Download 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
• 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. (seafloor)
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 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.
http://www.seed.slb.com/en/scictr/lab/buoy_
exp/buoyancy.html