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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.

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chapter 20 mountain building
Chapter 20 – Mountain Building

Topics to be Covered:

  • Crust – Mantle Relationships
  • Convergent Boundary Mountains
  • Uplifted Mountains
  • Fault – Block Mountains
earth s topography
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)
why two modes
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.
elevations on the crust
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 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!
  • 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
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 rebound1
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.
  • 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
Quick Review of Convergent Boundaries
  • What happens at these boundaries?

Plates are colliding

  • What type of force is involved?


  • What are the results of these boundaries?

Folding, faulting, metamorphism, volcanoes

convergent boundary mtns
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 mtns1
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 mtns2
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
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.