Prepared by Mark R' Noll

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• Prepared by Mark R. Noll
• SUNY College at Brockport

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Continental Landscapes

• Landscape evolution influenced by tectonics,
  climate differential weathering
• Most landforms developed within the last 2 my
• System moves toward an equilibrium condition

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Continental Landscapes

• Continents are composed of 3 main tectonic
  features
• Large flat shield area of ancient igneous
  metamorphic rocks
• Stable platform where shield is covered by
  sedimentary rocks
• Folded mountains formed at convergent margins

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Continental Landscapes

• Climate controls the action of the hydrologic
  cycle
• Weathering, erosion transport are results of
  climate
• Climate is controlled by latitude and topography
• Conversely, topography is influenced by climate

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Continental Landscapes

• Differential erosion works at all scales
• Erosion is a selective process
• Impact of erosion on hard soft rock creates
  variations in topography
• Tectonic activity, folding faulting, may
  exaggerate differences in erosion

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Fig. 23.1
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Evolution of Shields

• Shields are the eroded remnants of folded
  mountain belts
• Isostatic adjustment and erosion have nearly
  reached equilibrium
• Local relief is usually lt 100 m

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Fig. 23.4. Evolution of a Shield
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Rates of Uplift Erosion

• Uplift erosion are contemporaneous processes
• Uplift estimates average 6 mm/yr
• 6 km in 1 my
• 5 to 10 my to form a large mountain belt
• Erosion rates in mountain areas 1 to 1.5 mm/yr
• Uplift is 5x erosion

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Rates of Uplift Erosion

• Erosion rate is dependent on uplift elevation
  differences
• Rates of erosion decrease with decreasing
  elevation
• New mountains deeply eroded by the time mountain
  building is complete
• Erosion continues at a progressively slower rate

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Fig. 23.5. Rates of erosions of a mountain belt
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Canadian Shield

• The Canadian Shield is typical of shields
  worldwide
• Covers 1/4 of NA, over 3 million km2
• Basic structural features well exposed
• Only relief is resistant rocks up to 100 m above
  adjacent surface
• Evidence shows the core of several different
  mountain belts

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Stable Platforms

• Areas of sedimentary rock covering a shield
• Landforms are the result of erosion of flat lying
  sedimentary rocks
• Dendritic drainage is common
• Small warps in crust influence pattern

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Stable Platforms

• Dendritic drainage develops on flat lying
  sedimentary rocks
• Homogeneous surface covers large area
• Little or no structural control
• Streams develop equally in all directions
• Variations in landforms develop as streams downcut

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Stable Platforms

• Differential erosion develops vertical variations
  - cliffs and slopes
• Resistant rock layers form cap rocks on plateau
• Steep cliffs may form in resistant layers
• Plateau may be cut into smaller mesa

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Stable Platforms

• Differential erosion is most pronounced where
  beds are tilted
• Strike valleys form as weak layer is eroded away
• Resistant layers remain as asymmetrical ridges
• Cuestas - gently inclined
• Hogbacks - steeply inclined

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Stable Platforms

• Major structural features are large domes and
  basins
• Form while area is covered by shallow seas
• Sedimentary layers may dip up to 30o along flanks

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Stable Platforms in Arid Climates

• Differential erosion produces an array of
  features easily seen
• Buttes, pinnacles, pillars columns form by
  erosion on receding cliffs
• Joints play a role
• Natural arches form where groundwater causes
  erosion in a cliff face

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Fig.23.10. Differential erosion in horizontal
strata
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Stable Platform of NA

• Three distinct regions
• Paleozoic strata in the east
• Great Plains underlain by Mesozoic Cenozoic
  strata
• Atlantic Gulf coastal plains
• Note areas north of the Ohio and Missouri Rivers
  have been glaciated

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Stable Platform of NA

• Major dome structures include
• Cincinnati arch
• Wisconsin dome
• Ozark dome
• Basins form between these domes

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Stable Platform of NA

• Strata dip gently westward in the Great Plains
• Erosion forms cuestas and intervening lowlands
• The Black Hills (SD) are an exception
• Dome structure
• Surrounded by hogbacks strike valleys

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Stable Platform of NA

• Coastal plains are characterized by alternating
  layers of sandstone shale dipping gently
  seaward
• Topography is low cuestas and wide strike valleys
• Trellis stream patterns are common

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Folded Mountain Belts

• Folded mountains have complex structures
  including tight folds, thrust faults, accreted
  terranes and igneous intrusions volcanics
• Landforms are variable
• Depend on stage of development
• Differential erosion carves out weak zones

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Appalachian Mountains

• Ridge valley province
• Classic example of landscapes on folded and
  thrust faulted strata
• Paleozoic folding, covered by Cretaceous and
  Tertiary sediments
• Renewed erosion removed sedimentary cover and
  superposed east flowing streams - trellis patterns

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Fig. 23.15. Differential erosion in folded rocks
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Continental Rifts

• Dominant structure is normal faulting parallel to
  rift
• Large vertical displacements
• Horst graben structures develop
• Fault scarps form steep cliffs
• Stream erosion cuts into cliffs forming faceted
  spurs

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Continental Rifts

• Drainage feed isolated block faulted valleys
• Lakes form in central basins
• In arid regions playa lakes are temporary
• Weathering produces high sediment loads
  alluvial fans develop
• Bajada form as fans grow merge
• Pediment form as mountain front retreats

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Basin Range Province

• Large region of uplift extension
• Block faulting produced alternating mountain
  ranges basins
• System is in early stages of development as
  basins are large in north
• Continued uplift is evident from complex faceted
  spurs

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Fig. 23.19. Basin Range model landscape
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Fig. 23.18d. Faceted spurs in block faulted
mountains Wasatch Mountains, UT
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Flood Basalts

• Basaltic plains form from flood basalts
• Lava flows disrupt stream patterns
• Streams follow margins of basalt flows
• Cinder cones are eroded leaving volcanic necks
• Inverted valleys form where lava filled old
  stream valley - erode into mesas

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Basaltic Plains of NA

• The Columbia Plateau and Snake River Plain are
  one of the largest basalt plains in the world
• Fluid basalt covered wide areas, filling valleys
  and covering mountains
• Created new surface, currently eroding
• Columbia Plateau is older, more eroded

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Fig. 23.21. Landscape development in basalt
plains
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Magmatic Arcs

• Dominated by volcanic landforms
• Erosion removes volcanoes, leaving deeper igneous
  intrusions metamorphic rocks
• Circular landforms are common
• Drainage patterns are complex and difficult to
  establish

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Cascade Volcanic Chain

• Magmatic arc built on continental crust
• String of large composite volcanoes 80 km wide by
  500 km long
• Smaller volcanoes fill in gaps
• Volcanic activity has been persistent since the
  middle Tertiary
• Wide range of landforms present

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End of Chapter 23