Long Valley Caldera (IMAGE) University of Wisconsin-Madison Caption A new study looks at rock from the titanic eruption that formed Long Valley Caldera in California 765,000 years ago. Calderas occur when a volcano collapses after an eruption. Long Valley has been studied by Wes Hildreth (in background), an author of the new PNAS study, for decades. The study signals that we don't fully understand these giant eruptions. Credit U.S. Geological Survey Usage Restrictions Credit U.S. Geological Survey License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.