When Magma’s On the Move

In California’s Long Valley, the earth trembles every day where a volcano once exploded

For the past 20 years, what's on Dave Hill's mind has been literally beneath his feet. The U.S. Geological Survey (USGS) seismologist tracks swarms of earthquakes that occur daily in a 10-by-20-mile crater on the eastern face of the Sierra Nevada in California's Long Valley. But as writer Mark Wheeler discovers, it's not earthquakes that keep Hill up at night — it's the volcanic eruption that they portend.

Today Long Valley is home to Mammoth, the second biggest ski resort in the West after Vail, Colorado. But 760,000 years ago, a giant volcanic eruption here blew out 150 cubic miles of hot molten rock called magma. The underground magma chamber then collapsed, forming the crater known as the Long Valley caldera.

In 1968, Hill's USGS colleague Roy Bailey identified Long Valley as a "resurgent" caldera, containing a growing, magma-fed bulge. Decades of observation and study later, no one doubts that the caldera will erupt again. The earthquakes are one sign, as are trees killed by large amounts of carbon dioxide released into the soil by magma, suffocating the trees' roots.

But exactly when the caldera will erupt is difficult to predict. To get a better idea of how it behaves, USGS scientists like Hill constantly monitor the caldera's earthquakes and growing bulge. In the meantime, longtime residents remain cautious but hopeful. "We're just banking on the geologic timescale," says a new homeowner with a laugh.