Author Question: Use Figure 8.20 to compare the P-wave velocities of oceanic and continental crust at a depth of 10 ... (Read 98 times)

luvbio · **on:** Jul 16, 2018

Use Figure 8.20 to compare the P-wave velocities of oceanic and continental crust at a depth of 10 km. Why are these velocities different?

What will be an ideal response?

Question 2

Use data graphed in Figures 8.18 and 8.20 to write a paragraph explaining why geologists interpret the lower continental crust to consist of igneous rocks that are more mafic than the rocks composing the upper continental crust.

What will be an ideal response?

livaneabi · **Reply #1 on:** Jul 16, 2018

Answer to Question 1

The P-wave velocity of oceanic crust at a depth of 10 km is 7 km/s. The P-wave velocity of continental crust at a depth of 10 km is 6 km/s. The difference is due to composition (rock-type). The oceanic crust is made up of mafic rocks and the continental crust at 10 km is made up of felsic rocks.

Answer to Question 2

Figure 8.20 shows that P-wave velocity increases downward in the crust in a gradual way until about 35 km where there is an abrupt increase in velocity. The gradual increase could simply be a result of the deeper rocks experiencing higher pressure. The abrupt increase at about 35 km, however, would suggest a change in composition. Figure 8.18 shows that mafic igneous rock (e.g., basalt) has higher seismic velocities than felsic rocks (e.g., granite).