Metamorphic Rock Gneiss

Associated Smithsonian Expert: Michael A. Wise, Ph.D.

Michael Wise investigates granite and pegmatite dikes that intruded into metasedimentary rocks near Mount Antero, Colorado.

Courtesy of Jennifer C. Kelly

Dr. Michael Wise is a geologist and an expert on pegmatites, which are coarse-grained igneous rocks rich in the minerals quartz and feldspar. A native of Smithfield in southeastern Virginia, Dr. Wise first got interested in mineralogy as an undergraduate at the University of Virginia, when rock-hunting trips to central Virginia and North Carolina strengthened his interest in this type of rock. He obtained his doctorate from the University of Manitoba in Canada in 1987 and joined the Smithsonian National Museum of Natural History Department of Mineral Sciences one year later. He has traveled widely in the Appalachian mountains of the eastern United States to study pegmatites, which are good sources of not only semiprecious gemstones such as aquamarine, rose quartz, and topaz, but also rare elements, such as beryllium, lithium, and cesium, with important economic applications. His research has also taken him to Nevada, Colorado, California, and the Northwest Territories of Canada. In the laboratory, he uses a scanning electron microscope, cathodoluminescence microscope, X-ray diffractometer, and other tools to investigate the composition, crystal structure, and evolution of pegmatites.

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

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Deformed conglomerate rock, Death Valley, California, USA
Courtesy of Marli Bryant Miller

How Metamorphic Rocks Are Formed

High pressures and temperatures within the Earth's crust can change rocks from one type to another. Scientists call this process metamorphism, which comes from the Greek words for "after" and "form." At the junctions where the Earth's crustal plates collide, pushing up mountain ranges, or where one plate slides underneath another, the original rocks, or protoliths (from the Greek words meaning "first" and "rock") undergo reactions that change the chemical or crystal structure of the rocks with little or no actual melting. The grains of minerals within many metamorphic rocks are aligned in parallel due to the forces pushing on them. The pressure-temperature combination causes some sedimentary rocks to lose water molecules or become anhydrous.

Detailed geologic time scale.
From the Geologic Society of America, used with permission

About Morrison Quarry

The continental crust in Colorado was constructed from several island chains that collided and merged together about 1.75 billion years ago. It was then metamorphosed at depth under heat and pressure, and magma was injected through cracks in the rock. After several million years of erosion the rocks of the Morrison Quarry became exposed at the surface. Layers of sandstone were deposited directly on top of the exposed rocks forming what is known as the "Great Unconformity," a 1.4-billion-year gap missing between one rock layer and the next. These rocks were likely uplifted during the formation of the Ancestral Rockies between 320 and 270 million years ago. The Ancestral Rockies were eventually eroded away, buried by sediments and flooded by shallow oceans. They were brought to the surface once again during the growth of the current Rocky Mountains.

Touch a cliff face with rocks separated by 1.4 billion years in age, at a quarry in Morrison, CO. Sandstone on the left (south), schist on the right (north).
Photographed by Donald E. Hurlbert, Smithsonian Institution

About the Great Unconformity

Rocks record earth's history throughout time, and typically younger rocks are found above older rocks as sediments are deposited year after year. Occasionally erosion will erase rocks away and along with them a history of when they were formed. The Great Unconformity is a feature observed in Colorado where 300-million-year-old sandstone rests on top of gneiss that is about 1.7 billion years old. What happened to the 1.4 billion years in between? A record of that time was washed away, so no one can say for sure what happened in this area during that time.

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