Igneous Rock Larvikite

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.

Schematic cross-section of rocks that are later intruded by igneous rocks accompanied by volcanic activity
Courtesy of U.S. Geological Survey, Public Domain

About Intrusions in Igneous Rocks

Dikes, sills, and volcanic pipes are all types of intrusions, a word geologists use to describe places where magma has pushed its way upward through the Earth's crust and cooled close to the surface. Dikes are places where the molten rock has solidified in a sheet that cuts through layers of rock bodies. Sills, on the other hand, are intrusions that squeezed in between existing layers of rock. A volcanic pipe forms when a tube-like conduit of magma to a surface volcano solidifies after the volcano's eruption has ended. Because these large-scale intrusions are formed underground, humans must wait a long time, sometimes millions of years, until erosion, mining, or digging removes the surrounding rock, revealing the deep structures.