Igneous Rock Pahoehoe

Associated Smithsonian Expert: Richard Wunderman, Ph.D.

Dr. Richard Wunderman (right) with his graduate school adviser, Professor Bill Rose, at Michigan Technological University in 2013

Photo by Fraser Goff, Geologist

Dr. Richard Wunderman is a volcanologist in the Smithsonian?s Global Volcanism Program (GVP) at the Smithsonian National Museum of Natural History. Born in New York City, Wunderman grew up in Mountain View, California, part of the region dubbed ?Silicon Valley.? He admits he was not a stellar student when he was young, and his mother encouraged him to find a career that would allow him to spend time outdoors. He and his family traveled extensively. ?We had adventures in every sense of the word,? he says. In community college he was inspired by his geology, paleontology, and chemistry professors, and he later studied geology at the University of California at Berkeley. In graduate school at Michigan Technological University, he wrote his master?s thesis about a volcano in Guatemala and eventually earned his doctorate in 1988. Wunderman came to the Smithsonian in 1993, after working as a physics instructor at Front Range Community College in Colorado. As managing editor of the Bulletin of the Global Volcanism Network, he writes about active volcanoes and edits incoming reports from scientific colleagues all around the world.

Meet our associated expert

This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.

Make Field Book Cover

Image of Igneous Rock Pahoehoe

Create your own field book and fill it with images and object from Q?rius! When you create a field book, you can put this image on its cover.

or Sign up


Contact between igneous diabase (left) and metamorphic hornfels (right), which used to be sedimentary siltstone. Location: Manassas Quarry, Manassas, VA.
Photographed by Donald E. Hurlbert, Smithsonian Institution

About Extrusive Igneous Rocks

When molten rock (magma) reaches Earth's surface, it solidifies or hardens. Scientists call the resulting solid rocks "extrusive" igneous rocks. Extrusion is the process of pushing material out to the surface of the Earth's crust. At some volcanoes, the extrusive rock flows as lava across the ground before it hardens; the ripples in the lava may freeze in place. Hot, rapidly expanding gases within other volcanoes' vents can force the magma out explosively, forming pumice: low-density rock full of vesicles, or frozen bubbles. Extrusive igneous rocks are easy to find near many volcanoes, such as Mount St. Helens in Washington state. Hawai`i Volcanoes National Park, home of two active volcanoes, contains lava flows that cooled only a few decades, or minutes, ago.

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.