Download Metamorphic Rocks

METAMORPHIC ROCKS
Rocks that form from other pre-existing
rock (sedimentary, igneous, or metamorphic) that have
been changed from high temperature
and/or high pressure
Conditions that cause rock to undergo
metamorphism include:
• Heat - Under conditions of high temperature
from magma contacting pre-existing rock.
• Pressure - Deep burial and pressure from
mountain formation.
Types of Metamorphism
1. Contact Metamorphism
2. Regional Metamorphism
Contact Metamorphism
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Deep burial and pressure from mountain
formation.
The main metamorphic agent is heat.
Also forms when pre-existing rock comes
into contact with molten lava or magma.
The heat from the molten material is hot
enough to cause the minerals in the original
rock to re-crystallize, but not melt.
Regional Metamorphism
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Covers large areas typically associated with
mountains
Deep burial and pressure from mountain
formation.
Heat and Pressure change the rock
Regional Metamorphism
Most common
common form
form of
of metamorphism
metamorphism
•• Most
caused by
by large
large scale
scale forces
forces
•• caused
-- lithospheric
lithospheric plate
plate collision
collision
covers very
very large
large areas
areas
•• covers
-- metamorphic
metamorphic belts
belts or
or zones
zones
-- Zones
Zones are
are characterized
characterized by
by
Index Minerals
Minerals
Index
form under
under specific
specific temperatures
temperatures and
and pressures
pressures
>>form
metamorphic facies
facies
>>metamorphic
commonly associated
associated with
with
•• commonly
-- shields: stable areas of crystalline rocks
Shields of the World
Types of Metamorphic Rocks

Foliated: rocks with mineral crystals arranged
in cable-like distorted layers/structures
Mineral Alignment
 Banding
 Animation: Foliation


Nonfoliated: rocks with recrystallized
minerals; no layering

Recrystallization: This is the growth of new
mineral crystals from other rocks.
Foliated Texture
Foliated Textures
Slatey
•• Slatey
-- looks
looks like
like blackboard
blackboard
dull surface
surface
>>dull
-- smooth,
smooth, thin
thin layering
layering
-- breaks
breaks into
into flat
flat slabs
slabs
referred to
to as
as slatey
slatey cleavage
cleavage
>>referred
-- no
no mineral
mineral grains
grains visible
visible
Phyllitic
•• Phyllitic
-- looks
looks like
like waxed
waxed surface
surface
has aa "sheen"
"sheen" to
to itit
>>has
-- may
may have
have little
little "waves"
"waves" on
on
surface
surface
referred to
to as
as
>>referred
crenulations
crenulations
-- some
some small
small grains
grains visible
visible
Schistose
•• Schistose
-- distinct bands of minerals
-- visible
visible grains
mineral grains
garnets, staurolites
staurolites
>>garnets,
-- may
may have
have shiny
shiny
appearance
due to
to mica
mica minerals
minerals
>>due
Gneissic
•• Gneissic
-- larger
larger grains
grains
-- may
may look
look like
like igneous
igneous rock
rock
-- may
may have
have crude
crude banding
banding
intensely distorted
distorted
>>intensely
-- different
different minerals
minerals than
than
schistose
schistose
Scheme for Metamorphic Rock Identification
Texture
Composition
MINERAL
ALIGNMENT
BANDING
FOLIATED
Mica
Mica, Quartz,
Feldspar,
Amphiboles, Garnet
Mica, Quartz,
Feldspar,
Amphiboles, Garnet,
Pyroxene
Mica, Quartz,
Feldspar,
Amphiboles, Garnet,
Pyroxene
Type of
Metamorphism
Comment
Rock Name
Regional
Low-Grade
metamorphism of
SHALE
Slate
(Heat and
Pressure
increase w/
depth)
Foliation surfaces shiny
from microscopic mica
crystals
Platy mica crystals
visible
Compact, may split
easily
Phyllite
Schist
Gneiss
Progression of Metamorphism
Start with a shale and then hit
it with heat and pressure!
More
Heat
&
Pressure
Rock Name
Rock Type
Grade of
Metamorphism
Shale
Sedimentary
-----
Slate
Metamorphic
Low
Phyllite
Metamorphic
Low/Intermediate
Schist
Metamorphic
Intermediate/High
Gneiss
Metamorphic
High
Molten Rock
Cools into
Igneous Rock
-----
Shale
(Sedimentary Rock)
Heat
&
Pressure
Slate
(Metamorphic Rock)
Slate
(Metamorphic Rock)
Heat
&
Pressure
Phyllite
(Metamorphic Rock)
Phyllite
(Metamorphic Rock)
Heat
&
Pressure
Schist
(Metamorphic Rock)
With even more heat & pressure
(High-Grade Metamorphism)
… you end up with something
that is really Gneiss!
Scheme for Metamorphic Rock Identification
Texture
Composition
NONFOLIATED
Variable
Type of
Metamorphism
Contact
(Heat)
Quartz
Regional
Calcite and/or
Dolomite
(Heat
&
Pressure)
Various minerals in
particles and matrix
Comment
Various rocks changed
by nearby magma/lava
Metamorphism of
Quartz Sandstone
Metamorphism of
Limestone or
Dolostone
Rock Name
Hornfels
Quartzite
Marble
Pebbles may be
distorted or stretched Metaconglomerate
Where are metamorphic rocks found?

At the interface between igneous and
sedimentary rocks
• Mountainous regions
Minnesota Metamorphic Rocks
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Some of the oldest rocks in the
world include the gneiss found
in the Minnesota River Valley.
Morton Gneiss is 3.6 billion
years old, is a coarsely
crystalline, foliated
Large crystals = rock cooled
slowly beneath the Earth’s
surface.
Foliation = great heat and
pressure
Morton Gneiss
Minnesota Metamorphic Rocks
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Sediments, fine-grained sand
and mud, were deformed by
the same forces that caused the
uplift of mountains in northern
Minnesota.
Schist formed from sediments,
uplift, heat and pressure.
Schist is composed
predominantly of mica minerals,
which impart a platy or layered
texture to the rock.
Schist is common in central
Minnesota and across northern
Minnesota.
Mica Schist
Minnesota Metamorphic Rocks
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Stream deposits of reddish
quartz sand grains were
consolidated and slightly
altered = quartzite.
The reddish to purple Sioux
Quartzite is found at Blue
Mound State Park and the
Jeffers Petroglyphs in
southwestern Minnesota.
At the Pipestone National
Monument, the soft, red
pipestone the Indians favored
for carving is a thin claystone
layer between thick layers of
quartzite.
Quartzite
Minnesota Metamorphic Rocks