Page 52 - Laboratory Manual in Physical Geology

C. Seismology (the study of Earth’s structure and composition using earthquake waves), mantle xenoliths, and laboratory
3

experiments indicate that the upper mantle is peridotite rock. The peridotite has an average density of about 3.3 g>cm and
is capable of slow flow. Seismology also reveals the thicknesses of crust and mantle layers.

1. Seismology indicates that the average thickness of basaltic ocean crust is about 5.0 km. Use the average density of basalt

(from part A above) and your isostasy equation ( Activity 1.5 , item D) to calculate how high (in kilometers) basalt floats in
the mantle. Show your work.

2. Seismology indicates that the average thickness of granitic continental crust is about 30.0 kilometers. Use the average

density of granite (from part B above) and your isostasy equation ( Activity 1.5 , item D) to calculate how high (in

kilometers) granite floats in the mantle. Show your work.

3. What is the difference (in km) between your answers in C1 and C2?

4. How does this difference between C1 and C2 compare to the actual difference between the average height of continents

and average depth of oceans on the hypsographic curve ( FIGURE 1.11C )?

D. REFLECT & DISCUSS Reflect on all of your work in this laboratory. Explain why Earth has a bimodal global topography.

37