Chapter 11: Mountains, Basins, And Continents

Density of materials in the crust. Temperature of the crust and mantle. Thickness of the crust.

orogeny

passive

A (subduction zone) Crust is thickened by subduction zone magma, crustal shortening, and heating and replacement of lithosphere by athenosphere. B (other one) Crustal thickness increases as one continent is shoved over another. C (arrows pushing up a mountain) Asthenosphere moves upward into the lithosphere and causes uplift; may occur near hot spots or plate boundaries.

Continental crust is thicker than oceanic crust. Oceanic crust averages 7km in thickness. Continental crust averages 30 to 50 km in thickness.

Intrusion that cuts across both terranes. Single rock unit overlying the two terranes.

Sediment from one terrane deposited over the other.

Different fossils in two terranes of the same age.

a wedge-shaped zone of faults, folds, and metamorphosed rocks formed along the upper parts of a subduction zone as material is scraped off.

Rising mantle material first encounters the base of the lithosphere. Domal uplift forms at the surface, stretching the crust. Rift arms are formed by faulting, and eruptions of lava begin. Complete rifting of the continent occurs along two arms of the rift, while the third arm becomes inactive. Seafloor spreading continues and a passive continental margin is created as the mid-ocean ridge moves farther out to sea.

the Afar region in East Africa, and Yellowstone National Park in North America.

Local mountains are small features and do not involve regionally thickened crust. Regional mountain ranges are typically very long and involve regionally thickened crust.

A (subduction zone) Extension occurs in front of the arc, causing the crust to thin by normal faulting; forms a forearc basin. B (on island) Extension occurs behind the arc, where the crust is hot and weak; thins the crust by normal faulting. C (in water behind island) Extension behind the arc may be great enough to form a back-arc basin with a small-scale version of a mid-ocean ridge.

erosional remnant

the uplift of crust that occurs after an overlying mass has been removed, such as when an ice sheet melts away or when erosion strips material off the top of a thick crustal root of a mountain

decreases

Western Canada Convergent margin collisions; thickened crust. Andes Above a subduction zone; magmatic additions and crustal thickening. Alps Collisions between Europe and smaller continental blocks from the south. East African Rift Magmatic heating of the crust, thinning of the lithosphere, mantle upwelling. Tibetan Plateau and Himalaya Continental collision of India and Asia. Great Divide Range in eastern Australia No plate boundary currently there; geologists investigating the cause of the uplift.

Subduction zones Continental collisions Mantle upwellings

30 to 50 km 7 km

Central region consists primarily of Precambrian metamorphic and igneous rocks.

Metamorphic and igneous rocks underlie much of the continent.

Boundary between flat-lying platform sedimentary rocks and underlying crystalline rocks separates rocks with very different ages and geologic histories.

Broad region surrounding the central area is characterized by nearly horizontal sedimentary rocks that were deposited on top of the basement.

A large desert to the north of the Tibetan Plateau; this desert is 3,000m lower than the plateau and is partially filled by sediment from the adjacent highlands.

The largest, highest, and flattest plateau on Earth; average elevation is 5 km.

The world's highest mountain range, with many peaks more than 8 km above sea level.

The world's highest mountain, rising 8,850 m above sea level.

As a result of normal faulting. Along plate margins. On both oceanic and continental plates.

Normal faults dip in opposite directions, cutting the crust into wedge-shaped blocks. Fault movement form basins on the dropped blocks and mountains from the upthrown blocks. Only small amounts of extension occur with nonrotating fault blocks.

Normal faults dip in the same direction, cutting the crust into book-shaped blocks. Blocks and faults rotate, causing down-corners to become basins and up-corners to become mountains or ridges. Greater amounts of extension may occur with rotating faults and blocks.

Sierra Nevada Moderately high elevation. Basin and Range Largely due to very thin lithosphere. Colorado Rockies compression and shortening of the North American Plate. Great Plains Elevation decreases toward the east, as lithosphere gets cooler and thicker. Appalachian Mountains Ancient collision of North America and Africa; elevation lessened by erosion.

Folding can cause warping and uplift. A folded, hard layer can be uplifted and more resistant to erosion than surrounding rocks.

Formed of piled volcanic material, such as scoria, ash, lava, flows, debris flows, and mudslides. Sometimes local and not accompanied by regional increases in crustal thickness. Varied in size from small cinder cones to large shield and composite volcanoes.

Causes broad uplifts and basins within the continent in response to loading of sediments along the margin and the emplacement of thrust sheets.

Sea-level rise can cause significant flooding well into the interior of a continent.

Weather patterns can change, causing a change in climate; as an example, rain-shadow deserts may form.

Latitude can be changed over time, causing drastic changes in climate of the area.

tectonic terrane

Largest type of basin; underlain by thin, previously rifted crust; receives sediment from the continent.

Normal faults create downdropped blocks that accumulate sediment; may evolve into a passive margin.

