Laboratory Manual for Introductory Geology 4e

LEARNING
OBJECTIVES
■■
Understand the changes that
occur during formation of
metamorphic rocks
■■
Understand the agents that
bring about metamorphism
■■
Become familiar with
metamorphic textures and
mineral assemblages
■■
Interpret the history of
metamorphic rocks from
their textures and mineral
assemblages
MATERIALS
NEEDED
■■
A set of metamorphic rocks
■■
Magnifying glass or hand lens
and, ideally, a microscope and
thin sections of metamorphic
rocks
■ ■ Standard supplies for
identifying minerals (streak
plate, glass plate, etc.)
7.1 Introduction
Biologists use the term metamorphosis (from the Greek meta, meaning change, and
morph, meaning form) to describe what happens when a caterpillar turns into a butterfly.
Geologists use the similar term metamorphism for the processes of change
that a rock undergoes when exposed to physical and chemical conditions significantly
different from those under which it first formed. The original rock, called
the protolith, may be any kind of igneous, sedimentary, or older metamorphic rock.
Logically, the end product of metamorphism is a metamorphic rock. Metamorphism
differs from sedimentary and igneous rock-forming processes because it is
entirely a solid-state phenomenon. This means that the original rock remains in a solid
state without breaking apart or melting. By definition, metamorphism occurs at
temperatures higher than those involved in sedimentary rock diagenesis and lower
than those at which melting takes place.
Metamorphic rocks form at depths below those at which sedimentary rocks
generally form and above those where melting generally occurs, so they may yield
important clues about parts of the Earth for which neither igneous nor sedimentary
rocks can provide information. For example, what happens to rocks as the opposite
sides of the San Andreas fault grind past one another? Or to rocks along the west
coast of South America as the Nazca Plate is subducted beneath the continent? Or
to rocks deep below Mt. Everest as India collides with Asia? How did rocks in California
change when they were intruded by the granitic magma that today forms the
enormous Sierra Nevada batholith?
This chapter shows how we can answer these questions and unravel the geologic
history of rocks that have been changed—sometimes dramatically—from their original
forms. We’ll begin by describing metamorphic changes and the agents that
cause them, and then examine metamorphic textures and minerals for clues to the
type of metamorphism that occurred and the temperature and pressure conditions
under which it took place.
7.2 What Changes during Metamorphism?
Nearly all characteristics of the protolith may change during metamorphism,
including its texture, its mineral composition, and even its chemical composition.
In some cases, only one of these properties may change, but in others, two or all
three of them may change so much that the metamorphic rock looks nothing like
its protolith.
7.2.1 Changes in Texture
A rock’s texture includes the size, shape, and relationships of its grains. These properties
may change during metamorphism by any of three processes:
1. Recrystallization (change in grain size and shape): Mineral grains may regrow
to form new grains of the same mineral, changing in shape, size, or both
2. Pressure solution (change in grain shape and relationships): If water is present
during metamorphism, some minerals may partially dissolve and the ions may
re-precipitate, changing the shapes and relationships among grains.
3. Alignment of grains (change in grain orientation): In most instances, minerals
in the protolith are oriented randomly, but during certain kinds of metamorphism,
they become aligned parallel to one another. The resulting textures,
called foliation and lineation, are described in section 7.3.3.
174 CHAPTER 7 INTERPRETING METAMORPHIC ROCKS