Metamorphic

Granite schist
Granite schist from City of Rocks National Reserve.

NPS Photo

Metamorphic Rocks
Metamorphic rocks form when high temperatures and pressure act on a rock to alter its physical and chemical properties (metamorphism means 'to change form'). These conditions often stretch, twist and fold the rock as it cools. In metamorphic rocks some or all of the minerals in the original rock are replaced, atom by atom, to form new minerals. Types of metamorphic rocks include gneiss, quartzite, marble, schist, soapstone, and phyllite.
Parks with examples of metamorphic rocks include Rocky Mountain National Park, Shenandoah National Park, and Catoctin Mountain Park.
Key Terms
  • Contact Metamorphism: A type of local, thermal metamorphism caused by the intrusion and extrusion of magmas; takes place in rocks at or near their contact with a body of igneous rock.
  • Parent Rock: Rock from which soil, sediment, or other rock is derived.
  • Regional Metamorphism: A type of metamorphism that affects an extensive region, as opposed to local metamorphism that is effective only in a relatively restricted area.
Foliated gneiss
Foliated metamorphic gneiss.

NPS photo by Tim Connors

Metamorphic Rock Types

Foliated Metamorphic Rocks

As pressure squeezes on a parent rock during recrystallization it causes the platy or elongated minerals within the rock to become aligned, or foliated. Foliated rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied in. Types of foliated metamoprhic rocks include slate, schist, and gneiss. Gneiss found at Antietam National Battlefield, Chesapeake and Ohio Canal National Historical Park, and Harpers Ferry National Historical Park record the ancient beginnings of the Appalachian Mountain belt.

soapstone
Non-foliated metamorphic soapstone.

Photo courtesy of Tina Kuhn

Non-foliated Metamorphic Rocks
Not all parent rocks have platy or elongated minerals and when these rocks undergo metamorphism the individual mineral grains do not align. Types of non-foliated metamorphic rocks include marble, quartzite and hornfels. Soapstone is found in Rock Creek Park and was used by the American Indians for tools and implements; City of Rocks National Reserve is well-known for its extensive quartzite deposits.

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