Chapter 20 – Mountain Building

1. Chapter 20 – Mountain Building Topics to be Covered: • Crust – Mantle Relationships • Convergent Boundary Mountains • Uplifted Mountains • Fault – Block Mountains

2. Earth’s Topography • Most of the Earth’s elevations are concentrated around 2 modes. • The 1st mode is elevations that are between 0 to 1 km. (land) • The 2nd mode are elevations that are between 4 to 5 km below sea level. (sea-floor)

3. Why two modes? “Because of different densities and thickness of continental and oceanic crust.” • Remember: Oceanic crust is thinner but more dense than the thicker continental crust! • Oceanic crust is mainly basalt (2.9 g/cm3) • Cont. crust is mainly granite (2.8 g/cm3) • Cont. crust will “float” higher on the mantle.

4. Elevations on the Crust • Elevations of the crust depend on the thickness of the crust. • Higher elevations (mountains) require thicker crust to support them. • A large mountain is held up by a large “root” of crust that extends into the mantle to support it. (p.525 Fig. 20-3) • This is all due to the principle of isostasy.

5. Isostasy • Isostasy is the displacement of the mantle by the crust until an equilibrium between crust and mantle is reached. • Downward force of gravity on the crust is balanced by the upward force of buoyancy in the mantle. • Think of it as a boat floating on water!

6. Example • As more mass is added to a boat, what happens to the height of the boat floating in the water? Sinks lower in the water • As more mass is added to the crust, it sinks lower into the mantle. • As mass is decreased on the crust, the crust can rise.

7. Isostatic Rebound • Erosion can decrease the mass of the crust. • This will cause less mass to be forced on the mantle. • What will happen to the crust?

8. Isostatic Rebound The process of the crust rising due to the removal of overlying material. • Erosion reduces the mass of mountains. • This decreased mass allows the crust to “float” higher on the mantle.

9. Orogeny • An orogeny is the process that forms mountain ranges. • Orogeny results in broad, linear regions of deformation, called orogenic belts. • Most of the orogenic belts are found along plate boundaries. (mainly convergent boundaries)

10. Quick Review of Convergent Boundaries • What happens at these boundaries? Plates are colliding • What type of force is involved? Compression • What are the results of these boundaries? Folding, faulting, metamorphism, volcanoes

11. Convergent-Boundary Mtns. • Oceanic-Oceanic Convergent boundary: - one oceanic plate is subducted under another, cause the sinking plate to melt. - As magma rises upward an island arc of volcanic mountains is formed. Example: Mt. Pinatubo, Philippine Islands

12. Convergent-Boundary Mtns. • Oceanic-Continental Convergent boundary: - Oceanic crust is subducted under continental crust. - Subducted oceanic crust melts. - Magma rises upward forming a volcanic mountain range on the continental crust. Example: Andes Mountains, South America

13. Convergent-Boundary Mtns. • Continental-Continental Boundaries: - Both pieces of crust have relatively low density and cannot be subducted. - Plate will instead push together and get deformed (folded, faulted). - This will build up thick areas of crust, thus creating the tallest mountains on Earth. Example: Himalayas

14. Nonboundary Mountains • Uplifted Mountains – large areas of gradual upward movement. (Adirondacks) • Fault-block mountains – blocks of crust lifted or tilted by faults. • Volcanoes over hotspots.

15. http://www.seed.slb.com/en/scictr/lab/buoy_exp/buoyancy.html