Download Ch 5 Sec 1: Classifying Rocks Guide for Reading

Name ____________________________ Date ____________________ Class ____________
Ch 5 Sec 1: Classifying Rocks
Guide for Reading
■ What characteristics do geologists use to identify rocks?
■ What are the three main groups of rocks?
The rock of Earth’s crust forms mountains, hills, valleys, beaches, and
the ocean floor.
When studying a rock sample, geologists need to look at the inside of
the rock since the effects of ice, water, and weather can changer the
outer surface of a rock. Geologists look at:
1. mineral composition- Rocks are made of mixtures of minerals and
other materials, although some rocks may contain only a single
mineral. The 20 minerals that make up most of the rocks of Earth’s
crust are known as rock-forming minerals. Appendix B on pp. 200201 lists some of the most common rock-forming minerals.
2. color- A rock’s color provides clues to the rock’s mineral
composition. Granite is generally a light-colored rock that has high
silica content. It contains the minerals quartz, feldspar, hornblende and
mica (figure 2 p. 145). Basalt is a dark colored rock that is low in
silica. Geologists also observe the shape and color of crystals in a rock
to identify its minerals. They may perform other tests, such as testing
the surface of a rock with acid, to determine the presence of certain
compounds. Color alone does not provide enough information to
identify a rock. Geologists use the shape and color of crystals to
identify the minerals in the rock. Other tests are used as well, ex. Acids
to test for carbonates.
3. texture- A rock’s texture, the look and feel of the rock’s surface, is
very useful in identifying a rock.
Most rocks are made up of grains, particles of minerals or other rocks.
Grains give the rock its texture.
Geologists use a number of terms to describe a rock’s texture based on
the: Figure 4 p. 146
1. grain size
Often, the grains in a rock are large and easy to see. These
are said to be coarse-grained. Ex. Diorite
In other rocks, the grains are so small that they can be seen
only with a microscope. These are said to be fine-grained.
Ex. Slate
Some rocks have no visible grain even when they are examined
under a microscope. Ex. Flint
2. grain shape
Rock grains vary widely in shape. Some look like tiny particles
of sand, while others look like small seeds or exploding
stars. In some rocks (granite), the grain shapes result from
the shapes of the crystals that form the rock. In other rocks,
the grain shapes result from fragments of other rock which
can be smooth & rounded (conglomerate) or jagged (breccia).
3. grain pattern
Some grains lie in flat layers (banded/ foliated). Other grains form
swirling patterns. Some rocks have grains of different colors
in bands (gneiss), while in others the grains occur randomly
throughout (quartzite)- non-foliated/ non banded.
Geologists classify rocks into three major groups:
These terms refer to how the rocks in each group formed / origin.
1. igneous rock forms from the cooling of molten rock.
2. sedimentary rock forms when particles of other rocks or the
remains of plants and animals are pressed & cemented together.
3. metamorphic rock forms when an existing rock is changed by
heat, pressure, or chemical reactions. Most form deep
underground.