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

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

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/cm 3 ) Cont. crust is mainly granite (2.8 g/cm 3 ) Cont. crust will “float” higher on the mantle.

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.

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!

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.

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?

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.

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)

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

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

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

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

Nonboundary Mountains 1.Uplifted Mountains – large areas of gradual upward movement. (Adirondacks) 2.Fault-block mountains – blocks of crust lifted or tilted by faults. 3.Volcanoes over hotspots.

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