Igneous Rock Bishop Tuff

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.

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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Lava fountains erupt from Krafla volcano in Iceland
Photographed by Michael Ryan, U.S. Geological Survey, Public Domain

How Igneous Rocks Are Formed

Earth's crust, or outermost rocky layer, sits on top of a deeper layer called the mantle, which stores heat from two sources: the formation of the Earth 4.65 billion years ago and the radioactive decay of uranium, thorium, and potassium. When cracks between huge crustal plates open up, the gap causes the underlying mantle to rise up. The upwelling partially melts that region of the mantle; scientists call that decompression melting. The molten rock, or magma, is less dense than solid rock, so it moves upward, the way a cork bobs to the surface of water. As the magma reaches the upper layers of the crust or even Earth's surface, it cools and hardens into a solid known as igneous rock. Scientists categorize igneous rocks according to their chemical composition, the method of their formation, and their degree of crystallization.

Deposits from pyroclastic flows from Oshima volcano in Japan
Photographed by Richard Fiske, Smithsonian Institution, Public Domain


About Explosive Igneous Rocks

Inside some volcanoes, gases such as water vapor, carbon dioxide, and sulfur dioxide are dissolved within the melted rock (magma), lowering the density of the magma and increasing its buoyancy. If the magma contains a large fraction of trapped gases when it reaches the open air at the mouth of the volcano, the rapid decompression fragments the magma into pumice and ash (pieces of foamed-up magma) that explode from the volcano and rapidly harden in the air. (Think of the spray from bottles of soda or champagne if you shake them hard and open their tops.) Sometimes explosive eruptions release dangerous currents of hot gas, ash, and rock down the sides of volcanoes. These fast-moving currents, called pyroclastic flows, killed 16,000 people in the Roman towns of Pompeii and Herculaneum when Mount Vesuvius erupted in 79 C.E.