower temperatures if magma
- more explosive eruption
What type of lava is most likely to erupted at mid ocean ridges an mid plate hotspots and why
Basaltic (pillow lava)
- partial melting
- mantle/asthenosphere/peridotite
- pillow lava - erupted underwater
What processes enable solid rock at 110km at mid ocean ridges to produce lava at the surface
- upward convection currents
- decompression melting
- as it crosses the solidus line
- liquid magma - less dense/buoyancy
- fluid pressure - if it exceeds confining pressure
Why might there be a hotspot
- rising
- mantle plume
- hot rock (not magma)
How might mid plate volcanism in continental areas (flood basalts) be responsible for mass extinctions in the fossil record
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
How does the average geothermal gradient in the mantle compare with the crust
- lower in the mantle
- gradient is steeper in crust
- rate if temperature changes with depth decreases
Why is it not possible for dry granite to begin melting at the base of the crust
- 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
What is the importance of gas content in the emplacement and extrusion of magma
Affects
- viscosity
- buoyancy
- explosion
- density
- fluid pressure
What is the variety if plagioclase feldspar to crystallise at the highest temperature
Anorthite (calcium rich)
Why are there different compositions in a zoned crystal
- 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
Why are plagioclase ground mass crystals not zoned
- 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
What process during the crystallisation of an igneous rock would produce a magma with a different composition form the original melt
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
How do we know an igneous body is a sill rather than a lava flow or dyke
Evidence
- concordant
- upper baked margin
- dolerite
Explanation
- sill not a dyke (discordant)
- no upper baked margin in lava
- not cooled fast - not lava
Why is there an olivine layer above the lower chilled margin in a sill
- olivine crystallises first/ highest temps/ high melting point
- olivine crystals more dense
- gravity differentiation - settling/sink
- above the chilled margin - already solidified
Why might there be peridotite rock fragments in a sill
- xenoliths
- inclusions from country rocks
- mantle rock
- accept idea of currents ripping up cumulate layer
Why is the composition of olivine wishing the chilled margin of a sill more Mg rich than that within the Fe rich olivine layer
- early formed Mg rich olivine in chilled margin cooled fast
- no time for early crystals to react back with melt
Why are there olivine rich layers near the base of some intrusions
- olivine is the first to crystallise/ at highest temp
- high density minerals
- thus gravity settling
- cooling too quick to allow olivine to react back
How do we know if an igneous rock had crystallised very slowly
- large grain size/euhedral/well formed thus slow cooling
-olivine would of have had enough time to complete react back/ reaching equilibrium
Average crystal sizes in sills
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
When and why might dolerite have a similar composition to the original magma
- chilled margins
- faster cooling
- less time to differentiate
Why is dolerite more prone to chemical weathering than granite
- granite contains silicic minerals e.g. quartz/Muscovite
- lower in bowens reaction series
- keep in equilibrium than mafic minerals
