Laboratory Manual for Introductory Geology 4e

EXERCISE 5.1
A First Look at Igneous Rocks
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There are many ways to classify igneous rocks, but for now let’s use three easily observable criteria: grain size, color, and
specific gravity. Using the set of igneous rocks provided by your instructor, first separate the specimens into coarse- and
fine-grained categories. Record the specimen number in the appropriate column in the following table. Then regroup the
specimens using color and using specific gravity.
Grain size Color Specific gravity (heft)
Coarse Fine Light colored Dark colored Relatively high Relatively low
(a) Which two properties seem to be related to each other? Which is not related to the other two?
(b) Think about which properties of magma and which rock-forming processes control the grain size, color, and
specific gravity of an igneous rock. Which two properties have a common cause and most likely explain the
relationship you discovered in question (a)? Explain your reasoning. This connection will be helpful to keep in mind
as you identify and interpret the histories of igneous rocks.
5.2 Interpreting the Cooling Histories
of Igneous Rocks
Imagine that you are looking at an outcrop of ancient igneous rock. That rock
might have formed from volcanic debris blasted into the air, lava that frothed out of
a volcano or flowed smoothly across the ground, magma that cooled just below the
surface, or magma that solidified many kilometers below the surface. But millions
of years have passed since the rock formed, and if a volcano was involved, it has long
since been eroded away. If the rock is intrusive, kilometers of overlying rock must
have been removed to expose it at the surface. How can you determine which of
these possibilities is the right one?
The key to understanding the cooling history of an igneous rock is its texture: the
size, shape, and arrangement of its grains. The texture of an igneous rock formed
by the settling of ash and other volcanic fragments is very different from that of a
rock formed by magma cooling deep underground or by lava cooling at the surface.
Specimens of igneous rock composed of interlocking grains—whether those
grains are large enough to be identified or too small to be identified—are called
5.2 INTERPRETING THE COOLING HISTORIES OF IGNEOUS ROCKS
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