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Lecture 1 (Geothermal Gradient (Boundary Zone (transition of rheology,…
Lecture 1
Geothermal Gradient
Rate of temperature increases
with increasing depth
5 degrees C/km and 75 degrees C/km
Thin Lithosphere(Oceanic)
adiabatic convection of heat
in mobile asthenosphere
steeper gradient
effective
less efficient conductive heat transfer
Rigid lithosphere
shallower gradient
Boundary Zone
transition of rheology
heat transfer
partial melts occur
Thick Lithosphere(Continental)
Heat convection is not allowed
to shallower depths as under thin lithosphere
Heat flow is related to thin lithosphere
Temperature rises faster
under thin lithosphere (oceanic)
slightly lower
thick lithosphere (Continental)
High gradient
Observed along oceanic spreading centers
Mid-Atlantic rift
rising of magma
low gradient
subduction zones
thrusting of cold water-filled sediments
Depends on age of crust
Under old (oceanic crust), deep
decreases as the area is cooled down
Under young (shallow) oceanic crust
high
Earth layered structure
Crust
Upper mantle
Transition zone
lower mantle
outer core
Inner core
Rheological order
Lithosphere
Crust
Uppermost mantle
Asthenosphere
ductile mantle
Mesosphere
rigid mantle
Outer core
Inner core
Earth's Mantle
peridotitic composition
Upper Mantle
Olivine - spinel
low velocity layer: 60 - 220km
Transition zone
Seismic waves velocity increases
Spinel - perovskite
Si IV - Si VI
Lower Mantle
Gradual increase seismic waves velocity
includes "strange" D layer
post- perovskite phase
Earth's Core
Fe-Ni alloy
Outer core
liquid
no S waves propagation
Inner core
Solid
Earth's crust
Continental Crust
not homogenous
Tonalitic
2.7 g/cm3 (less dense)
Oceanic crust
relatively homogenous
ophiolite
3.3 g/cm3 (denser)
Geobarometric gradient
Rate of Pressure increase
increasing depth
P = pgh
1GPa at crust base
rate increase = 30MPa/Km
Core density increases
Metal alloy is denser
Fe-Ni alloy
where do subducted slabs go?
Stall and stagnate
Boundary layer
probably move downwards
D-layer
graveyard of subduct slabs
heating causes upwelling plumes
Mantle: Earth's Thermal Engine
Heat transfer
Radiation
Conduction
Convection
What drives movement of plates?
Convection currents in Mantle
Continental shift
Recycling of Oceanic crust