Lecture Outlines Physical Geology, 10/e

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Lecture OutlinesPhysical Geology, 10/e

• Plummer, McGeary Carlson

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Mountain Belts and the Continental
CrustPhysical Geology 10/e, Chapter 20
Steve Kadel, Glendale Community College
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Mountain Belts and Earths 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

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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

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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

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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

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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

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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

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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

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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

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End of Chapter 20