And visitors can feel the earth move under their feet. There is active magma below the Sierra in this region. Mammoth Mountain, though it has not been active in thousands of years, could still erupt some day, according to the U.S. Geological Survey. Located just 3 miles west of the town of Mammoth Lakes, it is venting huge plumes of carbon dioxide. The gas has been so concentrated that it has killed more than 20 acres of conifers near Horseshoe Lake.
Scientists say there is no danger at the moment because the carbon dioxide is cool.
Mount St. Helens in the Pacific Northwest belched hot carbon dioxide before it blew 20 years ago. But people get nervous about Mammoth Mountain anyway. The mountain is one of a series of volcanoes on the shoulder of a massive and ancient blast that left a valley 19 miles long and 10 miles wide. The blast that created Long Valley Caldera, as it is now called, occurred 730,000 years ago. Mammoth Mountain began building on the edge of the caldera about 180,000 years ago as lava filled with feldspar and mica poured out of it.