Question

Though we have no volcanoes in India but still there are igneous rocks Why​

Answer 1

Answer:

This is good question which has been asked many times on Quora itself. To explain, I will take you little bit into the cause of volcanoes and the phenomenon of subduction.

Volcanoes form when magma below the earth's surface rises and discharges into the earth's crust. This happens because the molten rockmass expands in volume and tries to find its way out. Volcanoes can form by either the plumes in the lithosphere or as a result of subduction (subduction happens when two sections of crust collide, forcing one slab deeper into the earth) when the subducting plate melts and expands.  The second clause applies to Himalayas but still there are no volcanoes.

Next come to the formation of the Himalayas. This mountain range is formed by squeezing of old oceanic margin by the action of fast moving plates. As a result most of oceanic crust got squeezed, folded and risen up in the form of the present Himalayan Mountain in a geologic time period of roughly 80 million years. Further movement is pushing Indian plate down. So there is continent-continent collision in the Himalayas in which the density of colliding plates are the same (both are continental crust) , whatever oceanic crust was there in between had risen in the form of the Himalayas (still rising).

There is an established subduction zone below the Tibetan Plateau, where the NE moving Indian plate is subducting under the Eurasian plate. The situation here is quite different from continental margin subduction wherein the denser  oceanic plate subducts below the continental plate.  Since oceanic crust is 30-40% denser than the continental crust, there is volume expansion after melting (following deep subduction). In case of the Himalayas, the densities of colliding plates are same. Therefore, even if the subducting Indian plate melts, there will not be any volume expansion or additional pressure generation to create a volcano formation like situation (refer cause of volcano in the first para).  The second reason is a massive crustal thickness in the Himalayas which needs to be ruptured for volcanic eruption. Because of these two basic reasons – same density of colliding plates and a massive crustal thickness, we do no see any volcanoes in the Himalayas even though the Indian plate is quite active and the Himalayas are an active seismic zone.

Despite this, we have record of volcanic activity in the southern Tibet known as Kulun volcano, which had erupted mildly in 1951 and was somewhat active (no eruption) in 1988. Because of low pressure generated by the molten subducting plate, the volcanic activity in this region can never be violent.

Regards.