Lithosphere and Asthenosphere

Question 1

Ridges

Answer 1

At isostatic equilibrium since no gravity anomaly.

All have the same shape when water depth is plotted against age of sea-floor. So, all form by the same process.

Lack a gravity anomaly.

Seismic velocity of mantle is smaller right beneath the ridge. So, it’s lower density, hot material there. Deformation of the geotherm leads to melt from intersecting the solidus. Low density, so no gravity anomaly despite height of ridge.

Observed heat flow by conduction is lower than predicted since cooled by hydrothermal circulation.

Question 2

Oceanic crust thickness

Answer 2

Nearly the same everywhere - 7km

Isostatic compensation occurs by lateral density changes, Pratt’s isostasy.

Question 3

Water depth and age of crust relationship

Answer 3

Water depth proportional to sqrt(age) if age < 70Ma.

So, can estimate age of sea-floor.

Question 4

Constant water depth

Answer 4

age of crust > 70Ma

Question 5

Plate-cooling models

Answer 5

As lithosphere moves away from the ridge axis, it cools and contracts i.e. becomes denser. So, there’s greater water depth to retain Pratt’s isostasy.

Ocean deepens with age. Shape of ridge is expression of its thermal structure. Ridges are high since they are hot and less dense.

Since lithosphere/asthenosphere boundary is a temperature boundary, it follows an isotherm.

Sea-floor flattens at 70Ma, so columns of oceanic crust eventually have the same density and temperature at any given depth. So, they’re at the same temperature. Plate-cooling model.

The lithosphere loses heat by conduction. If the oceanic crust reaches a stable temperature, it must be cooled from below by a constant heat source. So, the asthenosphere has a constant potential temperature of 1300 degrees celsius (if adiabatic gradient was extrapolated to the surface).

If asthenosphere were hotter, the final density of the columns would be lower and the ocean floor wouldn’t be at shallower depths.

Question 6

Heat flow by conduction

Answer 6

Conductive heat flow = conductivity * vertical temperature gradient

Question 7

Thermal plate thickness

Answer 7

Heat transfer changes from conduction to convection.

Question 8

Elastic thickness

Answer 8

Measure thickness of strong outer layer of Earth by how it bend to support volcanoes.
Homologous temperature < 0.5 - retains elastic strength

Question 9

Seismic thickness

Answer 9

Depth at which LVZ occurs at base of plates.

LVZ is hard to detect and sometimes is missing.

Question 10

Melting at ridges

Answer 10

Adiabatic decompression melting:
Rising material follows adiabatic gradient and intersects solidus 50km below the surface. Loses some heat due to latent heat of fusion.

Melt increases as the mantle potential temperature increases

Question 11

Solidus

Answer 11

onset of partial melting

Question 12

Melting by adding water

Answer 12

Adding water deforms the solidus. It can be intersected twice, so wet melt often recrystallises at it rises.

Question 13

Failed rift

Answer 13

If continent is extended by rifting but this stops at the necking stage, the crust remain thinned.

Will sink after rifting stops and fills with sediment to make a sedimentary basin.