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Lecture 6 Metamorphic Rocks
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What are metamorphic rocks?
Metamorphic processes
Texture of metamorphic rocks
Types of metamorphic rocks
Engineering considerations of metamorphic rocks
Geologic rock cycle
Stephen Marshak
(a)
(b)
(a) This thin section of a limestone shows small fossil shells
distributed in a matrix of lime mud. (b) This thin section of
marble (same composition as limestone) shows interlocking
grains. Atoms have been completely re-arranged. (Yong II Lee)
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What are metamorphic rocks?
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Metamorphic rocks form from preexisting rocks (igneous,
sedimentary, or other metamorphic rocks) through the action of
heat and pressure. This process of the transformation of one rock
type into another is called metamorphism (Greek: "changed
form").
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Metamorphism most often occurs deep within earth. Under
increased temperature and pressure, the minerals of preexisting
rocks become unstable and recrystallize in a solid state to become
new minerals.
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Study of metamorphic rocks yields valuable information about
metamorphic conditions on rock and about the geologic history of a
region.
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This metamorphic rock, California exhibits a slaty cleavage. The type of
rock cleavage allows it to split easily in the flat plates visible in this
photo. Photo by E.J. Tarbuck.
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Metamorphic processes
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Heat and pressure (stress) are the primary agents of
metamorphism.
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Heat: Heat provides the energy to drive the chemical changes that
result in recrystallization of minerals.
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Where does heat come from to cause metamorphism? One way is
the intrusion by hot magma. In effect, the surrounding rock is
"baked" by the high temperature of the molten magma. This kind of
metamorphism is called contact metamorphism.
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Another important way to get heat is deep burial. Temperature
increases about 15 to 30 degrees for each kilometer of depth in the
crust (geothermal gradient). Gradual burial in a sedimentary
basin can bury rocks formed at the surface to several kilometers.
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Contact metamorphism occurs around hot magma intrusions. Increases
in temperature and inclusion of pore fluids cause preexisting minerals
to form new minerals.
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Metamorphic Processes (continued)
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Pressure: An increase in pressure reduces mineral space and drive
chemical reactions that produced new minerals with closer atomic
packing and higher density.
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Pressure increases with depth inside solid earth much like pressure
increases with depth in water. Tectonic processes (such as subduction
and continental collision) can bury rocks to tens of kilometers. In this
case, metamorphism can occur over large areas and is called regional
metamorphism.
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Regional metamorphism also occurs during mountain building when
great volume of rocks are subjected to directed stress. The greatest
volume of metamorphic rocks are best exposed in the deformed
mountain belts and in ancient stable continental interiors known as
shields, such as the Canadian Shield. Shields are assumed to be the
remnants of ancient periods of mountain building.
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Contact metamorphism and regional metamorphism are the two main
processes of metamorphism.
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A. Buried rocks are subject to pressure from the load above. B.
During mountain building, rocks are subject to directional stress
that shortens and deforms rock strata. In these cases,
metamorphism can occur over large areas and is called
regional metamorphism.
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Metamorphic rocks are widely distributed in continental shields (such as
the Canadian Shield) and in the cores of folded mountain belts.
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Metamorphic grade
Metamorphism occurs incrementally, from slight change
(low grade) to dramatic change (high grade) as the
intensity of heat and pressure increases.
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Textures of metamorphic rocks: How
metamorphism changes rocks?
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Foliation
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During deformation where stresses are not uniformly oriented, many
metamorphic rocks develop textures in which the mineral grains
have strongly preferred orientations in the direction of least stress.
The resulting mineral alignment often gives the rock a layered or
banded texture called foliation (Latin: "splitting into leaf-like
layers").
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Depending on the metamorphism grade and parent rocks, the types
of foliation include slaty cleavage, schistosity, and gneissic
texture.
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Foliation. Under directed stress, elongated minerals become reoriented or
recrystallized resulting in alignment along the direction perpendicular to
the stress. Under intense metamorphism, a granite (left) could transform
to gneiss with a foliation of alternating light- and dark-colored bands
known as gneissic texture.
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Foliated Metamorphic Rocks
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Names for foliated metamorphic rocks are typically based on their
foliated textures.
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Slate: If the cleavage planes are very thin and the rock is fine
grained, the cleavage is called slaty cleavage and the rock is called
slate. Slate is usually produced by low-grade metamorphism of shale
under directed pressure and low temperature.
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Schist: The grains in a schist are coarser than in slate and the
surface of cleavage planes are relatively rough.
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Gneiss is a coarse-grained rock with coarse light- and dark-colored
bands. Gneiss forms under high-grade metamorphism from granite
or diorite and other rocks.
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Slate
is a fine-grained foliated metamorphic rock with slaty (very
thin) rock cleavage. Slate is usually produced by low-grade
metamorphism of shale under directed pressure and low temperature.
(a) Note the bedding plane is not necessarily parallel to the
cleavage. (b) Slate easily splits into thin sheets, which have
been used as roof shingles for millennium. (S. Marshak)
Compression of a bed end-on to create slaty cleavage
perpendicular to the bedding. (W.W. Norton)
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Schist
is a strongly foliated rock with abundant platy and elongated
minerals (muscovite, biotite, …) that can be readily split into thin flakes.
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Gneiss
is a coarse-grained rock with coarse light- and dark-colored
bands. Gneiss forms under high-grade metamorphism from granite or
diorite and other rocks.
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Nonfoliated metamorphic rocks
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Not all metamorphic rocks have a foliated texture. Metamorphic
rocks composed of only one mineral having equidimensional crystals
usually are not visibly foliated. Mineral composition forms the basis
for naming nonfoliated metamorphic rocks.
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Marble is a coarse, crystalline metamorphic rock composed almost
entirely of calcite or dolomite.
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Quartzite is a nonfoliated metamorphic rock formed from quartz
sandstone.
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Marbles are coarse, crystalline metamorphic products of heat and
pressure acting on limestones and dolomites.
The marble in this
unfinished sculpture
by Michelangelo is
fairly soft and easy to
carve, but does not
crumble. (S. Marshak)
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Quartzite
sandstone.
is a nonfoliated metamorphic rock derived from quartz
Transition from quartz sandstone to quartzite. IN a
quartzite, the grains have grown. If the quartzite cracks,
the crack ignores the grain boundaries. (W.W. Norton)
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Engineering considerations of
metamorphic rocks
Foliated rocks possess prominent directional
properties. The strength is much weaker in the
direction of the foliation than in other directions. Care
should be taken that loads (bridges, dams, buildings) are
not transferred to foliated directions. In tunnel
construction, foliated metamorphic rocks are generally
more costly because of more steel supports needed.
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Geologic Rock Cycle
The types of rocks we talked about can be transformed
from one type to another. The various processes of the
rock cycle provide an excellent demonstration of the
Earth as a dynamic system.
The rock cycle