Granitic magmas are very different from basaltic magmas. They have about 20 percent more silica, and the minerals in granite include quartz (Si02) and the complex minerals mica (K,Na,Ca) (Mg,Fe,Al)2 AlSi4 O10 (OH,F)2 and amphibole ((Mg,Fe,Cah (Mg,Fe,Al)5 (si,Al)8 o22(oH)2), which both have a lot of water in their crystal structures. Also, granitic magmas are found almost exclusively in regions of continental crust. From these observations it is inferred 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 either by 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.
These granitic magmas rise slowly because of their high sio2 content 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 is about 3-6 miles (5-10 km) beneath the surface, which explains why large portions of the continental crust are not molten lava lakes. There are many regions with crust above large magma bodies (called batholiths) that are heated by the cooling magma. An example is Yellowstone National Park, where there are hot springs, geysers, and many features indicating that there is 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.
Continue reading here: Andesitic Magma
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