Why does it all happen?

Question 1

Energy source for plate motion

Answer 1

But at least account for energy loss by earthquakes

Mean power annually is 2*10^11 W

Question 2

Heat loss from Earth

Answer 2

4*10^13W

75% lost through plate creation in oceans

Question 3

Heat on Earth

Answer 3

Residual heat from separation of the core and the mantle
Heat from radioactive decay (predominantly in the continental crust - double the heat generation per m^3 than oceanic crust and 50 times the heat generation per m^3 than undepleted mantle

Question 4

Rate of heat conduction through rock

Answer 4

Measured with the thermal time constant which is inversely proportional to kappa. Kappa = 10^(-6) m^2*s^(-1) for most rocks
thermal time constant is proportional to distance over which conduction occurs i.e. the thickness of the lithosphere (125km). Thus mean thermal time constant is 62Myr

Question 5

Age of the Earth

Answer 5

If the Earth lost heat entirely by conduction, since its radius is 6371km, it would be 10^11 years old. This is far too old
No significant cooling by conduction has occurred to depths greater than 1000km - calculated from actual age of earth (4.5 billion years)

Question 6

Rayleigh number

Answer 6

Ratio of how long it takes to conduct heat over a certain distance compared to transferring heat by convection. Tells you how likely it is that a material will convect.

Rayleigh number for the mantle is 10^6. So convection is vigorous in the mantle

Question 7

How we know about the pattern of mantle convection

Answer 7

Difficult to observe from the surface since a thermal pulse from the base of the lithosphere would would take about 60Myr to reach the surface, by which time the plates will have moved significantly.
Convecting asthenosphere bends the lithosphere.

Question 8

Long wavelength gravitational anomalies

Answer 8

A mantle plume will bend the lithosphere above it and can raise it by up to 2km. But, the rising mantle is at 1300 degrees celsius, so is less dense than the surroundings. So, the excess topography above it has a smaller gravitational anomaly than would be expected.

e.g. long wavelength swell across hawaii

Question 9

Scales of convection

Answer 9

Plates move from ridges (hot and high) to subduction zones (low and cold). This is convection.

Since the ocean floor flattens, it’s heated at it’s base. So, there are small scale convection currents underneath it.

Separated by LVZ (probably low viscosity zone too) at the base of the plates

Question 10

Hotspots

Answer 10

Effectively stationary relative to each other due to rate of motion being much lower than the relative motion of plates.

Provide a reference for how the plates moved relative to the asthenosphere

Question 11

Shearing forces at base of plates

Answer 11

No significant correlation between size of plates and their velocities, so these forces are negligible.

Question 12

Slab pull

Answer 12

Length of subduction zone correlates to rate of plate motion. So, slab pull is a significant force.

Cold slab is more dense than the surrounding lithosphere.
Basalt transforms to eclogite and the density increases from 3000kg/m^3 to up to 3500kg/m^3