Plate tectonics is the surface expression of mantle convection
Plate tectonics provides the chemical sources of life
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
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
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
Layered Structure
1D Earth Model
Observed
Peter Shearer
PREM
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
Basalt
Gabbro
diorite
Small percentage melts form interconnected networks
QuickTime™ and a
TIFF (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
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
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
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.
Conductive heat transfer, thermal boundary layer
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
Note: this curve is made up. We don’t know the geotherm that well.
Note: these curves are made up. We don’t know these things that well.
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
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
Moving across layer boundaries
Kellog, van der Hilst
Clapeyron
curves