Metamorphic Rocks/EPCC/LM6

Chapter 8: Metamorphism and
Metamorphic Rocks:
New Rocks from Old

What is Metamorphism? (1)
 Metamorphism is the change in form that
happens in Earth’s crustal rocks in response to
changes in temperature and pressure.

What is Metamorphism? (2)
 There are six major factors in metamorphism:
 Chemical composition.
 The change in temperature.
 The change in pressure.
 The presence or absence of fluids.
 How long a rock is subjected to high pressure or high
temperature.
 Whether the rock is simply compressed or is twisted
and broken during metamorphism.

Chemical Composition of Original
Rock
 The greatest factor in determining the mineral
assemblage of a metamorphic rock.
 The chemical composition of the original rock
controls the mineralogy of the metamorphosed
rock.

Temperature And Pressure (1)
 The heat source is Earth’s internal heat.
 Rock can be heated by burial or by nearby
igneous intrusion.
 Burial is inevitably accompanied by an increase
in pressure due to the weight of the overlying
rocks.
 An intrusion may be shallow, resulting in low
pressure, or deep, resulting in high pressure.

Temperature And Pressure (2)
 Low-grade metamorphism is the result of
metamorphic processes that occur at
temperatures from about 100o
C to 500o
C, and at
relatively low pressures.
 High-grade metamorphism is the result of
metamorphic processes at high temperatures
(above 500o
C), and at high pressure.

Stress
 Stress is applied pressure that results in
deformation in a solid, and the development of
new textures.
 Uniform stress occurs if pressure is equal in all
directions.
 Differential stress occurs if pressure is different
in different directions.
 Texture is controlled by differential versus
uniform stress.

Fluids and Metamorphism (1)
 Sedimentary rocks have open spaces between
their grains filled by a watery intergranular
fluid.
 This fluid:
 Is never pure water.
 Always contains small amounts of dissolved gases and
salts.
 Contains traces of all the mineral constituents in the
enclosing rocks.

 Some of the fluid in sedimentary rock is retained
surface water buried with the rocks.
 Some of the fluid is released when hydrous
minerals (containing water in the formula) such
as clays, micas, and amphiboles, decompose and
lose water as the temperature increases on
burial.

 When the temperature and pressure change in a
rock that is undergoing metamorphism, so does
the composition of the intergranular fluid.
 The intergranular fluid is an important
transporting medium.

 When intergranular fluids are absent,
metamorphic reactions are very slow.
 When pressure increases due to burial of a rock,
and as metamorphism proceeds, the amount of
pore space decreases and the intergranular fluid
is slowly squeezed from the rock.

 Any fluid that escapes during metamorphism
will carry with it small amounts of dissolved
mineral matter.
 Minerals precipitated in a facture are called a vein.
 Metamorphic changes that occur while
temperatures and pressures are rising (and
usually while abundant intergranular fluid is
present) are termed prograde metamorphic
effects.

 Metamorphic changes that occur as temperature
and pressure are declining (and usually after
much of the intergranular fluid has been
expelled) are called retrograde metamorphic
effects.

Role of Time in Metamorphism
 Coarse-grained rocks are the products of long
sustained metamorphic conditions (possibly over
millions of years) at high temperatures and
pressures.
 Fine-grained rocks are products of lower
temperatures, lower pressures or, in some cases,
short reaction times.

The Upper And Lower Limits Of
Metamorphism
 At the lower end, metamorphism occurs in
sedimentary and igneous rocks that are
subjected to temperatures greater than about
100o
C, usually under pressures of hundreds of
atmospheres, caused by the weight of a few
thousand meters of overlying rock.
 At the upper end, metamorphism ceases to occur
at temperatures that melt rock.

Role of Water in Determining the
Limits of Metamorphism
 The water present controls the temperature at
which wet partial melting commences and the
amount of magma that can form from a
metamorphic rock.
 When a tiny amount of water is present, only a
small amount of melting occurs.
 Migmatites are composite rocks that contain an
igneous component formed by a small amount of
melting plus a metamorphic portion.

How Rocks Respond To
Temperature and Pressure Change In
Metamorphism
 Lower-grade Metamorphism: Slaty Cleavage.
 the newly forming sheet-structure minerals create
foliation that tends to be parallel to the bedding
planes of the sedimentary rock being metamorphosed.
 Higher-grade Metamorphism: Schistosity.
 At intermediate and high grades of metamorphism,
grain size increases.
 Foliation in coarse-grained metamorphic rocks is
called schistosity (the parallel arrangement of coarse
grains of the sheet-structure minerals).

Mineral Assemblage Change
 As temperature and pressure rise, one mineral
assemblage “morphs” into another.
 Each assemblage is characteristic of a given rock
composition.

