Laboratory Manual for Introductory Geology 4e

FIGURE 5.2 Grain sizes in light-colored igneous rock.
3 cm 1 cm 5 mm
(a) Very coarse-grained (pegmatitic
crystals).
(b) Coarse-grained (phaneritic crystals).
(c) Fine-grained (most grains are too
small to see with the naked eye).
EXERCISE 5.3
Interpreting Igneous Cooling History from Grain Size
Name:
Course:
Section:
Date:
Three simple thought experiments will help you understand how magma and lava cool and enable you to deduce two basic
rules that will allow you to interpret a rock’s cooling history.
(a) Imagine you have two balls of pizza dough, each 16 cm in
diameter. As shown in the figure on the right, one is rolled
out to form a crust 1 cm thick and 50 cm in diameter.
Both pieces of dough still have the same mass and
volume, even though their shapes are very different. Both
are baked in a 450°F oven for 20 minutes and removed.
●●
Which has the greatest surface area for its volume, the
crust or the ball?
●●
Which will cool faster?
Why?
16 cm
16 cm
Rule 1 of magma cooling: Circle the correct two choices in the
following statement. A thin sheet of magma loses heat (faster/
slower) than a blob containing the same amount of magma.
This is because the surface area available for cooling in the
sheet is (larger/smaller) than the surface area available in a
blob of the same volume.
50 cm
1 cm
(b) Equal amounts of hot coffee are poured into a thin plastic
cup and a Styrofoam cup.
●●
Which cools faster, the coffee in the plastic cup or that
in the Styrofoam cup?
(Hint: Which cup can you hold the longest before burning your fingers?) Explain.
(continued)
5.2 INTERPRETING THE COOLING HISTORIES OF IGNEOUS ROCKS
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