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 Eastern Nor American Mobile Bel Platform
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 km 2 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 30 o 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