Metamorphism of Shale and Mudstone
 Slate (low grade):
 The low grade metamorphic product of shale.
 Phyllite (intermediate grade):
 Pronounced foliation, larger mica grains.
 Schist and gneiss (high grade):
 Schist is a coarse-grained rock with pronounced
schistosity.
 Gneiss is a high grade, coarse grained rock with
layers of micaceous minerals segregated from layers
of minerals such as quartz and feldspar.

Metamorphism of Basalt
 Greenschist has pronounced foliation like
phyllite, but also a very distinctive green color
because of its chlorite content.
 Amphibolite and granulite.
 When greenschist is subjected to intermediate-grade
metamorphism, amphibole replaces the chlorite.
Foliation is present in amphibolites, but is not
pronounced because micas and chlorites are usually
absent.
 At the highest grade of metamorphism, amphibole is
replaced by pyroxene and an indistinctly foliated rock
called a granulite develops.

Metamorphism of Limestone
 Marble is the metamorphic derivative of
limestone.
 Coarsely crystalline.
 Pure marble is snow white.
 Pure grains of calcite.
 Many marbles contain impurities that result in
various colors.

Metamorphism of Sandstone
 Quartzite is the metamorphic derivative of
quartz.
 It is derived from quartz sandstone by filling of
the spaces between the original grains with silica
and by recrystallization of the entire mass.

Types of Metamorphism (1)
There are four types of metamorphism:
 Cataclastic metamorphism
 Dominated by mechanical deformation.
 Contact metamorphism
 Dominated by recrystallization due to contact with
magma.

Types of Metamorphism (2)
 Burial metamorphism
 Dominated by recrystallization aided by water.
 Regional metamorphism
 Both mechanical deformation and chemical
recrystallization.

Cataclastic Metamorphism
 Mechanical deformation of a rock can occur with
only minor chemical recrystallization.
 Usually localized and seen in igneous rocks when
a coarse-grained granite undergoes intense
differential stress.
 Grain and rock fragments become elongated and
a foliation develops.

Contact Metamorphism (1)
 Occurs when bodies of hot magma intrude into
cool rocks of the crust.
 Vapors given off by the intruding magma play a
role.
 Mechanical deformation is minor or absent.

Contact Metamorphism (2)
 Rock adjacent to the intrusion becomes heated,
developing a metamorphic aureole.
 Hornfels.
 Aureoles reach more than 100 m in thickness.
 Metamorphism that involves a lot of fluid and a
large change in rock’s composition is called
metasomatism.

Burial Metamorphism
 When buried deeply in a sedimentary basin,
sediments may attain temperatures of a few
hundred degrees Celsius, causing burial
metamorphism.
 Zeolites are group of minerals with fully
polymerized silicate structures containing the
same chemical elements as feldspars, plus water.
 As temperatures and pressures increase, burial
metamorphism grades into regional
metamorphism.

Regional Metamorphism—A
Consequence of Plate Tectonics
 Regional metamorphism results from tectonic
forces that build mountains.
 It results from pronounced differential stresses
and extensive mechanical deformation in
addition to chemical recrystallization.
 Regional metamorphism produces greenschists
and amphibolites.

Metamorphic Facies
 Mineral assemblages caused by specific sets of
temperature/pressure conditions:
 Granulite facies, - hornfels facies
 Amphibolite facies, - zeolite facies
 Epidote-amphibolite facies,
 Greenschist facies,
 Blueschist facies,
 Eclogite facies

Metasomatism
 Metasomatism is the process in which rock
compositions are distinctively altered through
exchange with ions in solution.
 Metasomatic fluids may carry valuable metals
and form mineral deposits.

Plate Tectonics And Metamorphism (1)
 There are five geologic settings where plate
tectonics encourages metamorphism:
 Burial metamorphism.
 Subduction (blueschist and eclogite metamorphism).
 Regional metamorphism.
 Zone where wet fractional melting starts.
 Contact metamorphism.

Burial metamorphism occurs today in the sediment
accumulated in ocean-floor trenches, such as those off
the coasts of Peru and Chile.
 When oceanic crust with a covering of sedimentary
rocks is dragged down by a rapidly subducting
plate, pressure increases faster than temperature,
subjecting the rock to high pressure but relatively low
temperature.
 This is observed today along the subduction margin of the
Pacific Plate where it plunges under the coast of Alaska and
the Aleutian Islands.

Regional metamorphism: where continental crust is
thickened by plate convergence and heated by rising
magma, greenschist and amphibolite facies
metamorphic condition occur.
 Examples include the Appalachians, Alps, Himalayas, and
Andes.
 If the crust is sufficiently thick, when 10 percent or
more of the crust has melted the magma so formed
will rise forming stock or batholith.
 As the granitic magma formed by wet partial melting
rises, it heats and metamorphoses the rocks with
which it comes in contact.

Metamorphic Rocks/EPCC/LM6

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