Laboratory Manual for Introductory Geology 4e

EXERCISE 4.1
Settings in Which Rocks Form
Name:
Course:
Section:
Date:
Three settings are described below in which three different classes of rocks form. Fill in the blanks by indicating which class
of rocks is the result.
(a) Along the coast, waves carry sand out into deeper, quieter water, where it accumulates and gradually becomes
buried. A rock formed from sand grains cemented together is .
(b) Thick flows of glowing red lava engulf farms and villages on the flanks of Mt. Etna, a volcano in Sicily. The cold, black
rock formed when these flows are cool enough to walk on is .
(c) The broad tundra plains of northern Canada expose gray, massive rock that formed many kilometers below a mountain
belt. This rock, exposed only after the overlying rock was stripped away, is .
4.2.1 The Rock Cycle
An igneous rock exposed at the surface of the Earth will not last forever. Minerals
that crystallized from magma to form the rock can be broken apart at the surface
by weathering, undergo erosion (the grinding effects of moving ice, water, or air),
be transported by streams, be deposited in the ocean, and be cemented together to
form a sedimentary rock. These same minerals, now part of a sedimentary rock, can
later be buried so deeply beneath other sedimentary rocks that they are heated and
squeezed to form a metamorphic rock with new minerals. Erosion may eventually
expose the metamorphic rock at the Earth’s surface, where it can be weathered to
form new sediment and eventually a different sedimentary rock. Or the metamorphic
rock may be buried so deeply that it melts to produce magma and eventually
becomes a new igneous rock with still different minerals.
This movement of material from one rock type to another over geologic time is
called the rock cycle (FIG. 4.1). The rock cycle involves the reuse of mineral grains or
the breakdown of minerals into their constituent atoms and reuse of those atoms to
make other minerals. Rocks formed at each step of the rock cycle look very different
from their predecessors because of the different processes by which they formed.
4.3 A Rock Is More than the Sum of Its Minerals
The first thing a geologist wants to determine about a rock is whether it is igneous,
sedimentary, or metamorphic. Composition alone (i.e., the minerals that make
up the rock) is rarely enough to define a rock’s origin because some of the most
common minerals (such as quartz, potassium feldspar, and sodium-rich plagioclase
feldspar) are found in all three rock classes—for example, a rock made of these
minerals could be igneous (granite), sedimentary (sandstone), or metamorphic
(gneiss). Texture alone can identify which type of process was involved in forming
many rocks, but some textures can also develop in all three types of rock. The best
clue to the origin of a rock is its unique combination of texture and mineralogy.
4.3.1 Describing Texture
Geologists begin characterizing a rock’s texture by asking, does the rock consist
of a mass of glass or does it consist of grains? If it consists of glass, it will be shiny
and develop conchoidal (curving, ridged) fracture surfaces when broken. Rocks
4.3 A ROCK IS MORE THAN THE SUM OF ITS MINERALS
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