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Guide
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In any study of rocks, metamorphic rocks always
come last. This is logical; some other rock (igneous, sedimentary,
or even a different metamorphic rock) must exist first to get changed
into a metamorphic rock. There are limits to the changes that may
occur and still yield metamorphic rock. Note the wording in the title
of this chapter. It begins "Change in the Solid State," words chosen
to emphasize an important limit that is sometimes forgotten. No matter
how extreme the temperatures or pressures involved, the rock undergoing
change must remain essentially a solid. If it were to be broken down
into sediment or changed to a melt, you'd be back in the realms of
sedimentary or igneous rocks.
Once the above is explained and a basic definition
of metamorphic rock is established, the chapter examines the causes
of metamorphism and the features associated with different types
of metamorphism. The causes, known as agents of metamorphism, are
heat, hot groundwater, pressure, and differential stress. Under the
general topics of heat and hot groundwater (hydrothermal solutions)
you read about recrystallization, compositional banding, and metasomatism.
Learning about the roles of pressures and stresses in metamorphism
brings up discussions of differential stress, normal stress (compression
and tension), and shear stress. Temperature and pressure conditions
together determine whether a particular mineral is in a stable or
unstable condition, metamorphically speaking.
Classification of metamorphic rock comes next. It's
not very complex; there are only two fundamental divisions: foliated
and nonfoliated. Common foliated rocks (those that exhibit a layered
look) that the author describes are slate, metasandstone, metaconglomerate,
phyllite, schist, gneiss, and the "hybrid rock" (part igneous, part
metamorphic) migmatite. Common nonfoliated rocks described (which
don't have the layered look because they have neither preferred mineral
orientation nor compositional banding) are hornfels, quartzite, marble,
and dolomitic marble. Nature often manages to defy rigid classification.
As an example of this, the author points out the existence of two
rocks that exhibit contradicting characteristics, foliated quartzite
and foliated marble.
The significance of the existence of one kind of
metamorphic rock instead of another kind is addressed in index minerals,
metamorphic zones, grade of metamorphism (low, intermediate, or high
grade), and metamorphic facies, with its seven main subdivisions.
Just when you start to feel you have a firm grip
on what metamorphism is all about, you learn that a seemingly backward
version of everything you've just been taught can occur. Retrograde
metamorphism can happen to rocks under conditions of decreasing temperatures
and pressures, in direct contrast to the usual prograde metamorphism
associated with increasing temperatures and pressures.
The chapter concludes with a discussion of locations
where you can find metamorphic rocks (environments of metamorphism).
These include areas
- adjacent to plutons (contact or thermal metamorphism)
- in fault zones (dynamic metamorphism)
- beneath mountains adjacent to subducting plates
or between colliding plates (dynamothermal or regional metamorphism)
- in continental areas where rifting or transform
faulting is occurring
- at mid-ocean ridges
- in subduction zones
- on continental shields
If many of the above environments remind you of the
plate tectonic chapters, that's good! As you were told early in the
text, in any geologic discussion today it's difficult to avoid plate
tectonics. It so often provides the basic answer to the question
"Why does that happen?"
These first six chapters have presented the infrastructure
of Earth, what it is and how it got established. If you don't have
a firm grasp of the concepts presented so far, you may want to review
these fundamental chapters, as future chapters build on this material.
In the remaining chapters you will learn about activities that occur
within this infrastructure, beginning with some very dramatic action,
volcanic activity.
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