Volcanoes form when magma below the earth's surface rises and discharges into the earth's crust. As the magma rises, it gathers into reservoirs, and can eventually erupt onto the surface. Strong earthquakes occur during this event and the volcanic cone may appear to swell.
Volcanoes can form by either the plumes in the lithosphere -- the crust and hard upper mantle -- or as a result of subduction. Subduction happens when two sections of crust collide, forcing one slab deeper into the earth.
Oceanic-oceanic collision forces the edges of the upper mantle down due to the cold basalt in the ocean's crust. Oceanic-continental collisions form crust beneath a descending granite crust, which is too light to sink into the mantle.
However, continental-continental plate collisions have rocks on both sides that are too light to sink into the earth's mantle. This causes the edges to crumble and fold into mountain regions. The Himalayas are the best known example of this occurrence. This mountain region resulted when Asia and India crashed into each other.
Little melting occurred during this collision because few rocks were forced to great depths. This caused the formation of only a few volcanoes.
Though a majority of the Himalayas are nonvolcanic mountains, some volcanoes did form as the oceanic Indian plate collided with southern Asia. One of the most well-known volcanoes in this region is the Kunlun volcano in Tibet. The ocean floor north of India dragged the Indian continent toward Tibet over the course of 80 million years.
The plates collided, forming volcanoes in southern Tibet. These fast-moving plates nearly closed over the ocean between the two continents by squeezing the ocean floor sediment up to the surface. This is now known as the Himalayas.
The Kunlun volcano is the highest volcano in the Northern Hemisphere. Kunlun is an active volcano, and the last eruption was recorded in 1951 as a central vent explosion. Volcanic activity from this region caused a magnitude 7.2 earthquake on March 21, 2008, but no eruption was reported.
