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Lecture Outlines
Physical Geology, 10/e
Plummer, McGeary &
Carlson
Mountain Belts and the
Continental Crust
Physical Geology 10/e, Chapter 20
Steve Kadel, Glendale Community College
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
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
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
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
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, arccontinent, or continent-continent convergence
– Subsequent gravitational collapse and spreading
may allow deep-seated rocks to rise to the surface
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
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
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
End of Chapter 20