L8 - Plate Driving Forces

Question 1

What are the main forces involved in a subducting slab? (4)

Answer 1

Ridge push
Slab pull
Basal drag
Mountain range resistance

Question 2

What causes the driving forces of subduction? (1)

Answer 2

A consequence of gravity acting on lateral density contrasts

Question 3

What causes ridge push? (2)

Magnitude? (1)

Answer 3

At any depth above the level of isostatic compensation
Pressure beneath the MOR > beneath old seafloor
2-3TN/m along strike

Question 4

What is slab pull? (2)

Magnitude? (1)

Answer 4

Force is exerted on the plates by the sinking of cold material into the mantle
Material is colder than the surrounding mantle so buoyancy acts to drag plates towards subduction zones
Over 10TN/m

Question 5

What are the resistive forces on the subducting slab that mean net force transmitted < slab pull? (3)

Answer 5

Drag from the surrounding mantle as slab moves through
Friction on the subduction zone megathrust
Work done in bending the plate in the outer rise

Question 6

What does it mean that the outer rise of a subducting slab breaks in EQs? (1)

Answer 6

Force transmitted from a subducting slab to the horizontal part of the plate is limited by the strength of faults

Question 7

What is known about basal drag? (3)

Answer 7

Under fast-moving plates, the seismic anisotropy is aligned with plate motion direction
Relative motion between the plate and underlying mantle is in plate motion direction
Doesn’t say if force is driving or resistive

Question 8

What is the argument for basal drag being resistive? (1)

Where is it less clear? (1)

Answer 8

For fast-moving plates (India and Pacific), if basal drag was a driving force, not enough resistive force for the forces to balance
Under slow-moving plates (Africa) as there are few driving forces

Question 9

What does the size of basal drag depend on? (2)

Answer 9

Plate velocity relative to underlying mantle

Thickness and viscosity of the asthenosphere

Question 10

Where and why do mountain ranges exert a force? (2)
What does the force depend on? (1)
Magnitude? (1)

Answer 10

Exert it on surrounding lowlands
As P at a given depth beneath mountains > beneath lowlands (above isostatic compensation depth)
Thickness and density structure of the lithosphere
Up to 6TN/m

Question 11

What does the relationship between plate velocity and the percentage of the circumference by plate boundary type imply? (1)
What is the bi-modal nature seen? (1)
What is the problem with these conclusions? (1)

Answer 11

Slab pull may be the dominant force
Slow plates have no subduction, fast plates have subduction
Too few tectonic plates for meaningful statistics