Laboratory Manual for Introductory Geology 4e

EXERCISE 4.2
Describing a Rock’s Texture
Name:
Course:
Section:
Date:
Describe the texture of the rocks provided by your instructor Be sure to consider the size and shape of grains, the relationships
among grains, and whether grains are randomly oriented or aligned. Use your own words—the formal geologic terms
for the textures will be introduced later.
(a) Rock 1:
(b) Rock 2:
(c) Rock 3:
(d) Which textural features are common to all three rock samples, and which are not?
(e) Did all three rocks form by the same process? Explain your reasoning.
4.4 The Processes That Produce Textures
The best way to understand how a rock’s texture forms would be to observe the
rock-forming process in action. You can’t go inside a lava flow to watch an igneous
rock crystallize, deep below a mountain belt to see rock metamorphose, or
under the ocean floor to watch sediment become a sedimentary rock—but you can
model the formation of igneous, sedimentary, and metamorphic rocks well enough
in the classroom to understand how their textures form. You will use textural
features more fully later in detailed interpretations of igneous, sedimentary, and
metamorphic rocks. In Exercise 4.3 you will conduct simple experiments to understand
how igneous, sedimentary, and metamorphic processes create different and
distinctive textures. In Exercise 4.4 you will apply what you’ve learned to assign rock
samples to their correct class.
96 CHAPTER 4 MINERALS, ROCKS, AND THE ROCK CYCLE