1. 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

2. 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.

3. 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

4. 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.

5. 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

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

7. 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.

8. 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.

9. 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.

10. 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

11. FIGURE (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.

12. 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.)

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. The Himalayas Today Today the Himalayas: Contain all three rock types
Many sedimentary rocks from old sea-floor
Plateau approx 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

19. 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

20. 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.