Isostasy

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Isostasy

• Isostasy (Greek ísos "equal", stásis
  "standstill") is a term used in geology to refer
  to the state of gravitational equilibrium between
  the earth's lithosphere and asthenosphere such
  that the tectonic plates "float" at an elevation
  which depends on their thickness and density.
• Parts of the crust will rise or subside (sink)
  until stability is achieved

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• For example
• As a mountain erodes, mass is removed and the
  crust beneath the mountain rises to maintain
  equilibrium
• As sediment is deposited, the crust subsides
  (sinks) beneath its weight

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• The weight of the ice pushes down on the crust.
  When the ice melts the crust pop back up as the
  mantle rebounds
• This is an example of isostasy.

The same thing happens when a mountain erodes.
There is less weight pushing down on the mantle,
so it rebounds.
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Mountain Building
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Orogony

• Cycle of processes that form mountain ranges
• Mostly associated with converging plate boundaries

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Mountains
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Most commonly form at converging plate boundaries

• Ocean-Ocean
• Island arcs such as Japan and Indonesia
• Ocean-Continent
• Volcanic mountains such as the Andes
• Continent-Continent
• Folded and Faulted mountains such as the Himalayas

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Folded Mountains
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Orogenies can also happen far from plate
boundaries

• Fault-Block Mts.
• Crust is broken up due to tensional forces
• Grand Tetons in Wyoming

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Grand Teton Mt.
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Dome Mountains

• Nearly circular folded mountain
• Individual isolated structures that occur in
  areas of horizontal strata

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Adirondack Mts.
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Mt. Marcy, NY (5344 ft. highest peak in the
Adirondacks)
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• Dome Mountains can also be formed from hot molten
  material (magma) rising from the Earth's mantle
  into the crust that pushes overlying sedimentary
  rock layers upward to form a "dome" shape.
• Unlike a volcano, the magma typically does not
  reach the Earth's surface. Instead, the magma
  cools underneath the surface and forms the core
  of the mountains.
• Laccoliths

Algonquin Peak, 5115 ft. 2nd highest peak in the
Adirondacks
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• An example of a dome-shaped mountain is Half
  Dome in the Sierra Nevada range in California.
  It's made of granite, and was once a large blob
  of magma pushed up through the Earth. Granite is
  much harder than other rock, and so it doesn't
  erode as easily as the rest of the mountain. The
  softer layers of sedimentary rock were washed
  away, leaving the hard granite dome.