Life on the rocks

"Most people think of rocks as immutable," Susan Brantley told the noontime crowd of Research Unplugged attendees. But geologists know that rocks break down and dissolve. That is, they weather over geologic time scales.

Professor of geosciences and director of Penn State's Earth and Environmental Systems Institute, Brantley described both the geologic cycle and the interactions of rocks and living organisms at the March 23 conversation, held in the Penn State Downtown Theatre.

Emily Wiley

Geoscientist Susan Brantley describes life on the rocks.

Brantley gave several examples of the importance of the rock weathering process. For instance, the breakdown of rock forms soil, "which we need to grow food since we can't grow crops directly on rocks," she noted. Understanding how rocks dissolve in water is particularly important for our region because limestone—the bedrock of Centre County—is very soluble, she explained. Water dissolves or physically removes material from fractures in the limestone, forming the caves and sinkholes common in our region.

Brantley does not work on limestone problems; rather, she works on "rocks that people buy for countertops" —granites and basalts—to see how water changes them over geologic time. These rocks dissolve at a rate of approximately one meter per million years.

Contrary to their image as barren environments, rocks are host to many forms of life, Brantley explained. Every natural surface that touches water, including rocks, contains single-celled organisms like bacteria. Brantley's work includes growing bacteria on rock and mineral surfaces to observe the interactions. Geologists and biologists now know that bacteria can "eat" rocks to obtain iron, phosphorous, and other nutrients they need to survive. Sometimes the bacteria leave pits in the surfaces. "You can think of these pits as little sinkholes," said Brantley as she showed microscopic images of pits etched by bacteria into mineral surfaces.

However, not everything that comes out of rocks is good for living things, cautioned Brantley. Arsenic, for example, also dissolves out of rocks, contaminating ground water and causing serious health problems in some parts of Asia.

In addition to eating rocks, some bacteria can also "breathe" rocks. Brantley explained that when we breathe oxygen, we get energy by taking electrons from food and dropping them into oxygen. Some bacteria can "breathe rust" by dropping electrons into iron oxide minerals (rust). This process may also leave pits on the mineral surfaces.

Scientists are now trying to learn more about the diversity of rock-dwelling microorganisms and their activities. Wherever conditions permit life, "anywhere there are two species out of equilibrium, energy can be harvested, and there are likely to be organisms taking advantage of it," Brantley said. Researchers have found microorganisms 4000 feet below the sea floor, in dust particles high in the atmosphere, in ice cores in Antarctica, and in hot springs.

Considering the extreme environments where organisms have been found on Earth, researchers are increasingly confident about the possibility of finding life elsewhere in the solar system. "Geology on Mars is interesting to non-geologists because the rocks can tell us about life," Brantley said.

Citing a college trip to Yellowstone as an inspiration for her career direction, Brantley described geology as a "discovery field" because students typically don't come to college intending to major in geology, but rather discover the field along the way. When asked for advice on how to interest young students in the field, Brantley commented that in her personal experience, "more people are interested when they see the connections between geology and biology."

"Rocks are part of the bottom of the food chain," Brantley concluded, "and we actually know very little about the influence of rocks on living processes."

Susan Brantley, Ph.D., is professor of geosciences and director of the Earth and Environmental Systems Institute and the Center for Environmental Kinetic Analysis (CEKA) at Penn State. She can be reached at

Heather Buss is a graduate student in geosciences and a member of the Research Unplugged committee. She can be reached at

Last Updated March 23, 2005