9. Mountains and Mountain Ranges Orogeny – the process of mountain building Crustal thickening Subduction zones Sub-plate magmatic activity Continental collision Compressive forces “squeeze” crust together Thickening leads to isostatic adjustment Rising of less-dense continental crust Rising increases erosion, redistributes mass

FIGURE 9.15 (A) Several factors affect the height of the Himalayas and the Tibetan Plateau mountainous regions. Note the vertical scale on the left. The regions have been uplifted by underthrusting of the Indian Plate under the Asian Plate. As the mountains grew upward, they underwent erosion. In this cross section, the volume of rock removed by erosion is represented by the area between the “cumulative erosion level” and the modern ground surface. The intense tectonic compression also has caused the southward extrusion of plastically deforming rock from the Greater Himalayan Sequence, like toothpaste being squeezed from a giant tube (small red arrows). East–west stretching of Tibet, probably due to the north–south compression, also has thinned the upper crust there. Collectively, these processes have caused the entire region to rise isostatically. Mount Everest is located in the small red box.

Island Arcs Island arcs – volcanic mountain chain Ocean-ocean convergent zone Subduction complex – “squeegeed” sediments and fractured rock Underthrusting – subduction complex grows from bottom, forced up Forearc basin – down dip area between arc and complex

FIGURE 9. 16 Formation of an island arc FIGURE 9.16 Formation of an island arc. Saturated oceanic sediment and basalt is subducted and introduces water into the overlying asthenosphere and base of the overriding plate, causing partial melting and formation of basaltic magma. The magma rises and erupts, building submarine volcanoes that eventually grow above sea level to form a volcanic mountain chain. A subduction complex forms at the top of the subduction zone and consists of highly deformed slices of oceanic sediment, along with fragments of oceanic crust and upper mantle scraped from the top of the subducting oceanic plate. A forearc basin forms when sediment accumulates between the arc and the crest of the subduction complex.

The Andes Subduction at a continental margin Rising magma Some rises to form volcanoes Some cools inside forming plutons Thickening crust leads to isostatic rise Once thick enough, soft rock beneath oozed outwards creating thrust faults Foreland basin Filled with sediments eroding from risen landscape

FIGURE 9.17 The Cordillera Real mountain range in Bolivia’s Andes rises over 6,000 meters.

FIGURE 9.18 Development of the Andes, seen in cross section looking towards the north. (A) Subduction of oceanic lithosphere beneath the western side of the South American Plate has been ongoing for at least 140 million years. Basaltic magma produced by introduction of water from the downgoing slab rose to the overlying crust and partially melted it, forming andesitic magma. Eruption of this magma formed an arc on the overriding plate. Sediment eroded from the arc accumulated in a forearc basin between the arc and the subduction complex. Additional sediment accumulated on the east side of the arc in a foreland basin, formed where compression associated with the convergent margin caused thrust faulting, and the weight of the thrust sheets flexed the lithosphere downward. (B) As subduction continued, the size of the subduction complex and the width of the forearc basin grew, while arc magmatism migrated eastward. Continued compression in the foreland basin resulted in additional thrust faulting, downward flexure, and the accumulation of sediment there.

FIGURE 9.18 Development of the Andes, seen in cross section looking towards the north. (A) Subduction of oceanic lithosphere beneath the western side of the South American Plate has been ongoing for at least 140 million years. Basaltic magma produced by introduction of water from the downgoing slab rose to the overlying crust and partially melted it, forming andesitic magma. Eruption of this magma formed an arc on the overriding plate. Sediment eroded from the arc accumulated in a forearc basin between the arc and the subduction complex. Additional sediment accumulated on the east side of the arc in a foreland basin, formed where compression associated with the convergent margin caused thrust faulting, and the weight of the thrust sheets flexed the lithosphere downward. (B) As subduction continued, the size of the subduction complex and the width of the forearc basin grew, while arc magmatism migrated eastward. Continued compression in the foreland basin resulted in additional thrust faulting, downward flexure, and the accumulation of sediment there.

