Igneous Rock Porphyritic Basalt

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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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.

Granite from Livermore Falls, New Hampshire, USA
Photo by Smithsonian Institution, National Museum of Natural History, Department of Mineral Sciences

About Crystallization in Igneous Rocks

You can tell a lot about the history of igneous rocks by looking at the size of crystals within them. Rocks that cool quickly contain small crystals, while slow-cooled rocks are filled with large crystals. When magma erupts at the Earth's surface, heat radiates out from the lava allowing it to cool rapidly and the atoms and molecules do not have time to grow into large crystals before the lava solidifies. The resulting rock has such small crystal grains that humans have a difficult time distinguishing them, even with a handheld lens. Geologists describe the texture of these fine-grained igneous rocks as "aphanitic," from the Greek word meaning "unseen." Deep inside the Earth's crust, the magma cools much more slowly because the surrounding rocks insulate the magma from rapid heat loss. This allows the crystals to grow into mineral grains that are easier for humans to see. Geologists describe the resulting coarse-grained rock texture as phaneritic, from the Greek word meaning "visible." Some igneous rocks contain crystals that are much larger than the crystals in the matrix surrounding them. Scientists call these specimens, which resemble a chocolate-chip cookie, porphyritic rocks, and the larger crystals are called phenocrysts. The phenocrysts had started to form within the magma before it later cooled rapidly, probably due to that magma erupting at a volcano.