Metamorphic rocks

Metamorphic Rocks:
• Meta = change
Morph = form
• Metamorphic rocks form from changes to preexisting rock caused by heat and pressure
• Metamorphism can occur locally over a small area,
or over a large area covering many kilometers



Two Types of Metamorphism...
• Regional Metamorphism - heat and pressure are
significant over a wide area (many kilometers)
• Contact Metamorphism - occurs over a small area
when pre-existing rocks come into contact with hot
liquid magma from an igneous intrusion.


Metamorphic Rocks...
• Classified on the basis of type of metamorphism
(Regional or Contact) and texture:
– Foliated (occurs w/regional metamorphism)
– Non-foliated (usually occurs w/ contact metamorphism)



Regional Metamorphism
• Changes to rock over a large area
• Layers of rock deep in the Earth’s crust experience the
following:
– 1. High temperatures (depth and friction)
– 2. High pressures (weight and crustal movement)

– 3. Exposure to hot liquids, water, and gases
• Results:
– Mineral grains are compressed
– minerals “migrate” through rock and begin to separate
– minerals are aligned within rock in response to
pressures to produce foliation


Regional Metamorphism...
• Foliation - the alignment of minerals within a
rock caused by heat and pressure

This sample is
clearly foliated.
see the mineral
alignment???


Draw the foliation that results from regional
metamorphism
BEFORE

AFTER


Metamorphic Grade (degree of metamorphism)
Sedimentary
Rock

Low-Grade

Metamorphic
Rock

MediumGrade
Metamorphic
Rock

MediumGrade
Metamorphic
Rock

HIGH-Grade
Metamorphic
Rock

SHALE

SLATE

PHYLLITE

SCHIST

GNEISS

Clay minerals,
stratified, soft, and
fragile, dull in
appearance

Harder,
microscopic mica
minerals begin to
form, less air space;
dull shine

Micas still
Visible mica grains;
microscopic, but
foliation obvious;
begin to reflect
other minerals may
more light; “silky”
be seen.
sheen in appearance

Clearly “banded”
appearance as
minerals separate
into alternating
bands


This nice piece of
schist has large,
visible garnet crystals
in it.

This sample of schist
shows mica minerals

that have grown
large enough to
reflect light.


Here is another gneiss which shows distinct banding due to
mineral alignment. The arrows indicate how this sample
was squeezed by pressure at great depth below ground.


Contact Metamorphism
• Occurs over as small area around magma (igneous
bodies)
– country rock (pre-existing, in-place) is baked by the hot
temperatures associated with magma
– hot liquids and gases mix with country rock

• Results:
– minerals migrate and fuse together
– no foliation or alignment since pressure is not significant


Contact metamorphism
has occurred around this
igneous intrusion.


Draw layers of rock getting metamorphosed by
coming into contact with magma

Metamorphism – Parent Rock Definition
• Parent Rock - the pre-existing rock from which a
metamorphic rock forms


Metamorphism
Sedimentary
Material

Sedimentary
Rock

Metamorphic
Rock

Pebbles, gravel, sand

Conglomerate

Quartzite, Gneiss

Sand Grains (usually
quartz)

Sandstone

Quartzite

Clay (usually kaolin),

Silt

Shale, Mudstone

Slate, Phyllite,
Hornfels, Schist

Lime (shells,
fragments, grains)

Limestone

Marble


Metamorphic Rock Quiz...
• Metamorphic Rocks form from changes to other
rocks in response to pressure and heat

• TRUE


Metamorphic Rock Quiz...
• Metamorphic rocks are classified by their mineral
composition primarily

• FALSE. Metamorphic rocks are classified on the
basis of texture and type of metamorphism

Metamorphic Rock Quiz...
• Foliation in metamorphic rocks is the alignment of
minerals in response to high temperatures along
with great pressure.

• TRUE!


Metamorphic Rock Quiz...
• Only contact metamorphic rocks experience
foliation.

• FALSE. Regional metamorphic rocks become
foliated due to high pressure in addition to high
temperatures


Metamorphic Rock Quiz...
• Shale is a low-grade metamorphic rock

• FALSE. Shale is a sedimentary rock.


Metamorphic Rock Quiz...
• Middle to high grade metamorphic rocks appear
shiny because as mica minerals crystallize they
begin to reflect light.

• TRUE!

Metamorphic Rock Quiz...
• Extremely high grade metamorphic rocks
experience more alignment of minerals into distinct
bands.

• TRUE.


Metamorphic Rock Quiz...
• Rocks that come into contact with an igneous
intrusion will only change into igneous rocks.

• FALSE. The high heat near an igneous intrusion
will “burn” the surrounding rock and change it into
metamorphic rock.


Metamorphic Rock Quiz...
• The pre-existing rock that was changed into a
metamorphic rock is called the “Grandma” rock.

• FALSE. The original rock is called the parent rock.