last significant Mount Shasta eruption came from this cone and created a pyroclastic flow, a hot lahar (mudflow), and three cold lahars, which streamed down Mount Shasta's east flank via Ash Creek. A separate hot lahar went down Mud Creek. This eruption was thought to have been observed by the explorer La Pérouse, from his ship off the California coast, in 1786, but this has been disputed. Volcanic status During the last 10,000 years, Mount Shasta has erupted an average of every 800 years, but in the past 4,500 years the volcano has erupted an average of every 600 years. USGS seismometers and GPS receivers operated by UNAVCO form the monitoring network for Mount Shasta. The volcano has been relatively quiet for at least the past 15 years, with only a handful of small magnitude earthquakes and no demonstrable ground deformation. Although geophysically quiet, periodic geochemical surveys indicate that volcanic gas emanates from a fumarole at the summit of Mount Shasta from a deep-seated reservoir of partly molten rock. Mount Shasta can release volcanic ash, pyroclastic flows or dacite and andesite lava. Its deposits can be detected under nearby small towns. Mount Shasta has an explosive, eruptive history. There are fumaroles on the mountain, which show Mount Shasta is still alive. The worst-case scenario for an eruption is a large pyroclastic flow, similar to that which occurred in the 1980 eruption of Mount St. Helens. Since there is ice, such as Whitney Glacier and Mud Creek Glacier, lahars would also result. Ash would probably blow inland, perhaps as far as eastern Nevada. There is a small chance an eruption could result in a collapse of the mountain, as happened when Mount Mazama in Oregon collapsed to form what is now called Crater Lake, but this is of much lower probability. The United States Geological Survey monitors Mount Shasta and rates it as a very high-threat volcano. Climbing The summer climbing season runs from late April until October, although many attempts are made in the winter. In winter, Sargents Ridge and Casaval Ridge, to the east and west of Avalanche Gulch, respectively, become the most traveled routes, to avoid avalanche danger. Mount Shasta is also a popular destination for backcountry skiing. Many of the climbing routes can be descended by experienced skiers, and there are numerous lower-angled areas around the base of the mountain. The most popular route on Mount Shasta