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Question
Most of the volcanoes across the world are caused by the movement of tectonic plates. So why is that there is no volcanism in the Himalayas?
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Solution
The Himalayan mountain range and Tibetan plateau have formed as a result of the collision between the Indian Plate and Eurasian Plate which began 50 million years ago.
No volcanism in the Himalayas: - The Himalayas are also a plate-to plate collision tectonic boundary. In this case, the Indian Plate [of the Indian Subcontinent] is colliding head-on with the Eurasian Plate. Both plates are comprised of continental lithospheric crust, so there is no appreciable distinction in density.
Both have a density of approximately 2.7 g/cm³. - This as opposed to ocean crust with a mean density of 3.3 g/cm³. The plates try to compete in the plate-to-plate collision but the equal densities of the two plates cannot push one under the other very deep like that in a subduction zone. The result is largescale thickening of the continental crust in the region at and surrounding the collision boundary.
- At convergent plate boundaries, two situations are possible. First, both volcanoes and earthquakes form where one plate sinks under the other. This process, called subduction, takes place because one plate is denser than the other. The denser plate, which invariably has oceanic crust on its top, does the sinking. Second, only earthquakes occur when two plates collide (obduct), building a mountain range. The density of continental crust is too low for it to subduct, like wood floating on water. Instead, the two plates have a head on collision - building a mountain range.
- Much magma is generated at a convergent plate boundary where subduction is occurring. The sinking plate melts as it descends into the asthenosphere; this generates magma, which rises through the other plate to form volcanoes. As it rises, more melting takes place in the rocks it travels through, generating yet more magma.
A converging plate boundary where subduction occurs.
A converging plate boundary showing collision (obduction) and mountain range formation.
No volcanism in the Himalayas:
- The Himalayas are also a plate-to plate collision tectonic boundary. In this case, the Indian Plate [of the Indian Subcontinent] is colliding head-on with the Eurasian Plate. Both plates are comprised of continental lithospheric crust, so there is no appreciable distinction in density.
- This as opposed to ocean crust with a mean density of 3.3 g/cm³. The plates try to compete in the plate-to-plate collision but the equal densities of the two plates cannot push one under the other very deep like that in a subduction zone. The result is largescale thickening of the continental crust in the region at and surrounding the collision boundary.
- At convergent plate boundaries, two situations are possible. First, both volcanoes and earthquakes form where one plate sinks under the other. This process, called subduction, takes place because one plate is denser than the other. The denser plate, which invariably has oceanic crust on its top, does the sinking. Second, only earthquakes occur when two plates collide (obduct), building a mountain range. The density of continental crust is too low for it to subduct, like wood floating on water. Instead, the two plates have a head on collision - building a mountain range.
- Much magma is generated at a convergent plate boundary where subduction is occurring. The sinking plate melts as it descends into the asthenosphere; this generates magma, which rises through the other plate to form volcanoes. As it rises, more melting takes place in the rocks it travels through, generating yet more magma.
A converging plate boundary where subduction occurs.
A converging plate boundary showing collision (obduction) and mountain range formation.
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