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


Metamorphic Rocks


Metamorphic Rocks Formation


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





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






Covers large areas typically associated with
mountains

Deep burial and pressure from mountain
formation.
Heat and Pressure change the rock


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.


Scheme for Metamorphic Rock Identification

MINERAL
ALIGNMENT
BANDING

FOLIATED

Texture

Composition

Type of
Metamorphism

Comment

Rock Name

Regional
Mica

Low-Grade
metamorphism of
SHALE

Slate

Mica, Quartz,
Feldspar,
Amphiboles, Garnet

Foliation surfaces shiny
from microscopic mica
crystals

Phyllite

Mica, Quartz,
Feldspar,
Amphiboles, Garnet,
Pyroxene
Mica, Quartz,
Feldspar,

Amphiboles, Garnet,
Pyroxene

(Heat and
Pressure
increase w/
depth)

Platy mica crystals
visible
Compact, may split
easily

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

Rock Name

Various rocks changed
by nearby magma/lava

Hornfels

Metamorphism of
Quartz Sandstone
Metamorphism of

Limestone or
Dolostone

Pebbles may be
distorted or
stretched

Quartzite
Marble
Metaconglomerate




ESRT p. 7



Animation: Metamorphic Rock Scheme

Where are metamorphic rocks found?


At the interface between igneous and
sedimentary rocks

• Mountainous regions


THE ROCK CYCLE
How are rocks classified?


How are rocks classified?


By their origin (the way they are formed)


Major Rock Groups






Sedimentary
 Formed at the Earth’s surface
 Clastic (Mineral Fragments or grains, clays)
 Crystalline (chemical precipitates and evaporites)

 Bioclastic (formed organically from shells or
plants)
Igneous
 Formed from magma (molten rock)
 Plutonic (intrusive):slow cooling, coarse/very
coarse crystals
 Volcanic (extrusion): quick cooling at the surface,
glassy/fine crystals
Metamorphic
 Changed by heat and pressure


Rock Cycle Conclusion


Fig. 2.9

MAGMA


IGNEOUS

Solidification

MAGMA


IGNEOUS
Plutonic
Solidification

MAGMA


Volcanic
IGNEOUS
Plutonic
Solidification

MAGMA