2.4 The asthenosphere

Question 1

What are the physical properties of the crust and the uppermost part of the mantle?

the lithosphere

Answer 1

solid, brittle, rigid

Question 2

What are the physical properties of the layer beneath the lithosphere?

Answer 2

solid, plastic, rheid

Question 3

Define the term rheid.

Answer 3

a non-molten solid that deforms by viscous or plastic flow

Question 4

Define the term lithosphere.

Answer 4

the upper rigid layer of the crust and upper mantle

Question 5

Define the term asthenosphere.

Answer 5

a layer of the mantle below the lithosphere characterised by low seismic velocities / its rheid properties

Question 6

What happens to seismic velocities in the asthenosphere?

Answer 6

they decrease (due to the less rigid nature of the mantle material in this layer)

Question 7

What is the LVZ?

Answer 7

the Low Velocity Zone (where seismic velocities reduce in the upper mantle)

Question 8

Why does the asthenosphere behave as a rheid?

Answer 8

the upper mantle is at a high temperature resulting in 1-5% melting (partial melting) of the minerals with the lowest melting points, reducing the rigidity

Question 9

Define the term partial melting.

Answer 9

when a small proportion (1-5%) of a rock melts

the liquid fraction surrounds crystals which are still solid

Question 10

What is the approximate depth of the asthenosphere?

Answer 10

100 km (average)

the actual depth of the asthenosphere is taken as the 1300°C isotherm