plate tectonics is the surface expression of mantle convection
TRANSCRIPT
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Plate tectonics is the surface expression of mantle convection
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Plate tectonics provides the chemical sources of life
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• Mantle convection results in partial melts
• Volcanism delivers nutrients and gases to the crust, ocean and atmosphere
• Convection continually recycles nutrients
• Plate interactions maintain topography
Plate tectonics and geochemical cycles
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Whole Earth structure
Layered structure • structure we can most easily observe, via seismology
• seismic layers reflect chemical, thermal, and mechanical differences
Chemical structure• consequence of planet formation and
• ongoing differentiation through melting
• inferences about whole-earth chemistry can be made from melt products
Thermal structure• mostly adiabatic, but
• density (compositional) differences may restrict convection to “layers”
• thermal boundary layers separate layers that can’t mix
Mechanical structure• dependent on composition and temperature• controls convection• poorly understood
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Layered Structure
Global seismology • structure we can most easily observe, via seismology
• seismic velocity reflects chemical, thermal, and mechanical properties
• seismic layers reflect first-order differences in these things
• a reference, 1D (radially symmetric) velocity model exists for the earth
• tomography reveals structure relative to that reference model
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Layered Structure
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1D Earth Model Observed
Peter Shearer
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PREM
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Chemical structure
Effects of planet formation • Ken’s first lecture
• segregate the core, maybe lowermost mantle, early melt differentiation
Effects of melting• incompatible elements are enriched in continental crust
• mantle is depleted in these elements
• some parts of the mantle that haven’t yet melted may exist
Effects of convection• enriched crustal components are returned to the mantle via subduction
• subducted slabs may accumulate in the TZ or D’’
• it is still not clear how well mixed is the whole mantle
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Basalt Gabbro
diorite
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QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
3-D distribution of melt around grain boundaries for (a) dihedral angles less than 60° and (b) greater than 60°
For small dihedral angles there is a continuous, interconnected network of melt
Small percentage melts form interconnected networks
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Earth’s thermal structure
Geotherm • temperature as a function of depth in the earth
Internal heat sources and transfer mechanisms • sources: mostly radioactive decay
• transfer: conduction, convection/advection
Adiabatic gradient• the temperature gradient due to isentropic decompression
• can be determined if thermodynamic properties are known
• generally characterized by a potential temperature
Thermal boundary layers• temperature gradients in a region between non-mixing layers
• the lithosphere, transition zone, and CMB are examples
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The mantle adiabatic temperature gradient
An adiabatic temperature gradient is the temperature gradient resulting from isentropic pressure changes.
An isentropic pressure change involves a volume change (via compression or decompression) but no change in heat (i.e. no conductive heat transfer).
• When rising mantle decompresses, it expands
- work is done by the volume
- so potential energy is lost, and total energy decreases
Since no heat enters the system, T decreases
• For sinking mantle, it is the opposite
- work is done on the volume to compress it (by gravity)
T increases
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Mantle viscosity
• Viscosity is a measure of how much a material will deform under an applied shear stress. ( y = ˙dux/dy )
• The viscosity, , of mantle rock is temperature dependent.
• The temperature dependence of mantle viscosity, (T), is non-linear.
• (T) decreases rapidly as T approaches the solidus.
• The solidus is the temperature at which mantle rocks begin to melt.
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Conductive heat transfer, thermal boundary layer
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An internally heated solid with (T), away from conducting boundaries, will tend toward an adiabatic temperature gradient close to the melting temperature
• material rises dz along an adiabat• it cools with decompression• it is as the same temperature as it’s surrondings• T(z) is adiabatic
• material rises and cools• at T2 it is warmer than it’s surroundings• the material will continue to rise • unstable, convects• will establish a new geotherm as heat is transferred upward
• material rises and cools• at T2 it is cooler than it’s surroundings• the material will sink back to it’s point of neutral buoyancy• it will continue to heat internally
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Note: this curve is made up. We don’t know the geotherm that well.
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Note: these curves are made up. We don’t know these things that well.
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Earth’s thermal structure
Geotherm • temperature as a function of depth in the earth
Internal heat sources and transfer mechanisms • sources: mostly radioactive decay
• transfer: conduction, convection/advection
Adiabatic gradient• the temperature gradient due to isentropic decompression
• can be determined if thermodynamic properties are known
• generally characterized by a potential temperature
Thermal boundary layers• temperature gradients in a region between non-mixing layers
• the lithosphere, transition zone, and CMB are examples
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Mechanical structure
• For the most part, Earth’s mechanical structure is equivalent to it’s viscosity structure
• Viscosity structure is temperature dependent and non-linear, with viscosity decreasing near the melting temperature.
• Viscosity is also strongly dependent on water content
• Large mechanical contrasts exist between the lithosphere and asthenosphere, which are mechanically defined, and between the lower mantle and core and between the inner and outer core
• Phase transitions play an important role in the mechanical behavior of the convecting mantle
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Kellog, van der Hilst
Moving across layer boundaries
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Clapeyroncurves
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