Foreland basin occurs when crust is depressed by the weight of thrust sheets; forearc basins may arise form in accretionary prisms.

Huge basins form where a broad region drops in elevation; may be due to regional cooling or density changes.

Basins may develop if one block drops down relative to another in this mostly horizontal movement fault, or pull-apart basins may develop where motion along nearby faults causes the crust to pull apart.

thickened, causing uplift and an increase in elevation

Have a central region called a continental shield and a surrounding region called a continental platform. Be tectonically stable and far from plate boundaries. Contain sedimentary rocks that have important records of past environments and events.

Present-day configuration of the continents and oceans found. North America separated from Africa and South America; Atlantic Ocean forms; Laurasia and Gondwana are the supercontinents. Pangaea formed; Appalachian and Ouachita Mountains created. Europe and North America joined; Avalonia collided with North America. Continents dispersed; Gondwana was formed in the Southern hemisphere. All major continents were joined as Rodinia.

isostasy

Erosion. Deposition or burial by volcanic rocks. Type of crust (continental or oceanic). Deformation (such as compression or stretching).

A local mountain is created if overthrust block is uplifted faster than it is eroded or it is composed of erosion-resistant rocks.

One block slips down, forming a basin; the other remains high or is moved upward and can form a local mountain.

It warps and uplifts earth's surface as well as the underlying rock layers; uplift of a hard layer can create a local mountain.

A rock not easily weathered resists erosion and also may protect softer rocks beneath it from erosion; the area is left higher than the surroundings and may form an erosional remnant, ridge, or a mesa.

normal-fault

pull-apart

forearc

foreland

Folding Volcanism Differential erosion Thrust and normal faulting

Subduction creates deep oceanic trench, which acts as a deep oceanic basin.

Subduction-related mountain belt thickens by the addition of volcanic rock at and below the surface.

Convergence causes horizontal compression and creates a fold and thrust belt behind the main mountain belt.

Weight of the thrust sheets causes the continent to flex downward and forms a foreland basin.

As the mountain belt forms, uplift is faster than erosion and the mountain grows. Weathering and erosion begin to wear down the mountain as uplift slows. As erosion continues, less weight holds down the crustal root, and isostatic rebound causes uplift. Continuing erosion and isostasy cause rocks deep in the crust to be uplifted and exposed at the surface.

Different parts of the continents are of different ages.

Deep-ocean crust. Pieces rifted off another continent. Island arcs. Oceanic islands or plateaus.

An underthrust, as one continental plate is shoved beneath another. Faulted, folded, and cleaved rocks in high mountain belts. Sedimentary basins in front of the thrust sheets. Thrust and normal faults in the overriding plate.

accretion

Oceanic crust Pillow basalt, deep-sea sediment. Island arcs Andesitic volcanic rocks and volcanic derived sedimentary rocks. Piece rifted off another continent Thick granitic crust and continental sediment and rocks.

Europe collided with Siberia 200-300 mya, forming a range in the center of the new continent as the landmasses joined.

Rifting once again occurred around 200 Ma and created the Atlantic Ocean; North America underwent a cycle of rifting, collisions, and rifting. Convergence and collisions occurred between about 500 Ma to 220 Ma, causing the Taconic, Acadian, and Allgehnian orogenies. Rifting occurred to separate North America from Rodinia around 600 Ma.

Rifting thinned the crust along the western edge of North America; west became a passive margin with a broad shelf. Island arcs formed offshore and began to collide with the edge of the continent; Ancestral Rockies formed due to collisions in the east. Subduction along the western edge cause volcanism; inland, the continent flexed down, causing widespread flooding. Subduction began to occur at a lower angle and caused the Laramide Orogeny. Part of the convergent margin became transform (the San Andreas fault); Basin and Range Province and Rio Grande Rift formed.

Layers are thicker in the center of the basin, indicating the basin was subsiding during deposition. Rock layers form a bull's-eye pattern with the youngest rocks in the center, indicating the presence of the basin. It may have formed during an episode of continental rifting.

Dense lithosphere replaced by less dense asthenosphere. A region with thin lithosphere (such as the mid-ocean ridge)

Taconic About 450 Ma, an island arc collided with the North American continental margin. Acadian About 400 Ma, Avalonia collided with and was thrust over eastern North America. Alleghian About 300 Ma, Gondwana overrode the eastern edge of North America.

the overriding plate is either not moving toward the subjected slab relative to the asthenosphere or it is moving away

Probably due to Paleozoic collisional tectonics in the Appalachians.

Thick sequences of sedimentary rocks contained here, most likely related to Mesozoic rifting and the subsequent subsidence of the continental margin.

Sits between a volcanic arc and an offshore trench; receives abundant sediment from major rivers that drain into the sea.

Created by rifting of the western edge of North America; locally accumulated more than 10km of sediment.

The terranes have similar ages and sequences of rocks.

Composed of sediment eroded from the Appalachian Mountains. Contained within an ancient foreland basin. Sediment coarser and thicker to the east.

