DEFINITION OF GRANITIC MAGMA

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. Also, granitic magmas are found almost exclusively

in regions of continental crust. From these observations we

infer 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.

These granitic magmas rise slowly (because of their high

SiO2 and high viscosities), until they reach the level in the

crust where the temperature and pressure conditions are consistent

with freezing or solidification of a 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 the 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.

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