Granitic magmas are very different from basaltic magmas. They have about 20 percent more silica, and the minerals in granite (mica, amphibole) have a lot of water in their crystal structures. Granitic magmas are mostly exclusive to regions of continental crust. Inference from these observations leads to the conclusion that the source of granitic magmas is within the continental crust. Laboratory experiments suggest that when rocks with the composition of continental crust start to melt at temperature and pressure conditions found in the lower crust, a granitic liquid is formed, with 30 percent partial melting. These rocks can begin to melt by either the addition of a heat source, such as basalt intruding the lower continental crust, or by burying water-bearing minerals and rocks to these depths. The geological processes responsible for bringing the water-bearing minerals down to the level in the crust where the water can also the line of be released from their crystal structure, and lower the melting
Line of incipient partial melting of dry peridotite
temperature of surrounding rocks, are limited. Deep burial by sedimentation can cause the release of water, which escapes to the surface but does not usually cause the rocks to melt and form granite. In subduction zones water-bearing minerals can be carried to great depths in the Earth, and when they release water, the fluid can rise up into the overlying mantle and crust, and cause the granite melts to form in those regions.
These granitic magmas rise slowly (because of their high Si02 and high viscosities), until they reach the level in the crust where the temperature and pressure conditions are consistent with freezing or solidification of magma with this composition. This occurs about three to six miles (5-10 km) beneath the surface, which explains why large portions of the continental crust are not molten lava lakes. In many region, crust lies above large magma bodies (called batholiths) that are heated by the cooling magma. An example is Yellowstone National Park, where hot springs, geysers, and many other features indicate the presence of a large hot magma body at depth. Much of Yellowstone Park is a giant valley called a caldera, formed when an ancient volcanic eruption emptied an older batholith of its magma, and the overlying crust collapsed into the empty hole formed by the eruption.
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