Hi Canie. The following has been taken from USGS fact sheet 108-96 revised in May 2000. Take Care…Don in creepy town

In May of 1980, a strong earthquake swarm that included four magnitude 6 earthquakes struck the southern margin of Long Valley Caldera associated with a 25-cm, dome-shaped uplift of the caldera floor. It is not known it magma was the cause of these quakes, or the first quake was tectonic and the rest volcanic. These events marked the onset of the latest period of caldera unrest that continues to this day. This ongoing unrest includes recurring earthquake swarms and continued dome-shaped uplift of the central section of the caldera (the resurgent dome) accompanied by changes in thermal springs and gas emissions.

Measurements made by the scientists showed that the center of the caldera had risen almost a foot since the summer of 1979 after decades of stability. This continuing swelling, which now totals nearly 2 feet and affects more than 100 square miles, is caused by new magma rising beneath the caldera.

During the early 1990's, trees began dying off at several places on Mammoth Mountain on the southwest edge of Long Valley Caldera. Studies conducted by USGS and U.S. Forest Service scientists show that the trees are being killed by large amounts of carbon dioxide (CO2) gas seeping up through the soil from magma deep beneath Mammoth Mountain.

During the past 3,000 years the Mono-Inyo Craters have erupted at intervals of 700 to 250 years, the most recent eruptions being from Panum Crater and the Inyo Craters 500 to 600 years ago (Miller, 1985; Bursik and Sieh, 1986), and Paoha Island about 250 years ago (Stine, 1990). Evidence from both seismic soundings of the crust and studies of the fabric and composition of the lava indicate that these eruptions probably originated from small, discrete magma bodies rather than from a single, large magma chamber of the sort that produced the caldera-forming eruption 760,000 years ago.

• Long Valley Caldera