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.

Diorite (igneous)

Marble (metamorphic)

Limestone (sedimentary)

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

1. Foliation2. Bent/Distorted Layers

Key Identifying Features of Metamorphic Rocks

3. Key Identifier Minerals:– Garnet - Dark Red Color– Mica – Shiny, flaky mineral

Key Identifying Features of Metamorphic Rocks

Original Rock Add HEAT & PRESSURE

Metamorphic Rock

Shale Slate

Slate Phyllite

Phyllite Schist

Schist Gneiss

Quartz Sandstone

Quartzite

Limestone Marble

FOLIATED

NON

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

• 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.

Vocabulary