Igneous Rock Unakite

Associated Smithsonian Expert: Benjamin Andrews, Ph.D.

Geologist Ben Andrews on top of volcano Sant Maria, in Guatemala, looking down on Santiaguito.

Photographed by unknown source, Smithsonian Institution

Dr. Benjamin Andrews is a research geologist at the Smithsonian National Museum of Natural History who specializes in the study of volcanoes around the world. While growing up in Portland, Oregon, he often went hiking and backpacking in the nearby Cascade Range, home to Mount St. Helens and other volcanoes, and the Columbia River Gorge, lined with basalt. Prior to his senior year of high school, Andrews took a six-week geology field course with the Oregon Museum of Science and Industry; an experience that convinced him to make the study of volcanoes his career. After earning his doctorate from the University of Texas in 2009, he worked as a postdoctoral fellow at the University of California at Berkeley before joining the Smithsonian in 2011. In 2012 Andrews and researchers from Italy, Germany, and the United States traveled to Guatemala to study ongoing changes to the active lava dome of Santa Maria, an erupting volcano. At the Smithsonian, he runs experiments that simulate pyroclastic density currents of materials spewing from volcano vents, and he also is doing ongoing research on volcanoes in California and the Kamchatka Peninsula of Russia. Andrews and several of his colleagues participate in the Smithsonian’s Global Volcanism Program, which tracks the activity of volcanoes worldwide.

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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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.

Block of marble from Maine, USA
Photo by Smithsonian Institution, National Museum of Natural History, Department of Mineral Sciences

Mafic and Felsic Igneous Rocks

Igneous rocks may be classified by their overall color, which is a clue to their chemical and mineral composition. Geologists call most dark-colored igneous rocks, such as gabbro and basalt, mafic (pronounced "MAY-fic") rocks. In the word "mafic," the first syllable represents the element "magnesium" and the F stands for "ferric," an adjective describing the element iron. Mafic rocks have a relatively high content of iron, magnesium, and calcium and less silicon dioxide than other types of igneous rocks. They also begin to solidify at higher temperatures, usually above 1,000 degrees C (1,800 degrees F). By contrast, felsic rocks, such as granite and rhyolite, are lighter in color overall, though they may contain dark grains. "Felsic" is a word coined from the minerals "feldspar" and "silica" (quartz); felsic rocks contain 70 to 75 percent silica and do not start to crystallize until magma, or molten rock, cools to around 700 degrees C (1,300 degrees F). In other words, in a body of magma that is cooling down, the mafic minerals will tend to form before most of the felsic minerals.