1. Lecture Outlines Physical Geology, 10/e
Plummer, McGeary & Carlson

2. Steve Kadel, Glendale Community College
Mountain Belts and the Continental Crust Physical Geology 10/e, Chapter 20
Steve Kadel, Glendale Community College

3. Mountain Belts and Earth’s Systems
Mountain belts are chains of mountain ranges that are 1000s of km long
Commonly located at or near the edges of continental landmasses
Composed of multiple mountain ranges
Mountain belts are part of the geosphere
Form and grow, by tectonic and volcanic processes, over tens of millions of years
As mountains grow higher, erosion by running water and ice (hydrosphere) occur at higher rates
Air (atmosphere) rising over mountain ranges directly results in precipitation and erosion

4. Characteristics of Mountain Belts
Mountain belts are very long compared to their width
The North American Cordillera runs from southwestern Alaska down to Panama
Older mountain ranges (Appalachians) tend to be lower than younger ones (Himalayas) due to erosion over time
Young mountain belts are tens of millions of years old, whereas older ones may be hundreds of millions of years old
Even older mountain belts (billions of years) have eroded nearly flat and form the ancient stable cores (cratons) of the continents
Shields - areas of cratons laid bare by erosion

5. Rock Patterns in Mountain Belts
Mountain belts typically contain thick sequences of folded and faulted sedimentary rocks, often of marine origin
May also contain great thicknesses of volcanic rock
Fold and thrust belts (composed of many folds and reverse faults) are common, indicating large amounts of crustal shortening (and thickening) has taken place under compressional forces
Mountain belts are common at convergent boundaries
May contain large amounts of metamorphic rock
Erosion-resistant batholiths may be left behind as mountain ranges after long periods of erosion

6. Rock Patterns in Mountain Belts
Erosion-resistant batholiths may be left behind as mountain ranges after long periods of erosion
Localized tension in uplifting mountain belts can result in normal faulting
Horsts and grabens can produce mountains and valleys, respectively
Earthquakes common along faults in mountain ranges

7. Evolution of Mountain Belts
Rocks (sedimentary and volcanic) that will later be uplifted into mountains are deposited during accumulation stage
Typically occurs in marine environment, such as an opening ocean basin or convergent plate boundary
Mountains are uplifted at convergent boundaries during the orogenic stage
May be the result of ocean-continent, arc-continent, or continent-continent convergence
Subsequent gravitational collapse and spreading may allow deep-seated rocks to rise to the surface

8. Evolution of Mountain Belts
After convergence stops, a long period of erosion, uplift and block-faulting occurs
As erosion removes overlying rock, the crustal root of a mountain range rises by isostatic adjustment
Tension in uplifting and spreading crust results in normal faulting and production of fault-block mountain ranges

9. Evolution of Mountain Belts
Basin-and-Range province of western North America may be the result of delamination
Overthickened mantle lithosphere beneath old orogenic mountain belt may break off and sink (founder) into asthenosphere
Resulting inflow of hot asthenosphere can stretch and thin overlying crust, producing normal faults under tension

10. Growth of Continents
Continents grow larger as mountain belts evolve along their margins
Accumulation and igneous activity (e.g., when volcanic arcs plaster against continents during convergence) add new continental crust beyond old coastlines
New accreted terranes can be added with each episode of convergence
Western North America (especially Alaska) contains many such terranes
Numerous terranes, of gradually decreasing age, surround older cratons that form the cores of the continents

11. End of Chapter 20