2.9 Seismic tomography and Mantle plumes

Question 1

What technique can be used to identify differences in density within the mantle?

Answer 1

seismic tomography

Question 2

What is seismic tomography?

Answer 2

a technique in which a virtual 3D model is built up by analysing the behaviour of seismic waves as they pass through sections of a body

it involves looking for variations in P wave velocities

Question 3

How does seismic tomography work?

Answer 3

variations in P wave velocity are recorded by analysing anomalies where earthquake waves do not arrive at the seismograph at the expected times

Question 4

What is the significance of P waves that are faster than average for the Earth?

Answer 4

waves travel faster in colder rocks

colder rocks are more rigid

Question 5

Why is the seismic velocity higher than expected under mountain ranges?

Answer 5

this observation supports the theory of isostasy which suggests that mountains have cold ‘roots’ which extend into the mantle

Question 6

Why is the seismic velocity higher than expected near ocean trenches?

Answer 6

seismic anomalies indicate the presence of cold rock - the subducting plate

Question 7

What is the generally accepted explanation of negative (slower) anomalies in seismic velocity in the mantle?

Answer 7

mantle plumes
these anomalies extend to the core-mantle boundary and are associated with surface hotspots

Question 8

What is a hotspot?

Answer 8

an area of high heat flow and volcanic activity at the surface associated with mantle plumes

Question 9

What is a mantle plume?

Answer 9

a stationary area of high heat flow in the mantle, which rises from great depths and produces magma that feeds hot spot volcanoes

Question 9

What is a mantle plume?

Answer 9

a stationary area of high heat flow in the mantle, which rises from great depths and produces magma that feeds hot spot volcanoes