FIGURE 9.18 Development of the Andes, seen in cross section looking towards the north. (A) Subduction of oceanic lithosphere beneath the western side of the South American Plate has been ongoing for at least 140 million years. Basaltic magma produced by introduction of water from the downgoing slab rose to the overlying crust and partially melted it, forming andesitic magma. Eruption of this magma formed an arc on the overriding plate. Sediment eroded from the arc accumulated in a forearc basin between the arc and the subduction complex. Additional sediment accumulated on the east side of the arc in a foreland basin, formed where compression associated with the convergent margin caused thrust faulting, and the weight of the thrust sheets flexed the lithosphere downward. (B) As subduction continued, the size of the subduction complex and the width of the forearc basin grew, while arc magmatism migrated eastward. Continued compression in the foreland basin resulted in additional thrust faulting, downward flexure, and the accumulation of sediment there.

The Himalayas Collision between continents Oceanic lithosphere between two continents “used up” – then collision between continents Both continental crust, neither can sink. India thrusts beneath Asia, crustal thickness doubles

FIGURE 9.20 (A) The Indian subcontinent was part of Gondwana 200 million years ago and was separated from the southern margin of Asia by the Tethys Sea.

FIGURE 9.20 (B) About 150 million years ago, Gondwanaland began to break apart through the process of tectonic rifting, and the Indian Plate was a major fragment resulting from this tectonic breakup. India began drifting northward by about 120 million years ago, as oceanic lithosphere forming its leading edge subducted beneath the southern continental margin of Asia. This process formed an oceanic trench, a subduction complex, and an Andean arc along Asia’s southern margin. (In this reconstruction about 100 million years ago, the amount of oceanic crust between India and Asia is abbreviated to fit the diagram.)

FIGURE 9.20 (C) By 50 million years ago, Indian continental lithosphere had begun to collide with and subduct under Asia. Great thicknesses of shallow marine oceanic sediments from the collapsed Tethys Sea were deformed by folding and thrust faulting in the collision zone. Compression from the continent–continent collision caused crustal thickening and melting of large volumes of magma in the middle crust. Collectively, these processes elevated the Tibetan Plateau.

FIGURE 9.20 (D) By 15 million years ago, continued convergence of India with Asia led to the plastic extrusion of midcrustal-level (red) rocks upward and to the south, out of the collision zone. Today, these rocks form the Greater Himalayan Sequence.

FIGURE 9.20 (D) By 15 million years ago, continued convergence of India with Asia led to the plastic extrusion of midcrustal-level (red) rocks upward and to the south, out of the collision zone. Today, these rocks form the Greater Himalayan Sequence.

FIGURE 9.20 (D) By 15 million years ago, continued convergence of India with Asia led to the plastic extrusion of midcrustal-level (red) rocks upward and to the south, out of the collision zone. Today, these rocks form the Greater Himalayan Sequence.

FIGURE 9.20 (D) By 15 million years ago, continued convergence of India with Asia led to the plastic extrusion of midcrustal-level (red) rocks upward and to the south, out of the collision zone. Today, these rocks form the Greater Himalayan Sequence.

The Himalayas Today Today the Himalayas: Contain all three rock types Many sedimentary rocks from old sea-floor Plateau approx. 4000 m; mountains rise from this Crustal mass caused faulting as it spread Extensional (normal) faults in mountains Compressional (reverse/thrust) faults in foothills India is still moving north The Alps, Urals, and Appalachians all happened under similar circumstances

Mountains and Earth Systems Mountains rise due to tectonic forces Interact with hydrosphere, atmosphere, biosphere Air rises over mountains Moisture rains out Rise of Himalayas coincides with global cooling period Soil erosion can be a problem for human habitation in mountains

FIGURE 9.21 As the human population in mountainous regions increases, people have cut forests on steep hillsides to plant crops. This terraced hillside in Nepal collapsed during heavy monsoon rainfall.