Magma Types Percent SiO2Volcanic Rock Plutonic Rock

Ma c 45-52% Basalt Gabbro

Intermediate 53-65% Andesite Diorite

Felsic > 65% Rhyolite Granite from increasing the pressure on a dry mineral—it decreases the melting temperature.

PARTIAL MELTING

If a rock melts completely, the magma has the same composition as the rock. Rocks are made of many different minerals, all of which melt at different temperatures. Therefore if a rock is slowly heated, the resulting melt, or magma, will first have the composition of the first mineral that melts and then the first plus the second mineral that melts, and so on. If the rock continues to melt completely, the magma will eventually end up with the same composition as the starting rock, but this does not always happen. Often the rock only partially melts so that the minerals with low melting temperatures contribute to the magma, whereas the minerals with high melting temperatures did not melt and are left as a residue (or restite). In this way the end magma can have a composition different from the rock it came from.

The phrase "magmatic differentiation by partial melting" refers to the forming of magmas with differing compositions through the incomplete melting of rocks. For magmas formed in this way, the composition of the magma depends on both the composition of the parent rock and the percentage of melt.

Temperature (°C)

1,200

20-

40-

60-

_____

E

80-

-C

Q. 01

100-

O

120-

140-

160-

1,200

1,500

G Infobase Publishing

Typical geothermal gradient beneath the oceans, showing incipient partial melting of a dry peridotite

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