Presentation on theme: "Plate tectonics is the surface expression of mantle convection"— Presentation transcript:
1 Plate tectonics is the surface expression of mantle convection
2 Plate tectonics provides the chemical sources of life
3 Plate tectonics and geochemical cycles • 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
4 Whole Earth structure Layered structure Chemical structure • structure we can most easily observe, via seismology• seismic layers reflect chemical, thermal, and mechanical differencesChemical structure• consequence of planet formation and• ongoing differentiation through melting• inferences about whole-earth chemistry can be made from melt productsThermal structure• mostly adiabatic, but• density (compositional) differences may restrict convection to “layers”• thermal boundary layers separate layers that can’t mixMechanical structure• dependent on composition and temperature• controls convection• poorly understood
5 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
12 Chemical structure Effects of planet formation Effects of melting • Ken’s first lecture• segregate the core, maybe lowermost mantle, early melt differentiationEffects 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 existEffects 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
16 Small percentage melts form interconnected networks 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
19 Earth’s thermal structure Geotherm• temperature as a function of depth in the earthInternal heat sources and transfer mechanisms• sources: mostly radioactive decay• transfer: conduction, convection/advectionAdiabatic gradient• the temperature gradient due to isentropic decompression• can be determined if thermodynamic properties are known• generally characterized by a potential temperatureThermal boundary layers• temperature gradients in a region between non-mixing layers• the lithosphere, transition zone, and CMB are examples
20 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 decreasesSince 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
21 Mantle viscosity• Viscosity is a measure of how much a material will deform under anapplied 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.
23 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 asit’s surrondings• T(z) is adiabatic• material rises and cools• at T2 it is warmer than it’ssurroundings• the material will continue to rise• unstable, convects• will establish a new geothermas heat is transferred upward• material rises and cools• at T2 it is cooler than it’ssurroundings• the material will sink back to it’spoint of neutral buoyancy• it will continue to heat internally
24 Note: this curve is made up. We don’t know the geotherm that well.
25 Note: these curves are made up. We don’t know these things that well.
26 Earth’s thermal structure Geotherm• temperature as a function of depth in the earthInternal heat sources and transfer mechanisms• sources: mostly radioactive decay• transfer: conduction, convection/advectionAdiabatic gradient• the temperature gradient due to isentropic decompression• can be determined if thermodynamic properties are known• generally characterized by a potential temperatureThermal boundary layers• temperature gradients in a region between non-mixing layers• the lithosphere, transition zone, and CMB are examples
27 Mechanical structure• For the most part, Earth’s mechanical structure is equivalent toit’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 andasthenosphere, which are mechanically defined, and betweenthe lower mantle and core and between the inner and outer core• Phase transitions play an important role in the mechanicalbehavior of the convecting mantle
28 Moving across layer boundaries Kellog, van der Hilst