Igneous Rock Pumice

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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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.

Some types of pumice are less dense than water, so they float
Photo by Marli Bryant Miller

About Vesicles in Igneous Rocks

The bubbles in a bottle of carbonated beverage do not exist until someone removes the sealed cap, thus releasing the external pressure on the liquid. Likewise, when magma erupts from beneath the Earth's surface, the sudden release of pressure causes dissolved gases to form bubbles within the molten rock. Geologists call these bubbles "vesicles." As the bubbles grow larger, due to decreasing pressure and more gas coming out of the magma, the magma becomes less dense, so it may rise faster and spew out of the ground. Rapid cooling of the lava may solidify it into a porous, cavity-filled rock before all the bubbles have a chance to reach the surface of the liquid and burst. Pumice and scoria are two types of rock rich in vesicles. In some cases, heated groundwater may deposit other minerals such as calcite into empty vesicles after the erupted magma has solidified, so the resulting rock may resemble a cookie filled with nuts. Scientists call this texture "amygdaloidal."

10-mm graded crushed basalt rock or aggregate for use in concrete
Photo by Bill Bradley

Building Materials That Come from Rocks

Many building materials - concrete, tiles, brick, glass, paint, plaster, and drywall - contain rocks or components derived from minerals. Quarries, or open-pit mines, produce crushed rocks, gravel, and sand of different grain sizes, known as aggregates. Coarsely crushed rocks and gravel are mixed with cement, a binding material that holds the aggregate in place to form concrete. The ancient Romans invented concrete, but after their empire fell, concrete technology was forgotten until the 18th century. Limestone, a sedimentary rock, and gypsum, a mineral in sedimentary deposits, are two key ingredients of cement. Sand and smaller particles of crushed rock go into making bricks. Finely ground gypsum is filler in paint, plaster, and drywall. While different types of glass used in buildings may have specialized ingredients and coatings, they are all mostly silica, or melted quartz sand.