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Guide
to Reading 
Sometime in your previous school years you probably
learned there are three basic kinds of rocks: igneous, sedimentary,
and metamorphic. The classification is based on the origin of the
rock. Igneous rocks, the fire-formed ones, are the logical group
to start with for two reasons: (1) they make up the greatest part
of planet Earth, and (2) they were the first rocks to exist on Earth.
The igneous category is subdivided into two main
branches: intrusive igneous rocks and extrusive igneous rocks. In
the first, molten rock below Earth’s surface (magma) hardens
and creates various igneous rocks (including granite, diorite, and
gabbro) and structures (irregular shapes called plutons and tabular
intrusions). In the second, molten rock spills out on the surface
as lava that produces rocks (including rhyolite, andesite, basalt,
obsidian, pumice, and pyroclastic rocks), structures (volcanoes and
lava flows), and some wild and exciting events, for “extrusive
igneous activity” is synonymous with the word “volcanics.”
This chapter concentrates on magmas and rocks and touches only briefly
on volcanic activity. You’ll have to wait until Chapter 7 for
the wild and exciting events; that’s the chapter devoted to
volcanic eruptions and their importance to humans throughout history.
Any study of igneous activity begins with a discussion
of magma. As you learned in Chapter 1, Earth’s interior is
not composed chiefly of molten rock. Instead there are relatively
few interior areas that are liquid, and there has to be some special
reason for the solid rock of these areas to have become molten rock,
that is, magma. Your author organizes his discussion of magma formation
as follows:
- the conditions that cause melting of mantle and
crustal rocks (decreased pressure, addition of volatiles, and heat
transfer)
- the chemical composition of magmas
- the four major types of magmas (silicic, intermediate,
mafic, and ultramafic)
- movement of magma, why it goes where it does,
and why it has different viscosities
Once formed, magma doesn’t always stay molten.
Why and how does it harden? Your author discusses
- the sequence of hardening of a melt (fractional
crystallization, Bowen’s Reaction Series)
- factors that control the cooling rate
- structures that result when magma solidifies within
the Earth (plutons, tabular intrusions, laccoliths, batholiths,
xenoliths and the stoping process, sills, and dikes)
- classification of igneous rocks (based on chemical
compositions and textures)
A word of advice: A discussion of magma is not conceptually
difficult, but it can be confusing. After all, the intrusive activity
is happening in the unseen and unfamiliar world of Earth’s
interior, and extrusive lava is not something the average person
has had experience dealing with. Keep in mind the following two sets
of word associations to help you follow the discussions of solid
rock melting and liquid rock freezing.
Set one: A chemical composition high in silica,
low in iron and magnesium (silicic); light-colored, light-weight
rocks; low-temperature but high-viscosity melts
Set two: A chemical composition high in iron and
magnesium, low in silica (mafic); dark-colored, heavy rocks; high-temperature
but low-viscosity melts
Earth is a dynamic place; given enough geologic time,
nothing on it remains unchanged. Once igneous rocks exist, they become
part of that rock cycle you no doubt learned about long ago, at the
same time you learned to recite “igneous, sedimentary, and
metamorphic.” Chapters 5 and 6 continue the story, as natural
forces change igneous rock into sedimentary and metamorphic rocks.
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