Laboratory Manual for Introductory Geology 4e

EXERCISE 4.3
Understanding the Origin of Rock Textures
Name:
Course:
Section:
Date:
In this exercise you will conduct simple experiments to better understand rock textures.
(a) Crystalline igneous rock. Partially fill ( 1 2
inch) a glass petri dish with the thymol powder provided by your instructor.
Heat on a hot plate until the material melts completely. Carefully remove the dish from the hot plate using forceps,
and allow the liquid to cool. Observe the crystallization process closely. You may have to add a crystal seed if
crystallization does not begin in a few minutes. Congratulations—you have just made a “magma” and then an
“igneous rock” with an interlocking crystalline texture.
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Describe the crystallization process. How and where did individual grains grow, and how did they eventually join with
neighboring grains?
Sketch and describe the texture of the cooled “rock.”
Sketch:
Describe:
This crystalline texture formed when crystals in the cooling “magma” grew until they interfered with one another and
eventually interlocked to form a cohesive solid. Crystals grow only when the magma cools slowly enough for atoms to have
time to fit into a crystal lattice.
(b) Glassy igneous texture. Add sugar ( 1 2
inch) to another petri dish, melt it on a hot plate, and allow it to cool.
Describe the cooling process and sketch the resulting texture.
Sketch:
Describe:
(continued )
4.4 THE PROCESSES THAT PRODUCE TEXTURES
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