By Sir Rahim-Ud-Din

1. Metamorphic Rocks

2. Metamorphic Rocks
Metamorphic rocks are produced from preexisting igneous,
sedimentary, or from other metamorphic rocks. Every metamorphic
rock has a parent rock – the rock from which it was formed.
Metamorphism, which means to “change form,” is a process that
leads to changes in the mineralogy, texture, and sometimes the
chemical composition of rocks. Factors that might cause a rock to
alter from one form to another includes changes in temperature,
pressure (stress), and the introduction to chemically active fluids.
Metamorphism often progresses incrementally, from slight changes
(low-grade metamorphism) to substantial changes (high-grade
metamorphism). An example of low-grade would be shale turning
into slate when put under pressure (stress). In high-grade changes,
slight melting may occur, as well as folds or obliteration of fossils.

3. Examples of
Metamorphic Rocks
Gneiss
Schist
Slate
Marble
Quartzite
Phyllite

4. What Drives Metamorphism?
The agents of metamorphism include heat, pressure (stress), and
chemically active fluids. During metamorphism, rocks may be
subjected to all three metamorphic agents simultaneously.
Heat is the most important agent of metamorphism because it
provides the energy to drive chemical reactions that result in the
recrystallization of existing materials and/or the formation of new
materials.
Earth’s internal
heat comes
mainly from
radioactive
decay within the
Earth’s interior.

5. What Drives Metamorphism?
Pressure, like temperature, increases as you get deeper into the
Earth. Buried rocks are subjected to this pressure, called confining
pressure, which causes the spaces between mineral grains to close,
producing a more compact rock having a greater density. This
pressure may ultimately cause minerals to recrystallize into new
minerals that a display a more compact form.
Unlike confining pressure, which
“squeezes” rock equally in all
directions (and does not fold or
deform them), differential
stress, where forces pushing on
the rocks are unequal, can create
folds and deformation. This is
especially prominent at
convergent plate boundaries.

6. Metamorphic Rock Textures
Texture is used to describe the size,
shape, and arrangement of grains
within a rock.
Most igneous and sedimentary rocks
consist of mineral grains that have a
random orientation.
By contrast, deformed metamorphic
rocks that contain platy minerals
(micas) and/or elongated minerals
(amphiboles) typically display some
kind of preferred orientation in which
the mineral grains exhibit a parallel or
specific alignment.
This preferred orientation of a rock’s
minerals is called a foliated texture.
Mica Schist
Gedrite

7. Examples of Foliated Textures
Various types of foliation exist, depending largely upon the grade
of metamorphism and the mineralogy of the parent rock. We’ll look
at three main types of foliation: rock or slaty cleavage; schistosity;
and gneissic texture.
Slaty Cleavage: This type
of foliated texture describes
a rock’s tendency to break
or cleave along a specific
crystal plane. Slate is a rock
with excellent rock
cleavage, as it breaks in flat
slabs.

8. Examples of Foliated Textures
Schistosity: This type of
foliated texture describes a
rock created with large,
platy minerals (such as
mica and chlorite) that
have grown large enough to
be seen by the unaided eye.
In addition to platy
minerals, schist often
contains deformed quartz
and feldspar grains that
appear as flat, or lens-
shaped, grains hidden
among the mica grains.

9. Examples of Foliated Textures
Gneissic Texture: During
high-grade metamorphism,
ion migrations can result in
the segregation of minerals.
Although foliated, gneisses
will not usually split as
easily as slates and schists.
Gneisses that do cleave
tend to break parallel to
their foliation and expose
mica-rich surfaces that
resemble schist.

10. Foliated Metamorphic Rocks
Gneiss
Schist
Slate
Phyllite

11. Other Metamorphic Textures
Metamorphic rocks that do not exhibit a foliated texture are referred
to as nonfoliated. Nonfoliated textures usually form in
environments where parent rocks are composed of minerals that
exhibit equidimensional crystals, such as quartz or calcite.
Another texture common to
metamorphic rocks consists of
particularly large grains, called
porphyroblasts, that are
surrounded by a fine-grained
matrix of other minerals.
Porphyroblastic textures develop
in a wide range of environments
and result in very large
specimens of certain minerals,
such as garnets.

12. Nonfoliated Metamorphic Rocks
Marble Quartzite

14. The exterior of the Taj Mahal is constructed mainly of the
metamorphic rock marble.

15. Metamorphic Environments
There are a number of environments in which metamorphism
occurs. Most are in the vicinity of plate margins, and many are
associated with igneous activity.
Contact or thermal
metamorphism
occurs when rocks
immediately
surrounding a molten
igneous body are
“baked” and therefore
altered from their
original state.

16. Hydrothermal metamorphism occurs when hot fluids circulate
through fissures and cracks that develop in rock. This hot fluid
chemically alters rocks and is closely related to igneous activity.
Metamorphic Environments

17. Metamorphic Environments
Regional metamorphism occurs where rocks are squeezed
between two converging lithospheric plates during mountain
building.

18. The typical transition in mineralology that results from progressive
metamorphism of shale.