Schedule • Basic equations you need to know in geodynamics • Fundamentals of fluid dynamics • Vectors and tensors • Conservation laws and constitutive law • 1D examples • Dynamic similarity • Fundamentals of mantle convection • Boussinesq approximation • Rayleigh number and Nusselt number • Stability analysis of mantle convection Examples: Post-glacial rebound
Initial: h(t=0) = h0 cos(2π/Lx) Assumption: L >> h0
Coastline photo from Turcotte & Schubert, 2002) Example: Ductile structures
http://www.sjsresource.com.au/images/Photo%20Galleries/Structural%20Geology/Folds/multilayer %20behaviour%20Folding,%20Ticino%203.jpg
O(1) m Example: Ductile structures
http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-1002/HTML.Lect4/LargeFold.jpg
O(100) m Example: Ductile structures
http://en.wikipedia.org/wiki/Fold_mountain#mediaviewer/File:Zagros_1992.jpg
O(1000) km A convecting Earth Plate tectonics provides a framework to understand global seismicity, volcanism and mountain building processes.
Plate motion and the interaction of these plates underpin this theory
* Earth has a heat source coming from the core! ! * Heat is converted into motion via density variations
* Drifting continents * Spreading centres * Subduction zones * Post glacial rebound * Hotspots * Heat budget Thermal convection http://dreamtigers.wordpress.com/2011/05/11/plate-tectonic-metaphor-illustrations-cmu/
Variations in temperature cause small changes in the fluid density
Buoyancy forces cause cold http://asapscience.tumblr.com/post/50419005208/the-earths-center-is-out-of-sync-we-all-know (compressed, i.e. higher density) material to sink http://www.see.leeds.ac.uk/structure/dynamicearth/convection/models.htm Early experiments
Brittle material
Convection = flow driven by internal forces (buoyancy)
In the Earth, density variations are attributed to pressure, Viscous fluid “convection” temperature and composition
Convection drives large-scale dynamics within in the Earth’s mantle Modern experiments Fundamentals of fluid dynamics Frames of reference DT D = 2T Material derivative Dt r Dt
Stationary frame (Eulerian) convective rate of change due to transport DT @T @T := + v of material to a different position (x) with Dt @t x @x respect to a fixed coordinate system local rate of change due to DT @T = + v T temporal variations at a Dt @t ·r position x Moving frame (Lagrangian)
DT @T coordinate@T system is transported with the := + v material, thus only the local rate of change Dt @t x @x remains Conservation of mass • General continuity equation: @⇢ ! + (⇢v)=0 ! @t r·
• Incompressible fluid For many geological material (such as the Earth’s crust and mantle), over long time-spans, one may assume an incompressible condition (e.g. the density of each material point does not change with time) D⇢ =0 v =0 Dt r· Valid assumption when pressure and temperature changes are not very large and no phase transformations (e.g. no large volume changes) occur within the medium Strain
original shape Tensile strain - or - direct strain
✏xx,✏yy deformed shape
Shear strain ✏xy
y Volumetric strain (dilatation)
✏xx + ✏yy x Tensor: Physical meaning ⌧ij ⌧yy ⌧xy direction face normal ⌧yx
⌧xx y
x Strain-rate tensor
• Time rate of change of the strain tensor
! ✏˙xx, ✏˙yy, ✏˙xy [1/s] x! =(x, y) position !v =(vx,vy) velocity
! @vx @vy 1 @vx @vy ✏˙ = ✏˙yy = ✏˙xy = + xx @x @y 2 @y @x ! ✓ ◆ 1 @v @v ✏˙ = i + j ! ij 2 @x @x ✓ j i ◆ • A deviatoric tensor has a mean (average) of zero, e.g.
2 1 1 1 (˙✏xx +˙✏yy)=0 ✏˙kk =0 ✏˙kk =0 2 2 ! 2 kX=1 Stress tensor