Metamorphic Rocks
Metamorphic Rocks
Metamorphism:
The transition of one rock into another in the solid
state under conditions unlike those under which it
formed
Metamorphic rocks are produced from:
Igneous rocks
Sedimentary rocks
Other metamorphic rocks
Agents of Metamorphism

Heat- The most important agent
Recrystallization results in new, stable
minerals
 Two sources of heat



Contact metamorphism – heat from magma
Regional Metamorphism - increase in
temperature with depth due to the
geothermal gradient
Agents of Metamorphism

Pressure
Increases with depth
 Confining pressure applies forces equally in
all directions
 Rocks may also be subjected to differential
stress which is unequal in different directions
and causes deformation

Agents of Metamorphism

Pressure
Types of Metamorphism
Contact
metamorphism
From Understanding
Earth, Press and Siever.
Freeman.
Types of Metamorphism
Regional metamorphism
Types of Metamorphism
Regional metamorphism
More
Uplift
Types of Metamorphism
Regional metamorphism
From Understanding
Earth, Press and Siever.
Freeman.
Metamorphic Textures
Foliation - any planar arrangement of features
within a rock
Foliation can form in various ways:
 Rotation of platy and/or elongated minerals
 Recrystallization of minerals in the direction
of preferred orientation
 Changing the shape of equidimensional
grains into elongated shapes that are aligned
Metamorphic Textures
Foliation can form in various ways:
Metamorphic Textures

Foliated textures
 Slaty cleavage

Closely spaced planar surfaces along which
rocks split
Metamorphic Textures

Foliated textures
 Schistosity


Platy minerals are discernible with the
unaided eye and exhibit a planar or layered
structure
Rocks having this texture are referred to as
schist
Metamorphic Textures

Foliated textures
 Gneissosity


During higher grades of metamorphism, ion
migration results in the segregation of
minerals into layers
Gneissic rocks exhibit a distinctive banded
appearance
Metamorphic Textures
Metamorphic rocks that lack foliation are
referred to as non-foliated



Develop in environments where
deformation is minimal
And/or composed of minerals that
exhibit equidimensional crystals
General name is granofels
Metamorphic Rocks
Foliated Metamorphic Rocks
Slate: compact, very finegrained, metamorphic rock
with a well-developed
cleavage. Freshly cleaved
surfaces are dull
Phyllite: a rock with a
schistosity in which very fine
phyllosilicates
(sericite/phengite and/or
chlorite), although rarely
coarse enough to see unaided,
impart a silky sheen to the
foliation surface. Phyllites
with both a foliation and
lineation are very common.
a
b
Metamorphic Rocks
Foliated Metamorphic Rocks
Schist: a metamorphic rock
exhibiting a schistosity. By
this definition schist is a
broad term, and slates and
phyllites are also types of
schists. In common usage,
schists are restricted to those
metamorphic rocks in which
the foliated minerals are
coarse enough to see easily in
hand specimen.
Metamorphic Rocks
Foliated
Metamorphic
Rocks
Gneiss: a metamorphic rock
displaying gneissose
structure. Gneisses are
typically layered (also called
banded), generally with
alternating felsic and darker
mineral layers. Gneisses may
also be lineated, but must
also show segregations of
felsic-mineral-rich and darkmineral-rich concentrations.
Metamorphic Rocks
Specific Metamorphic Rock Types
Marble: a metamorphic rock composed
predominantly of calcite or dolomite. The protolith
is typically limestone or dolostone.
Metamorphic Rocks
Specific Metamorphic Rock Types
Quartzite: a metamorphic rock composed
predominantly of quartz. The protolith is typically
sandstone.
Metamorphic Rocks
Greenschist/Greenstone: a low-grade metamorphic
rock that typically contains chlorite, actinolite,
epidote, and plagioclase. Such a rock is called
greenschist if foliated, and greenstone if not. The
parent is either a mafic igneous rock or graywacke.
Metamorphic Rocks
Specific Metamorphic Rock Types
Amphibolite: a metamorphic rock dominated by
hornblende + plagioclase. Amphibolites may be
foliated or non-foliated. The parent is either a
mafic igneous rock or graywacke.