||Locations of Volcanoes
The lava in plate collisions is formed from cold basaltic rocks being carried down to great depths where they are heated at high pressure. Even then, melting only occurs because the dissolved water and CO2 carried down with the basalt lowers its melting temperature. The lava in plate separations is formed by carrying already hot rock up to low pressures near the surface. This type of melting is called "depressurization melting" and works for nearly all solids. This type of melting is not observed in everyday life because the necessary pressure changes are very large.
We have now accounted for most of the volcanoes in the world, but there are few other types not associated with plate boundaries. These oddballs include some of the most continuously active volcanoes in the world, like Kilauea in Hawaii and La Fournaise on Reunion Island in the Indian Ocean. The lavas in these volcanoes are basaltic and contain little dissolved gas. This type of volcano is usually found at one end of a long chain of extinct volcanoes. For example, the active volcanoes in Hawaii--Mauna Loa, Kilauea, and the Loihi Seamount volcano off the SE side of the big island of Hawaii--form a small cluster of active volcanoes at one end of a linked chain of hundreds of extinct volcanoes nearly three thousand miles long. Curiously, the ages of the volcanoes in these chains increase systematically along the chain from one end to the other. In the Hawaiian chain, their ages range from zero for the active volcanoes on Hawaii to several million years for those on Niihau, the westernmost part of the major Hawaiian islands, to more than 100 million years for those at the far end of the chain. At the "old" end of such chains, one often finds a huge flood basalt type volcano. These gigantic formations include the largest deposits of volcanic rock in the world. One of these deposits, the Columbia River Basalts (CRB), covers more than one-third of the state of Washington. A chain of dead volcanoes leads eastward from the Columbia Basalts to the "live" one now located at Yellowstone. Volcanic chains and flood basalt deposits are scattered randomly in time and place around the world and range in size from single volcanoes to giant groups of volcanoes and flood deposits half the size of the United States.
The long chains of extinct volcanoes with a "live" one at one end motivated the idea of "hotspot" volcanism: a small, fixed source of lavas from deep in the mantle that continuously "burns through" the overlying lithospheric plate as it passes over. In this model the island chains would be similar to a string of burn spots left in a thin piece of wood which had been passed over a fixed torch. What could cause a hotspot? Can the hotspot idea account for the observed volcanoes?
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