Lecture 6 metamorphic rocks

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

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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 foliation planes are relatively rough.

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Gneiss is a coarse-grained rock with coarse light- and darkcolored 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 is a nonfoliated metamorphic rock derived from
quartz sandstone.