Laboratory Manual for Introductory Geology 4e

EXERCISE 5.11
Origin of Intermediate Magmas in Subduction Zones
Name:
Course:
Section:
Date:
Melting above a subducting slab produces mafic magma, as described above, yet intermediate rocks (andesite, diorite)
are common in many subduction zones. Considering the magmatic processes discussed in section 5.4 and the difference
between oceanic and continental lithosphere, explain how this intermediate magma forms.
(a) A relatively small amount of intermediate magma occurs in volcanic island arcs. Remembering that melting above
the subducting slab produces mafic magma, explain the origin of the intermediate magma.
(b) Much more intermediate magma erupts in continental volcanic arcs. Why?
5.5.4 Plate-Tectonic Settings of Felsic Rocks
(Granite and Rhyolite)
Granite and rhyolite are most abundant on the continents—in continental volcanic
arcs, continental collision zones, continental rifts, and continental hot spots. They
form largely by partial melting of the upper (granitic) layer of continental lithosphere
and to a lesser extent by differentiation of mafic magmas. Some rhyolite
and granite forms in subduction zones by differentiation of mafic and intermediate
magmas or by assimilation. Only very small amounts of felsic igneous rock are
found in the oceans, mostly rhyolite in oceanic hot-spot islands, formed by extreme
fractional crystallization.
EXERCISE 5.12
Origin of Granite and Rhyolite in Continental Rifts
Name:
Course:
Section:
Date:
Continental rifts typically contain large amounts of basalt and rhyolite. Your answer to Exercise 5.10b explained the origin
of the basalt, but fractional crystallization of mafic magma can produce only a very small amount of felsic magma. Explain
the origin of the large volumes of felsic magma in continental rifts.
5.5 IGNEOUS ROCKS AND PLATE TECTONICS
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