Ore Deposits and Global Volcanic Effects Flashcards
How do PDCs (Porphyry Copper Deposits) form?
Form in subduction zone magmatic systems.
Find ore deposits where they have been exhumed (eroded and uplifted) near to the surface.
PDCs are porphyritic intrusions of magma.
Cu comes from the magma.
What is porphyritic texture?
Phenocrysts - grow slowly at depth as the magma cools
Microlites - nucleate rapidly as magma ascends
How do PCD stocks contain more Cu than would come from the stocks alone?
The additional Cu comes from the magma body below the stock.
Magma circulates, when it rises it releases fluid containing Cu and then falls back down again.
Process repeats.
But magma is quite viscous in the deposit for this to be effective.
What are kimberlites?
Diamond deposits brought up quickly from depth in a volcanic eruption.
Get diamond mines.
Have to come up quickly otherwise diamonds turn to graphite.
Diamonds from at low temp, high pressure (depth).
Only found on Cratons (Archean geological terrains).
What is fluidisation?
The upward flow of gas which supports the weight of particles.
Means that particles are well mixed from all depths.
How do Ni-sulphide ores form in komatiite lava flows? (from the Archean, when temps were hot)
Magma incorporates sulphur rich sediments into melt.
Get blobs of sulphide melts (dense and tend to sink).
Ni partitions sulphide melts, settle to base and form ore deposit.
What volcanic processes tend to have global effects?
Explosive eruptions.
Mainly from the ash clouds produced.
How do volcanoes cause global cooling?
Sulphuric acid aerosols are released in volcanic eruptions (easy to measure).
Sulphur is responsible for atmospheric cooling.
e.g. Tambura, 1815 - no summer
How does Sulphur cause atmospheric cooling?
Sulphuric acid particles are small, ~0.5 microns.
Effect radiation budget:
- cause scattering of shortwave radiation
- this decreases direct radiation
- which decreases the net total radiative flux
- results in net global surface cooling e.g. volcanic winters
- main impacts is particles reach stratosphere (hence small particles are of concern)
Why do tropical eruptions have a greater climatic impact than high latitude eruptions?
- Can spread across equator and both hemispheres
- Increase equator-pole temperature gradient
- Change polar vortex
- Impact jet stream
- Results in warming at poles but anomalous warming everywhere else
Laki Case study facts?
Laki fissure eruptions, Iceland, 1783-84
- outpouring of lava from fissure
- small eruptions around the vent (bombs and scoria)
- caused high levels of gas emissions
Gas release caused:
- fluorine released, incorporated into grass, 60% livestock died
- 22% humans died from famine and disease
- SO2 acid rain
Global imapcts:
- fog over N.America and central Europe
- the first winter after had cold temperature anomalies
- ice core peaks showed evidence in Greenland (not Antarctica, didnt spread that far)
What classifies a super eruption?
> 400km^3 of magma erupted
e.g. Toba 74 kya
can cause destruction on continental scales, not likely globally.
What can cause mass extinctions?
Flood basalt super eruptions. e.g. Deccan traps
Get release of sulphur which may cause initially cooling.
But the release of CO2 is huge (basalts contain more CO2 than silicates), cause warming.
