Stress and Mountain Building - Mountains can stand alone or in ranges that formed at a similar time and in a similar way. Many processes can create mountains. Although most mountains form along plate boundaries, some result from intraplate activity. For example, volcanoes build upwards at hotspots within the Pacific Plate.
Most of the world’s largest mountains result from compression at convergent plate boundaries. The largest mountains arise when two continental plates smash together. Continental lithosphere is too buoyant to get pushed down into the mantle or subduct, so when the plates smash together, the crust crumples upwards, causing uplift. The stresses cause folds, reverse faults, and thrust faults, all of which allow the crust to grow thicker and rise upwards.
The world’s highest mountain range, the Himalayas, is growing from the collision between the Indian and the Eurasian plates. About 80 million years ago, the Indian plate was separated from the Eurasian plate by an ocean. As the Indian plate moved northward, a subduction zone formed beneath Eurasia. The seafloor was subducted and caused the formation of a set of continental arc volcanoes.
When the oceanic lithosphere was completely subducted, about 40 million years ago, the Indian plate began to collide with the Eurasian plate. Some of the Indian plate was thrust beneath Asia and some of Asia was thrust onto India. Rock also folded, which thickened the crust and formed the mountains. In places, the old seafloor that was between the two slabs of continental crust have been thrust over the Asian continent and are found high in the Himalayas.
Subduction of oceanic lithosphere at convergent plate boundaries also builds mountain ranges. The Andes Mountains are a chain of continental arc volcanoes that build up as the Nazca plate subducts beneath the South American plate. Rifting at a divergent plate boundary can also build mountain ranges.
When crust is pulled apart, it occupies more area. The crust breaks into blocks that slide up and down along the normal faults that separate them. The result is alternating mountain ranges and valleys, known as a basin-and-range. The state of Nevada is the center of a classic basin and range province.
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