Download Tectonics and Landscape

Continental Landscapes
• Landscape influenced by tectonics,
climate & differential weathering
– Most landforms developed within the last 2
million years
– System moves toward an equilibrium
Continental Landscapes
• 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
North
American
Craton
Shield
Western North
American Mobile
Belt
Platform
Eastern Nor
American
Mobile Bel
Accreted terranes
along convergent
margin
Growth of North American Continent
Continental Landscapes
• Climate controls the hydrologic cycle
– Weathering, erosion & transport are
results of climate
– Climate is controlled by latitude and
topography
– Conversely, topography is influenced by
climate
Continental Landscapes
• Differential erosion works at all scales
• Erosion is a selective process
– Impact of erosion on hard & soft rock
creates variations in topography
• Limestone valleys in the east; ridges in the west
• Sandstone and limestone cliffs; shale slopes
– Tectonic activity, folding & faulting, may
exaggerate differences in erosion
Fig. 23.1
Evolution of Shields
• Shields are the eroded remnants of
folded mountain belts
– Isostatic adjustment and erosion have
nearly reached equilibrium
– Local relief is usually < 100 m
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
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
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
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
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
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
The Michigan Basin
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
Fig.23.10. Differential erosion in horizontal strata
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
Stable Platform of NA
• Major dome structures include:
– Cincinnati arch
– Wisconsin dome
– Ozark dome
– Basins form between these domes
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
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
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
Fig. 23.1
Northern Rocky Mountains
Young folded and faulted mountains
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
Old folded and faulted mountains
Fig. 23.15. Differential erosion in folded rocks
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
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
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
Fig. 23.1
Fig. 23.19. Basin & Range model landscape
Fig. 23.18d. Faceted spurs in block faulted mountains
Wasatch Mountains, UT
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
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
Fig. 23.1
Fig. 23.21. Landscape development in basalt plains
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
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
Fig. 23.1
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
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
Fig. 23.5. Rates of erosions of a mountain belt