Metamorphic Rock Mica Schist

Associated Smithsonian Expert: Leslie Hale, B.S.

Leslie Hale at the Monterey Bay Aquarium (California, USA) during a conference of museum collections managers

Photo by Smithsonian Institution, National Museum of Natural History, Department of Mineral Sciences

Leslie Hale, the Smithsonian’s rock and ore collections manager at the Smithsonian National Museum of Natural History, tells people that she is a “rock librarian.” While growing up in Bowie, Md., not far from the Smithsonian, Hale collected rocks and took a summer class on lapidary art (making jewelry out of stone). Her career choice was greatly influenced by her attendance at a magnet high school for science and mathematics and taking geology as a senior-year elective. Hale joined the Smithsonian staff shortly after finishing her bachelor’s degree at the University of Maryland in 1989. Today, she supervises two full-time staff members as well as several volunteers, interns, and contractors. She assists Smithsonian scientists and visiting researchers who want to use the museum’s extensive collection of rock and ore specimens. Hale also sends out rocks on loan to geologists at distant universities; she keeps track of the objects’ whereabouts and sends out requests for return or loan renewal. Finally, she gives tours of the Smithsonian’s geology research facilities, conducts inventories, answers questions from the public, and identifies rock specimens.

Meet our associated expert

This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.

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Shear zone in gneiss, Grand Teton National Park, Wyoming, USA
Courtesy of Marli Bryant Miller

About Regional Metamorphism

Regional metamorphism takes place over large areas of Earth's surface where tectonic plates crash together, pushing up mountain ranges. The metamorphic processes typically happen between temperatures of 350-650 degrees C (660-1,200 degrees F) and at depths of 5-20 km (3-12 miles). High pressures from these enormous collisions cause the rocks to recrystallize and the new mineral grains align into an arrangement like a layer cake, or a sponge when it is squeezed; that texture is called foliation. Low-grade metamorphic rocks, like slate, split easily into sheets. High-grade metamorphic rocks, like gneiss, may be foliated but do not break into sheets.

Gneiss outcrop, San Bernardino Mountains, California, USA
Photo by J.C. Matti, U.S. Geological Survey, Public Domain

About Foliation in Metamorphic Rocks

Thanks to high pressures in a particular direction, the grains in some metamorphic rocks become aligned parallel to each other, giving the rock a layered appearance. Scientists call this property foliation, from the Latin word folium, which means "leaf." In slate, which is a fine-grained metamorphic rock, the foliation occurs at a small scale due to the alignment of tiny flakes of mica and other silicate materials. Geologists say slate has good cleavage because it tends to break into parallel thin sheets. In coarse-grained metamorphic rocks such as gneiss (pronounced "nice"), the foliation may occur as alternating light- and dark-colored bands. These layers may also appear wavy or curved because the forces that changed the rock acted unevenly on the rock. Rocks that metamorphose due to high temperature rather than high pressure may not show any foliation.