Metamorphism and Metamorphic Rocks - The Changing Earth: Exploring Geology and Evolution - page 167

Metamorphism is associated with all three types
of plate boundaries but is most widespread along
convergent plate boundaries.
Many metamorphic minerals and rocks are valuable
metallic ores, building materials, and gemstones.

morpho, “shape”), which result from the transformation of
other rocks by processes that typically occur beneath Earth's
surface (see Figure 1.14). During metamorphism , rocks are
subjected to sufficient heat, pressure, and fluid activity to
change their mineral composition, texture, or both. These
changes result in metamorphic rocks that usually do not look
anything like the original rock before it was metamorphosed.
These transformations take place below the melting tempera-
ture of the rock; otherwise, an igneous rock would result.
A useful analogy for metamorphism is baking a cake. Just
like a metamorphic rock, the resulting cake depends on the
ingredients, their proportions, how they are mixed together,
how much water or milk is added, and the temperature and
length of time used for baking the cake.
Except for marble and slate, most people are not famil-
iar with metamorphic rocks. We are frequently asked by stu-
dents why it is important to study metamorphic rocks and
processes. Our answer is always, “Just look around you.”
A large portion of Earth's continental crust is composed
of metamorphic and igneous rocks. Together, they form the
crystalline basement rocks underlying the sedimentary rocks
of a continent's surface. These basement rocks are widely ex-
posed in regions of the continents known as shields , which
have been very stable during the past 600 million years
(

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INTRODUCTION
Its homogeneity, softness, and various textures have made mar-
ble, a metamorphic rock formed from limestone or dolostone,
a favorite rock of sculptors throughout history. As the value of
authentic marble sculptures has increased over the years, the
number of forgeries has also increased. With the price of some
marble sculptures in the millions of dollars, private collectors
and museums need some means of ensuring the authenticity
of the work that they are buying. Aside from the monetary
considerations, it is important that forgeries not become part
of the historical and artistic legacy of human endeavor.
Experts have traditionally relied on artistic style and
weathering characteristics to determine whether a marble
sculpture is authentic or a forgery. Because marble is not very
resistant to weathering, however, forgers have been able to
produce the weathered appearance of an authentic work.
Using newly developed techniques, geologists can now
distinguish a naturally weathered marble surface from one
that has been artificially altered. Yet, there are examples in
which expert opinion is still divided on whether a sculpture
is authentic. One of the best examples is the Greek kouros
(a sculptured fi gure of a Greek youth) that the J. Paul Getty
Museum in Malibu, California, purchased for a reputed price
of $7 million in 1984. Because some of its stylistic features
caused some experts to question its authenticity, the museum
had a variety of geochemical and mineralogical tests performed
in an effort to determine the authenticity of the sculpture.
Although numerous scientifi c tests have not unequivocally
proved authenticity, they have shown that the weathered surface
layer of the kouros bears more similarities to naturally occurring
weathered surfaces of dolomitic marble than to known artifi cially
produced surfaces. Furthermore, no evidence indicates that the
surface alteration of the kouros is of modern origin.
Unfortunately, despite intensive study by scientists,
archaeologists, and art historians, opinion is still divided as
to the authenticity of the Getty kouros. Most scientists accept
that the kouros was carved sometime around 530 B . C . Point-
ing to inconsistencies in its style of sculpture for that period,
other art historians think that it is a modern forgery.
Regardless of whether the Getty kouros is proven to be
authentic or a forgery, geologic testing to authenticate mar-
ble sculptures is now an important part of many museums'
curatorial functions. To help geologists in the authentica-
tion of marble sculptures, a large body of data about the
characteristics and origin of marble is being amassed as more
sculptures and their quarries are analyzed.
Having examined igneous and sedimentary rocks, we
now turn our attention to the third major rock group, the
metamorphic rocks (from the Greek meta, “change,” and

Figure 7.1). Some of the oldest known rocks, dated at
3.96 billion years from the Canadian Shield, are metamor-
phic, which means that they formed from even older rocks!
Metamorphic rocks such as marble and slate are used as
building materials, and some metamorphic minerals, such
as garnets (used as gemstones or abrasives) and talc (used in
cosmetics, in manufacturing paint, and as a lubricant), are
economically important. Asbestos, another valuable meta-
morphic mineral, is used for insulation and fi reproofi ng and
has been the subject of much debate over the danger it poses
to the public's health (see Geo-Focus on pages 172 and 173).

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THE AGENTS OF METAMORPHISM
The three principal agents of change that cause metamor-
phism are heat , pressure , and fl uid activity . Time is also im-
portant to the metamorphic process because chemical
reactions taking place in rocks undergoing metamorphism
proceed at different rates and thus require different amounts
of time to complete. Reactions involving silicate compounds
are particularly slow, and because most metamorphic
rocks are composed of silicate minerals, it is thought that
metamorphism is a very slow geologic process.
During metamorphism, the original rock, which was
in equilibrium with its environment, meaning that it was
chemically and physically stable under those conditions,
undergoes changes to achieve equilibrium with its new en-
vironment. These changes may result in the formation of
new minerals, a change in the texture of the rock, or both.
In some instances, the change is minor, and features of the
original rock can still be recognized. In other cases, the rock
changes so much that the identity of the original rock can be
determined only with great diffi culty, if at all.