Exposure of explosive ash deposits (IMAGE) Proceedings of the National Academy of Sciences Caption Exposure of explosive ash deposits tens of meters thick, 30 to 40 kilometers north of the Toba caldera. Credit Image credit: Stephen Self (University of California, Berkeley, CA) Usage Restrictions None 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.