Driving Forces of Plate Tectonics Flashcards
Source of energy to drive plates?
- Heat, radioactive decay in core and mantle
- Surface by mantle convection
Possible mechanisms for plate motion
- Mantle drag
- Edge-Force model
Edge-force model
- Plates driven by forces applied to their margins/edges
- Slab-pull, Suction, Ridge-push
Mantle drag
- Plates dragged along by the mantle
- Plates move in response to viscous drag exerted on base of lithosphere by lateral motion of asthenosphere at top of convection cells
Relative importance of driving and resistive forces
- Plate characteristics vs. velocity
- Clues from stress field within plates
Why is mantle drag not a main mechanism at present?
- Poor Coupling: driving lithosphere at 40mm/yr requires 200 mm/yr asthenosphere motion, which is unreasonably fast
- Large cells of simple regular geometry cannot explain motion of small plates or plates with irregular margins
- But it was likely important for supercontinent breakup
Driving forces = resistive forces
Present velocities are constant
Features of Driving forces diagram?
- Trench suction
- Ridge push
- Slab pull
- Drag under continent
- Drag under ocean
- Negative buoyancy (downing slab)
Force: Ridge Push
Gravitationally sliding away from elevated (hot, buoyant) ridge
Force: Ridge Resistance
Resistance due to internal strength of elastic lithosphere (minor effect)
Forces beneath plate interiors
- Mantle drag (Force and Resistance), viscous shear stress between lithosphere and asthenosphere
- Drag under ocean and continent (also Force and resistance
Which type of plate (continent or ocean) has a greater force beneath it? Why
8 times greater beneath continents
- B/c it sticks down deeper into mantle where it is denser and has a greater grip on overlying plate (b/c the plate doesn’t float as high)
If velocity of asthenosphere > velocity of plate
Driving Force of drag under plate
If velocity of asthenosphere < than velocity of plate
Resistance Force of drag under plate
Force: Slab Pull
Due to negative buoyancy (Fnb) of cold dense slab
Force: Trench suction
Extensional force on landward side of subduction zone
Force: Slab Resistance
Mainly at tip of descending plate (where it is 5 - 8 times greater than viscous drag on upper and lower slab surfaces)
Force: Bending Resistance
Resistance to elastic flexure of plate
- Subducting plate must bend to subduct
Force: Overriding Plate Resistance
Friction between plates at subduction zone
When Force Slab Pull approximately = Bending Resistance plus Overriding Plate Resistance
- Downgoing slab achieves terminal velocity
When Force Slab Pull > Bending Resistance plus Overriding Plate Resistance
- Slab descends faster than terminal velocity
- Tension in slab
When Force Slab Pull < Bending Resistance plus Overriding Plate Resistance
- Slab descends slower than terminal velocity
- Compression in slab
What is the origin of trench suction force (Fsu)
Several possibilities
- Overriding plate collapses towards steepening plate (subducting plate)
- Slab ‘rollback’ (moves in time towards subducting plate)
- Secondary convective flow induced by motion of lithosphere
- Active volcanism (in back-arc) forces lithosphere apart and pushes plate back towards trench
Relative importance of driving forces
- Absolute plate velocity (NNR) versus plate area
- Plate velocity versus % plate circumference connected to subducting slab
- Plate velocity versus continental area of plate
Absolute plate velocity (NNR)
- Velocity is independent of plate area
- Inconsistent with mantle drag
- Mantle drag less important b/c it would expect that faster plates would be bigger but this is not the case
Plate velocity versus % plate circumference connected to subducting slab
- Plate velocity is larger for plates attached to big downgoing slabs
- Faster plates have more subduction along boundaries
- Indicates greater slab pull
Plate velocity versus continental area of plate
- Plate velocity is slower if attached to large continents
- Mantle drag inhibits plate motion rather than speed it up
- Oceanic plates faster b/c less mantle drag from being less deep in mantle
