Geoscience Reference
In-Depth Information
VISUAL CONCEPT CHECK
12.1
What two types of crust occur in landscapes
that look like this? Which of the two is denser
and why?
Rocks and Minerals in
the Earth's Crust
cal and atomic configurations that usually take some kind of crys-
talline form. A great example of a crystalline mineral that you
may be familiar with is quartz, which typically occurs as a clear,
six-sided prism (Figure 12.5).
When minerals become bonded in a solid state, they form
rock
. Most rocks are very old by human standards, with the old-
est being over 3.5 billion years old! Nevertheless, new rock is
probably being created someplace at the very instant that you are
reading this chapter. Is a major volcanic eruption occurring on
Have you ever picked up an interesting-looking rock when walk-
ing along the beach or hiking in the mountains? Have you ever
noticed a fantastic exposure of rock along a highway? If so, per-
haps you wondered how the rocks formed, where they came from,
or how old they were. In most cases the rocks were very old and
contained elements that had been recycled many times through-
out Earth's history. Some of these minerals may have even been
in the center of Earth at one time. This part of the chapter exam-
ines the basic kinds of rocks on Earth and how they form.
Rocks are composed of a variety of Earth elements. Figure
12.4 shows the approximate percentage by weight of the various
elements, including oxygen and silicon, with lesser amounts of
aluminum, iron, calcium, sodium, potassium, and magnesium.
These elements combine in various ways to form
minerals
,
which are naturally occurring substances with distinctive chemi-
Aluminum, 8.1%
Iron, 5%
Calcium, 3.6%
Sodium, 2.8%
Potassium, 2.6%
Magnesium, 2.1%
Others, 0.8%
Oxygen
47%
Silicon
28%
Figure 12.5 Quartz crystals.
Quartz is an excellent example of
a crystalline mineral. In small grains, it forms sediments in des-
erts, river deposits, and coastal areas. It is also a common part of
most rocks formed in the continental crust.
Figure 12.4 Chemical elements in the Earth's crust.
Percent-
ages refer to the percentage by weight of each element in the
Earth's crust.
Minerals
Naturally occurring substances with distinctive
chemical configurations that usually manifest themselves in
some kind of crystalline form.
An amorphous mass of consolidated mineral matter.
Rock