Accretionary prism of oceanic material. Displaced granitic rocks. Slices of Paleozoic and Mesozoic oceanic crust and sediment. Mesozoic island arcs.

Antarctica, Australia, South America, and Africa were joined together in the southern hemisphere; existed from nearly 600 Ma to about 150 Ma.

North America, Europe, and Asia were joined in the Northern Hemisphere; existed from 250 Ma to about 150 Ma.

all the continents were joined; existed from about 250 Ma to about 200 Ma.

all the continents were joined; existed from about 1.1 Ga to about 600 Ma.

Give direct measurements of uplift; parts of the Himalaya are rising a few centimeters per year.

Determine general amount of uplift; the top of Mount Everest contain limestone that was deposited below sea level and later uplifted to over 8,800 m.

As deep rocks are uplifted, they cool and lock in daughter products from radioactive decay; in the Himalaya, some of the show ages as young as several million years.

Age of uplift may be determined when pieces of rock from a mountain were added to the sedimentary sequence; sediment from the Himalaya first appeared around 45 million years ago.

A The region contains the oldest dated rocks (4.0 billion years old) in North America. B The area contains many terranes accreted during the Paleozoic, Mesozoic, and Cenozoic. C Late Precambrian and Paleozoic terranes accreted during the formation of the Appalachian Mountains. D The Grenville Province terrane was accreted 1.1 billion years ago. E Rocks on the Canadian Shield are 3.0 to 1.7 billion years old.

A A unit younger than a basin may overlap the edge of the basin and its faults, showing that the basin had stopped forming by the time the unit with developed. B Units deposited during formation of a basin may be very thick and may contain coarse sediments that record steep slopes along the flanks of the basin. C Units older than a basin typically have the same thickness across the area and were then tilted and faulted when the basin formed. D Rate of deposition of different units may be calculated by looking at their times of deposition and thicknesses; the time when sediment started to accumulate most rapidly shows when the basic began forming.

Warmer rocks are in the subsurface. Region has thick crust. Less dense materials are present in the crust.

isostatic rebound

was discovered by observations of surveying equipment that showed a smaller gravitational attraction by the the Himalaya than expected explains that higher mountains have thicker crustal roots, analogous to floating icebergs

Crust has been compressed from the sides; may create faults and/or folds. Crust is continental. Crust undergoes deposition of sediment or addition of volcanic rocks.

A nearly 0 km B 70 km C 100 km D 140 km

Tetonic activity along continental margins: It causes broad uplifts and basins within the continent in reponse to loading of sediments along the margin and the emplacement of thrust sheets. Changes in global climate: Sea-level rise can cause significant flooding well into the Interior of a continent. Rise of mountains along a coast: Weather patterns can change, causing a change in climate; as an example, rain-shadow deserts may form. (Consider US west coast mountain ranges) Movement of the tectonic plate on which the continent rides: Latitude can be changed over time, causing drastic changes in climate of the area.

A: A foreland basin can form in front of the collision zone as one plate is forced under the other. D: Crust that gets too thick or hot may begin to spread under its own weight, forming normal faults and the development of associated basins. E: Behind a collision zone, a basin can be formed as weight of the thrust sheets in adjacent crust pushes the region down.

False

C

Forearc basin: A sedimentary basin that lies between the volcanic arc and the trench in a convergent plate boundary Foreland basin: A basin that forms when crust is warped by the weight of thrust sheets, especially when formed between a mountain belt and continental interior Pull-apart basin: A basin that forms as the result of movement within a zone of strike-slip faulting Normal-fault basin: A low area that has been downdropped b one or more normal faults

The differences are what we would predict. Rule of thumb: increase of 6km crust thickness = increase in 1km elevation

A: Initial Rifting occurs along eastern N.A.: starts within Rodinia. B: Seafloor spreading creates new ocean basain; eastern coast of N.A. becomes passive margin. C: Convergent margin forms; creates an island arc that later collides with N.A. D: Avalonia collides with N.A. during the Acadian orogeny; foreland basin created for the Catskill Delta E: Alleghenian orogeny: Gondwana collides with N.A.: Appalacians form along eastern N.A., while Ouachitas form along sourthern N.A.

100 km; 0 km; 150 km

shaping the boundaries along which continents separate, due to location of the rift arms

Accretionary Prism

warping and uplifting Earth's surface and underlying rock layers

enters a subduction zone, where it is scraped off the subducting plate and added to the continent

Changes in global climate and the rise and fall of sea level Movement of the plate upon which it rides (change in latitude) tectonic activity along the edges of a continent

having rocks, structures, fossils, and other geologic aspects that are unlike those in adjacent regions being bounded by faults

+Thrust faulting +Adding magma to the crust +Crustal or mantle heating beneath a region

+Accretionary prism of oceanic material +Mesozoic island arcs +Slices of Paleozoic and Mesozoic oceanic crust and sediment +Displaced granitic rocks