Metamorphic Rocks
• Metamorphic Rocks: form
when heat and/or pressure
change pre-existing rocks.
– Metamorphism: the
process of forming
metamorphic rocks.
Formation of Metamorphic Rocks
• Recrystallization: the process of increasing the size
of the mineral crystals/clasts AND/OR changing
the mineral composition WITHOUT MELTING.
– HIGH heat and pressure allows atoms to
rearrange
Animation
Types of Metamorphism
1. Contact Metamorphism: caused by contact with
extreme HEAT (magma/lava OR hot fluids) that
will alter the rock.
– No foliation
When extreme heat is added to a Quartz sandstone, you get Quartzite.
When extreme heat is added to Limestone, you get Marble.
Limestone
(sedimentary)
Marble
(metamorphic)
Diorite
(igneous)
Marble
(metamorphic)
Limestone
(sedimentary)
A thick body of diorite was injected between layers of limestone of the
Helena Formation in northern Montana. The great heat of the
intrusion baked the limestone above and below it into white marble.
Types of Metamorphism
2. Regional Metamorphism: occurs over wide
regions when rocks are exposed to extreme heat
AND pressure.
– Foliated rocks
– Folded rocks
– Faulted rocks
Warped,
distorted
layers
Folded Rock
outcrop on I-68,
Maryland
Textures of Metamorphic Rocks
1. Foliated rocks: have layers of mineral crystals
– occurs when pressure aligns minerals into layers.
In SLATY FOLIATION, rocks may split into
flat, thin layers
In SCHISTOSE foliation, minerals have been
squished into flakes of mica
In GNEISSIC foliation, minerals have been
squished into bands of color (“zebra stripes”)
Textures of Metamorphic Rocks
2. Nonfoliated Rocks: not layered
– rocks were not subjected to a directional pressure
and/or
– minerals are not flat
Quartzite
Marble
Key Identifying Features of
Metamorphic Rocks
1. Foliation
2. Bent/Distorted Layers
Key Identifying Features of
Metamorphic Rocks
3. Key Identifier Minerals:
– Garnet - Dark Red Color
– Mica – Shiny, flaky mineral
Original Rock
F
O
L
I
A
T
E
D
N
O
N
Add HEAT &
PRESSURE
Shale
Metamorphic
Rock
Slate
Slate
Phyllite
Phyllite
Schist
Schist
Gneiss
Quartz
Sandstone
Limestone
Quartzite
Marble
Which metamorphic rock is nonfoliated, and
composed of calcite?
Which metamorphic rock is foliated, and
consists of ONLY mica?
Vocabulary
• Recrystallization: changing the size of
mineral crystals or mineral composition
without melting.
• Foliation: Mineral layering or banding
Vocabulary
• Slaty Foliation: Fine-grained and breaks into
thin, smooth layers.
• Schistose Foliation: Medium-grained with
high mica content.
• Gneissic Texture: Coarse-grained with
banded minerals.