Metamorphism and Metamorphic Rocks - The Changing Earth: Exploring Geology and Evolution - page 173

This process, which usually
occurs near Earth's surface, is
called hydrothermal alteration
(from the Greek hydro , “water,”
and therme , “heat”) and may
result in valuable mineral
deposits.
Geologists think that many
of the world's ore deposits result
from the migration of metallic
ions in hydrothermal solutions.
Examples are copper, gold, iron
ores, tin, and zinc in various
localities including Australia,
Canada, China, Cyprus, Finland,
Russia, and the western United
States.

Lava flow

Baked zone

Ash layer

◗ Figure 7.6 Contact Metamorphism from a Lava Flow A highly weathered basaltic lava fl ow near
Susanville, California, has altered an underlying rhyolitic volcanic ash by contact metamorphism.
The red zone below the lava fl ow has been baked by the heat of the lava when it fl owed over the
ash layer. The lava fl ow displays spheroidal weathering, a type of weathering common in fractured
rocks (see Chapter 6).

Dynamic Metamorphism
Most dynamic metamorphism
is associated with fault (frac-
tures along which movement
has occurred) zones in the
shallow crust where rocks are subjected to concentrated
high levels of differential pressure. The metamorphic
rocks that result from pure dynamic metamorphism are
called mylonites and typically they are restricted to nar-
row zones adjacent to faults. Mylonites are hard, dense,
fine-grained rocks, many of which are characterized by
thin laminations ( ◗ Figure 7.7). Tectonic settings where

mylonites occur include the Moine Thrust Zone in north-
west Scotland and portions of the San Andreas fault in
California (see Chapter 2), and parts of the northeastern
United States.

Image not available due to copyright restrictions

Regional Metamorphism
Most metamorphic rocks result from regional metamor-
phism , which occurs over a large area and is usually caused
by tremendous temperatures, pressures, and deformation all
occurring together within the deeper portions of the crust.
Regional metamorphism is most obvious along convergent
plate boundaries where rocks are intensely deformed and
recrystallized during convergence and subduction (Figure
7.4). These metamorphic rocks usually reveal a gradation
of metamorphic intensity from areas that were subjected to
the most intense pressures and/or highest temperatures to
areas of lower pressures and temperatures. Such a gradation
in metamorphism can be recognized by the metamorphic
minerals that are present.
Regional metamorphism is not, however, confi ned only
to convergent margins. It can also occur in areas where plates
diverge, although usually at much shallower depths because
of the high geothermal gradient associated with these areas
(Figure 7.4).

Index Minerals and Metamorphic Grade
From fi eld studies and laboratory experiments, certain min-
erals are known to form only within specifi c temperature and
pressure ranges. Such minerals are known as index minerals
because their presence allows geologists to recognize low-, in-
termediate-, and high-grade metamorphism ( Figure 7.8) .

Metamorphic grade is a term that generally character-
izes the degree to which a rock has undergone metamorphic
change (Figure 7.8). Although the boundaries between the

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