Mountain Building and
the Evolution of Continents

Pamela J. W. Gore
Georgia Perimeter College

Mt. McKinley, Alaska. Photo courtesy of Kip Tyler, Physical Geology student, Fall 1998.

Objective

Explain the process of isostasy and the concept of isostatic equilibrium.

Physiographic Provinces

• Appalachian belt
  • New England
  • Piedmont
  • Valley and Ridge
  • Blue Ridge
  • Appalachian Plateau
  • Ouachitas (Arkansas-Oklahoma)

• Coastal Plains
  • Atlantic Coastal Plain
  • Gulf Coastal Plain

• Central stable region
  • Canadian Shield
  • Central Interior
  • Great Plains

• Cordilleran belt
  • Rocky Mountains
  • Columbia River basalt flows
  • Colorado Plateau
  • Basin and Range Province (horsts and grabens)
  • Sierra Nevada (granite batholiths)
  • Pacific Coast Ranges
  • Cascade Range (volcanoes)

Know the FIVE physiographic provinces in Georgia. Be able to locate them on a map and say something about each (topography, basic rock types, and general age of rocks).

Mountain Belts

1. folding
   • anticlines
   • synclines
2. thrust faulting
3. metamorphism
4. igneous activity

There are a number of types of mountains, including:

• volcanic mountains
• folded and thrust-faulted mountains
• fault-block mountains
• mountains formed as a result of erosion of less resistant surrounding rocks (such as unearthed plutons)

An orogeny is a mountain building episode. Be familiar with the locations of the following:

• Appalachian Mountains
• Cordillera (Rocky Mountains, Cascades, Sierra Nevada, Coast Ranges)
• Andes Mountains
• Himalaya Mountains
• Alps
• Ural Mountains

Note that the longest mountain range on Earth is the mid-ocean ridge system, about 65,000 km (or 40,000 miles in length). The mid-ocean ridges are 500 to 5000 km wide and cover more than 20% of the Earth's surface.

Isostasy and Crustal Uplift

Consider low density crust floating on a denser mantle that flows.

When weight is applied to the crust, it subsides. When weight is removed, it rebounds (uplifts).

What are some ways in which weight could be applied to (or removed from) the crust?

The continental glaciers which once covered much of the northern part of North America were on the order of several thousand meters thick (judging by the thicknesses of modern glaciers). (See diagrams in glacier chapter.) A 3000 meter thick ice sheet is about 2 miles thick.
If we consider an ice sheet a mile or two thick, we realize that the added weight would have caused the Earth's crust to subside (or downwarp).
The continental glaciers melted about 10,000 years ago. In this amount of time, the land has been steadily uplifting due to the removal of the weight.
In the Hudson Bay region of Canada, as much as 330 m (about 1000 ft) of uplift has occurred.
This is a rate of uplift of roughly 3 - 4 cm/year.

How far has the land been elevated during your lifetime?

Evidence for crustal uplift.

• Fossil sea shells found in rocks thousands of feet above sea level.
• Roman temples with clam borings indicating subsidence followed by uplift.
• Wave-cut platforms (terraces) hundreds of meters above sea level in coastal regions of California.

Note that mountains have deep roots, as determined through seismological and gravity studies.

Continental crust averages about 35 km thick. Under some mountain chains, crustal thickness is approximately twice that thickness (about 70 km thick).

As erosion lowers mountains by erosion, isostasy raises them up again.
These processes operate until the mountain belt reaches "normal" crustal thickness again.

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This page created by Pamela J. W. Gore
Georgia Perimeter College
pgore@gpc.edu

Page created May 29, 1995
Modified March 10, 1997
Modified December 16, 1998
Last modified July 17, 1999