The Origin of Mountains g110

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The Origin of Mountainsg110

• Lecture prepared by Bob Hall.
• Revised 8/6/2000

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The Origin of MountainsPoints to Be Covered

• Simple Mountains examples
• Volcanoes
• Dome or Fold Mts.
• Erosional
• Fault-Block
• Complex Mountain Systems
• Characteristics of Complex Mtn. Systems
• Geosynclinal theory Strange, unworkable fiction,
  begging to be replaced with a nonfiction theory.

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Origin of MountainsPoints to Be Covered (cont.)

• Geosynclinal Theory (cont.)
• Basic Concept
• The Big Trough
• Sedimentation
• The Cycle
• Continental Accretion A Corollary
• The Past is the Key to the Present
• Plate Tectonics to the Rescue

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Origin of MountainsSimple Mountains - Volcanoes
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Origin of MountainsSimple Mountains

• Dome and Fold Mountains

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Origin of MountainsSimple Mountains

• Erosional Mountains

Zion National Park
Zion National Paark, Utah
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Origin of MountainsSimple Mountains

• Fault-Block Mountains

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Complex Mountains Appalachians An Example

• Eastern Part The Piedmont and BlueRidge
• Igneous and metamorphic rocks
• Complexly faulted.
• Thrust westward over many thousands of feet of
  sedimentary rocks.

Fig. 8-26
Eastern Appalachians Piedmont and Blue Ridge
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Complex Mountains Appalachian Example

• Middle Part Ridge and Valley
• Northern part intensely folded sedimentary rocks
• Southern part thrust faulting (more intense
  deformation)
• Ridges and valleys alternating because of
  differential erosion.

Middle Appalachians
Fig. 8-26
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Complex Mountains Appalachian Example

• Western Part Allegheny and Cumberland Plateaus
• Sedimentary Rocks in horizontal position.
• Carved into mountains by erosion.

Western Appalachians
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Complex MountainsCharacteristics

• Sedimentary rock thousands of feet thick,
  elsewhere represented by a few hundred ft.
• Volcanic rocks interbedded with sedimentary
  rocks.
• Metamorphic rocks.
• Granitic roots (batholiths)
• Many faults, especially thrust faults.
• Erosion of rocks of different hardness.

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Geosynclinal Theory Geological Fiction

• Geosyncline
• Slow subsiding trough thousands of miles long and
  100s of miles wide
• Along continental margin.
• Receives marine sediments

Island Arc


Continental Shelf Geosyncline
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Geosyncline Theory - Sedimentation

• Outer Trough
• Sediments from the continent.
• Volcanics from island arcs.
• Subsides more rapidly than inner trough.
• Much greater thickness of sediments.

• Inner Trough
• Sediments from continent
• Not many volcanics
• Subsides less rapidly
• Less thickness of sediments

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Geosynclinal TheoryThe Cycle

• Subsidence and sedimentation.
• - All deposition in shallow water as trough
  slowly subsides.
• - Sedimentation just keeping up with
  subsidence.
• Downwarp, compression, thrust faulting,
  metamorphism
• Intrusion of batholiths
• Isostatic uplift to form mountains.
• Erosion to carve out details.

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Geosynclinal Theory The Cycle

Island Arc

Continental Shelf Geosyncline
1.
Figure 14-32
2.
3.
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Origin of MountainsGeosynclinal Theory
ContinentalAccretion A Corollary

• Continents increasing in size through time by
  welding of geosynclinal sediments.
• Oldest rocks should be in center of continents.
• Successively younger rocks should be found
  outward.
• North America supported this view.

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Origin of MountainsGeosynclinal Theory
Continental Accr.
lt0.2
1.5-2.0
2.0-2.8
Figure 21-21
0.9 To 1.2
1.2-1.5
0.5
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Origin of MountainsGeosynclinal Theory The
Past is the Key to the Present

• Where are all the modern geosynclines?
• No area seemed to fit the classic model.
• Change the model.
• - No longer visualized as a trough.
• - Thick wedge along continental margin.
• But, do these areas subside?
• - Not for the most part too low density.

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Origin of MountainsPlate Tectonics to the Rescue

• Plate collisions
• Plate subduction
• Continental accretion

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Origin of MountainsTerms

• Complex mountains
• Continental accretion
• Dome mountain
• Erosional mountain

• Fault-block mountain
• Fold mountain
• Geosynclinal theory

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Origin of MountainsStudent Responsibilities

• Know about the different forms of simple
  mountains. Know the characteristics of complex
  mountains. What were the fallacies of the
  geosynclinal theory of complex mountain systems?
  How have these ideas been replaced by plate
  tectonic?

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Mt. Everest