Laboratory Manual for Introductory Geology 4e

5.5 Igneous Rocks and Plate Tectonics
Igneous rock types are not distributed equally or randomly on the Earth because
melting occurs in different ways in different plate-tectonic settings. As you read this
section, remember that assimilation, magma mixing, and magmatic differentiation
can produce exceptions to just about every generalization, so keep in mind the
phrase “are generally found.” With that warning, look at FIGURE 5.9, which illustrates
the plate-tectonic settings where igneous rocks form, and TABLE 5.3, which
lists the igneous rock types that are generally found in those settings. In Exercises
5.10, 5.11, and 5.12, you will apply what you’ve learned about the origin of igneous
rocks to suggest plate-tectonic settings for specimens in your rock sets.
5.5.1 Plate-Tectonic Settings of Ultramafic Rocks (Peridotite)
Several lines of evidence indicate that the mantle consists of the ultramafic rock
peridotite, composed of olivine and pyroxene, which is therefore the most abundant
rock on the Earth. But peridotite is not a commonly observed rock because its
magma is so much denser than the crust that it doesn’t have the buoyancy to rise to
the Earth’s surface. It is found at divergent boundaries and continental rifts, where
FIGURE 5.9 Tectonic settings for major igneous rock types.
Continental
flood basalts
Continental
rift zone
Ocean-continent
convergent boundary
Mid-ocean
ridge
Oceanic hot-spot
island
Volcanic
island arc
Continent-continent
collision zone
Upper
continental crust
Lower
continental crust
Oceanic crust
TABLE 5.3 Tectonic associations of igneous rocks
Type of Plate Boundary Geologic Setting Rock Types
Convergent
Volcanic island arcs (ocean-ocean
convergence)
Andean-type mountains
(ocean-continent convergence)
Continental collision zones
(continent-continent convergence)
Basalt, minor andesite, very minor rhyolite (and intrusive
equivalents)
Andesite, rhyolite, minor basalt (and intrusive equivalents)
Granite, rhyolite, peridotite in ophiolites
Divergent
Not associated with
plate boundaries
Mid-ocean ridge
Continental rift
Oceanic hot-spot islands
Ocean floors
Continents
Basalt—a special kind called MORB (mid-ocean ridge basalt)
Rhyolite and basalt
Basalt
Basalt (MORB), peridotite (locally along faults)
Granite, rhyolite, basalt, and gabbro; andesite and diorite by
magma mixing
5.5 IGNEOUS ROCKS AND PLATE TECTONICS
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