Geology Reference
In-Depth Information
earlier, all have the same chemical composition but differ in
crystal structure, are used in manufacturing high-temperature
porcelains and temperature-resistant minerals for products
such as sparkplugs and furnace linings.
Asbestos is a metamorphic mineral that is widely
used for insulation and fireproofing and is widespread in
buildings and building materials. It is also generally regarded
as an environmental hazard because the concentration of
amphibole asbestos fibers in your lungs could have fatal
consequences (see Geo-Focus on pages 172 and 173).
Chapter Summary
Metamorphic rocks result from the transformation of
other rocks, usually beneath Earth's surface, as a conse-
quence of one or a combination of three agents: heat,
pressure, and fl uid activity.
Heat for metamorphism comes from intrusive magmas,
extrusive lava fl ows, or deep burial. Pressure is either
lithostatic (uniformly applied stress) or differential
(stress unequally applied from different directions).
Fluids trapped in sedimentary rocks or emanating from
intruding magmas can enhance chemical changes and the
formation of new minerals.
The three major types of metamorphism are contact,
dynamic, and regional.
Contact metamorphism takes place when a magma or
lava alters the surrounding country rock.
Dynamic metamorphism is associated with fault zones
where rocks are subjected to high differential pressure.
Regional metamorphism occurs over a large area and is
usually caused by tremendous temperatures, pressures,
and deformation within the deeper portions of the crust.
Metamorphic grade generally characterizes the degree to
which a rock has undergone metamorphic change.
Index minerals—minerals that form only within specifi c
temperature and pressure ranges—allow geologists
to recognize low-, intermediate-, and high-grade
metamorphism.
Metamorphic rocks are primarily classifi ed according
to their texture. In a foliated texture, platy and elongate
minerals have a preferred orientation. A nonfoliated
texture does not exhibit any discernable preferred
orientation of the mineral grains.
Foliated metamorphic rocks can be arranged in order of
increasing grain size, perfection of their foliation, or both.
Slate is fi ne grained, followed by (in increasingly larger
grain size) phyllite and schist; gneiss displays segregated
bands of minerals. Amphibolite is another fairly common
foliated metamorphic rock. Migmatities have both
igneous and high-grade metamorphic characteristics.
Marble, quartzite, greenstone, hornfels, and anthracite
are common nonfoliated metamorphic rocks.
Metamorphic zones are based on index minerals and are
areas of rock that all have similar grades of metamor-
phism, that is, they have all experienced the same inten-
sity of metamorphism.
A metamorphic facies is a group of metamorphic rocks
whose minerals all formed under a particular range of
temperatures and pressures. Each facies is named after its
most characteristic rock or mineral.
Metamorphism occurs along all three types of plate bound-
aries, but is most common at convergent plate margins.
Metamorphic rocks formed near Earth's surface along
an oceanic-continental convergent plate boundary
result from low-temperature, high-pressure conditions.
As a subducted oceanic plate descends, it is subjected to
increasingly higher temperatures and pressures that result
in higher-grade metamorphism.
Many metamorphic rocks and minerals, such as marble,
slate, graphite, talc, and asbestos, are valuable natural
resources. In addition, many ore deposits are the result of
metamorphism and include copper, tin, tungsten, lead,
iron, and zinc.
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