DEFINITION OF BASALTIC MAGMA

Partial melting in the mantle leads to the production of

basaltic magma, which forms most of the oceanic crust. By

looking at the mineralogy of the oceanic crust, which is dominated

by olivine, pyroxene, and feldspar, we conclude that

very little water is involved in the production of the oceanic

crust. These minerals are all anhydrous, that is without water

in their structure. Thus dry partial melting of the upper mantle

may lead to the formation of oceanic crust. By collecting

samples of the mantle that have been erupted through volcanoes,

we know that it has a composition of garnet peridotite

(olivine + garnet + orthopyroxene). By taking samples of this

back to the laboratory and raising its temperature and pressure

so that it is equal to 62 miles (100 km) depth, we find

mafic 45–52% basalt gabbro

intermediate 53–65% andesite diorite

felsic 65% rhyolite granite

that 10 percent to 15 percent partial melt of this garnet peridotite

yields a basaltic magma.

Magma that forms at 50 miles (80 km) depth is less

dense than the surrounding solid rock, so it rises, sometimes

quite rapidly (at rates of half a mile per day measured by

earthquakes under Hawaii). In fact, it may rise so fast that it

does not cool off appreciably, erupting at the surface at more

than 1,832°F (1,000°C); this is where basalt comes from.

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