Igneous Rock Granite

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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Cripple Creek granite at Florissant Fossil Beds National Monument in Colorado
Photographed by Joseph Hall, U.S. National Park Service, Public Domain

About Intrusive Igneous Rocks

Molten rock, or magma, does not always reach Earth's surface. It may flow upward through cracks that end below the surface, where it gets trapped and cools slowly. Some intrusions, called plutons, are several kilometers or miles wide. During the slow cooling process, the magma freezes into crystals. Magma containing higher iron, magnesium, and calcium levels is the first to turn solid and forms dark, coarse-grained rocks such as gabbro. The lighter-colored granite comes from magma with high levels of silica (silicon dioxide) and relatively little iron and magnesium. Humans cannot witness the formation of intrusive igneous rock in the same way we see volcano eruptions. However, over millions of years, the crustal rocks above some intrusions wear away, leaving the solidified magma exposed to the environment in places such as the Sierra Nevada Mountains and Yosemite National Park in California.

A variety of tektite forms from different locations
Photo by Linda Welzenbach, Smithsonian Institution

About Shocked Rocks and Tektites

A collision between a celestial body and the Earth creates sudden, enormous pressures, far beyond those generated by volcanoes and plate tectonic processes. The violent blow from the impact changes the crystal structure of the quartz and feldspar found in normal, unshocked rocks. A microscope reveals tiny, distinctive parallel cracks in the silicate minerals; these cracks are called planar deformation features. Quartz may also transform into its high-pressure forms, such as suevite and coesite. The "shock metamorphism" may also create diamonds out of carbon. Huge impacts may throw off bits of melted rock debris, which quickly cool with a glassy texture and a round or elongated shape. Scientists call these glassy stones tektites and they provide evidence of long-ago impact events. Some evidence of shocked silicate rocks has also been found at atomic bomb testing sites.