Topic 4: Geologic processes and resource origins Flashcards
What are the three components of the earths structure?
- core (inner and outer)
- Mantle (upper and lower)
- Lithosphere (crust and uppermost mantle)
What is the lithosphere?
- the ‘rock sphere’
- contains the igneous, sedimentary and metamorphic rocks
- variable thickness
- forms a semi-rigid tectonic plate
What are the two types of crust? What are their differences?
- oceanic
- higher density, but thinner
- mafic materials
- silica and magnesium (SiMa) - continental
- lower density, but thicker
- felsic
- silica and aluminium (SiAl)
What is plate tectonic theory? the basic concepts?
- a unifying idea that connects continental drift, sea floor spreading, seismic activity, crustal structure, volcanism, and resource distribution
- basic concepts of it is that there is a thin rigid lithosphere that is broken into a series of plates atop a mobile convecting asthenosphere
- plates move independently driven by mantle convection
How does resource distribution connect to plate tectonic theory?
- different resources available in different areas due to plate tectonics
- resources available in areas where they shouldn’t be, because of our current day
- ex. Halite in Germany and Poland
- doesn;t make sense now- as Europe has a cooler climate. This suggests that Europe used to have a hotter climate - it was further south
What are the three types of plate boundaries? explain ?
- divergent (2 plates spreading away from each other, constructive)
- convergent (2 plates moving toward each other/subducting - destructive)
- transform (2 plates sliding past each other)
What are divergent plate boundaries?
- fractures in the lithosphere where two plates move apart
- dense mafic magma upwells along the rift
- creates thin dense oceanic crust
- causes deep abyssal plains
- ex. midatlantic ridges (like in iceland, or east african rift)
What is interesting about rift valleys?
- they are essentially very young rifts
- create weird lavas due to very partial melting
- carbonatite (carbon and oxygen) lava
What are oceanic convergent plate boundaries?
- destructive boundaries
- oceanic lithosphere forced beneath continental lithosphere or other oceanic lithosphere
- causes a subduction zone, lithosphere descends into asthenosphere
- creates ocean trenches
- felsic and intermediate volcanism
- creates island arcs and mountain uplift, also major earthquakes
What are continental-continental convergent boundaries?
- destructive boundaries
- where continental lithospheres are forced together, large mountains are created
- continental crust too buoyant to subduct
- folding, faulting, and metamorphism occurs
- ex. uplift of himalayas, uplift of rockies
What are transform plate boundaries?
- strike-slip
- plates slide past one another
- often link off-set constructive margins
- high earthquake potential
- creates ocean ridges, faults
- ex. san andreas fault
How are felsic/grantitic intrusions formed? (igneous processes)
- formed from destruction of plate boundaries
- partial melt of subducting oceanic crust
- felsic melt is much less dense than surrounding crust, migrates upwards via stoping
- melt stabilizes
- slow cooling permits magmatic differentiation
- slow crystallization to produce coarse-grained materials
What is ‘stoping’ movements?
- how felsic melt migrates upwards thru surrounding crust
- when magma moves up buoyantly, moves thru fractures, melts material on the way, the the entire magma body moves up
What is the resource potential of felsic/granitic intrusions?
- important for emplacing hydrothermal and porphyry-type deposits
- minerals such as copper, gold, silver, zinc, lead and tin all settle near or in felsic igneous deposits
- many dissolved elements carried in hot fluids into faults and fractures surrounding granite batholith, dissolved copper, zinc, silver, etc. Minerals are precipitated in vein networks
what are porphyry-type deposits?
- where dissolved elements are carried in hot fluids into faults and fractures surrounding granite batholith
- cooling, decrease in pressure, reaction with country rock may cause steam explosions/expansions, creating more fractures
- solubility decreases, minerals precipitate into the vein network
- creates literal ‘veins’ of minerals (such as gold)
What are pegmatites?
- felsic/granitic intrusions
- very coarse grained granites formed at the margins of fluid-rich granitic intrusions
- apatite, beryl, spodumene
- major economic sources for these elements
how are mafic/basaltic intrusions formed?
- formed by mid ocean ridges, constructive plate boundaries and mantle plum hotspots
- partial melt of the mantle
- extrusive basalts have little mineral potential
- intrusive basalts made due to slow cooling mafic magmas and thus are higher potential
- fall out of heavy minerals in sequence to bottom of the magma chamber produces stratified intrusion
What are some potential resources of mafic/basaltic intrusions?
- dunite as a source for olivine
- metal sulfides formed during late-stage crystallization, sink to bottom of magma chambers and form rich deposits of iron, nickel, and copper sulfides
- magma may become saturated in chromium
What is metamorphism?
- the alteration of preexisting rock
- original compounds and or textures have been transformed
- solid state process
- result of high temperature and/or pressure
- may cause foliated or unfoliated rocks
What are foliated rocks? unfoliated?
foliated: mineral grains align themselves to differential pressure, layered appearance. formed from differential stress and re-orientation of mineral grains perpendicular to that stress
unfoliated: mineral grains grow into each other, equal pressure, massive appearance. alteration due to temperature with limited differential stress
What are metamorphic grades? how do we determine this?
- degree of alteration from parent rock (protolith)
- depends on extent of heat and pressure
- higher heat and pressure = higher grade (typically larger crystal size and more developed foliation)
- new minerals form at different grades of metamorphism
