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
what are some common foliated rocks? (in order of least metamorphized to most)
slate –> phyllite –> schist —> gneiss
what doe we use slate for?
- construction and roofing
What are some common non-foliated metamorphic rocks?
- quartzite
- marble
- amphibole
- hornfels
What is regional metamorphism? Explain the typical conditions and some examples
- it is large scale metamorphism
- associated with mountain-building events
- major heating and differential stressing of rocks during continental collisions, and continental metamorphism
- causes mechanical deformation
- ex. alps, himalaya, appalachians
What are some resources we get from regional metamorphism ?
slate, phyllite, schist, gneiss = construction, decorative tiles, countertops
marble and quartzite = dimension stone, sculpture
garnet = used in abrasives, ceramics
kyanite = ceramics
What are some metamorphic settings?
- regional metamorphism
- contact metamorphism
What is contact metamorphism
- alteration of rock immediately in contact with intruded igneous rock
- smaller scale than regional metamorphism
- less pressure involved
- highly dependent on release of hydrothermal fluids
- typically no foliated rocks
What are skarns? Where do they occur?
- metalliferous ores (skarns- carbonate replacement)!
- these occur at margins of felsic intrusions
- iron oxides, copper, lead, zinc, iron sulfides, gold and silver
- occurs when felsic magma intrudes into limestone
What are some resources of contact metamorphism?
- metalliferous ores (skarns)
- precious stones and abrasives (beryl, corundum)
What is weathering?
- an in situ breakdown or alteration of rocks and minerals
- depends on the intrinsic properties of rocks
- also depends on environmental variables
- can be mechanical or chemical
What are intrinsic properties of rocks that may influence weathering?
- mineralogy/chemical
- strength of cementation
- rock structures and loading histories
- thermal expansion rates and specific heat capacity
what are environmental variables that may influence weathering
- temperature variability
- moisture availability
- rain/ground/soil water pH
- vegetation
What are types of mechanical weathering?
- unloading
- crystal growth
- thermal stressing
What is unloading
- type of mechanical weathering
- brittle expansion of unloaded rocks
- spalling and exfoliation
What is crystal growth?
- salt formation in cavities, mineral grain boundaries
- ice formation with freeze-thaw cycles
what is thermal stressing?
- differential expansion-contraction of rocks and minerals
- solar heating and cooling, fire, lightening
What is the net effect of mechanical weathering?
- produces sediment
What types of chemical weathering?
- any changes in minerals thru addition or subtraction of chems
- dissolution
- hydrolysis
- oxidation
What is dissolution? (in terms of chemical weathering)
- acidic rainwater dissolves calcium carbonate
- water saturated in calcite can be re-precipitated
what is hydrolysis?
- H ions and H2O combine with K feldspar to produce kaolinite (china clay)
- granular disintegration of granites (feldspars and micas to clay minerals, resistant quartz left behind)
What is oxidation? (in terms of chemical weathering)
- reaction of ferromagnesian minerals with free oxygen
- oxidation of granites (felsic) and mafic rocks
What are some weathered products that we use as resources?
- soils (mechanical and chemical breakdown products)
- kaolinite (hydrothermal modification of granites)
- bauxite (aluminum ore)
What is kaolinite? how does it form? what do we use it for?
- type of ‘china’ clay!
- hydrothermal modification of granites
- weathered feldspars and micas turn to clay minerals
- we use it for ceramics, toothpaste, food, cosmetics
How is bauxite formed?
- 3 minerals together
- heavy, tropical weathering leaches out all the mobile minerals
- leaves residual mass enriched in aluminum hydroxides
What is deposition?
- a reduction in energy, where materials fall out of suspension
- sediments sorted according to grainsize and density of materials
What is entrainment?
- particles picked up by moving fluids
- size of particles entrained depends on the energy of the system
- fast flowing = bigger grains, gentle stream = smaller grains
what is transportation?
- sediment moved from original location
- different grain size transported depending on energy
- different types of ‘loads’ depending on size of grains (solute, suspended, bed loads)
- transport rounds and reduces size of grains
what are some examples of erosion and deposition products as resources?
- aggregates - unconsolidated deposits sand and gravel (some of the most important materials for construction, concrete and roads)
- placer deposits - dense ore minerals concentrated by fluvial sorting processes (typically gold, diamonds, tin, etc)
what is diagenesis?
- the physical and chemical processes that affect sedimentary materials after deposition and before metamorphism and between deposition and weathering.
- low temp and low pressure modification of sedimental including recrystallization and cementation - lithification
What is regolith?
- blanket of fractured weathered rock and soil at the earths surface
What are the economic importance of shallow subsurface processes? why do we care?
- Aquifers may contain various dissolved soilds, such as ions
- organic breakdown of sediments or organic molecules increases the ration of carbon to hydrogen (creates cellulose to coal, and marine plankton to petroleum/oil)
What are some marine-specific processes that we use as resources?
- limestone formation (coral reef lithified to produce limestone, we then use this for construction materials)
- phosphate deposition ( phosphate debris (fish bones and teeth) accumulate, we can extract this for use in fertilizers)
- manganese/ polymetallic nodules (form on very deep ocean floors, can extract for nickel, cobalt and copper)
- iron sulphide black smokers (essentially hydrothermal vents pumping out super hot water, mixes with cold ocean water, precipitates iron sulfides that look like black smoke)
