Geology Reference
In-Depth Information
Understanding how rocks form
Characteristics of rocks such as shape, color, and location tell a story of how and where
the rocks formed. A large part of geologic knowledge is built on understanding the pro-
cesses and conditions of rock formation. For example, some rocks form under intense
heat and pressure, deep within the earth. Other rocks form at the bottom of the ocean
after years of accumulation and compaction. The three basic rock types, which I discuss
in detail in Chapter 7, are:
Igneous: Igneous rocks form as liquid rock material, called magma or lava, cools.
Igneous rocks are most commonly associated with volcanoes.
Sedimentary: Most sedimentary rocks form by the compaction of sediment
particles that have settled to the bottom of a body of water, such as an ocean or
lake. (There are also chemical sedimentary rocks, which are not formed this way. I
describe these in Chapter 7 as well.)
Metamorphic: Metamorphic rocks are the result of a sedimentary, igneous, or oth-
er metamorphic rock being crushed under intense amounts of pressure or subjec-
ted to high amounts of heat (but not enough to melt it) that change its mineral
composition.
Each rock exhibits characteristics that result from the specific process and environ-
mental conditions (such as temperature) of its formation. In this way, each rock
provides clues to events that happened in earth's past. Understanding the past helps us
to understand the present and, perhaps, the future.
Tumbling through the rock cycle
The sequence of events that change a rock from one kind into another are organized in-
to the rock cycle. It is a cycle because there is no real beginning or end. All the different
types of rocks and the various earth processes that occur are included in the rock cycle.
This cycle explains how materials are moved around and recycled into different forms
on the earth's surface (and just below it). When you have a firm grasp on the rock cycle,
you understand that every rock on earth's surface is just in a different phase of trans-
formation, and the same materials may one day be a very different rock!
Mapping Continental Movements
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