The summit cone of Mount Spurr consists of a large lava dome built in the center of a horseshoe-shaped crater formed earlier by a large landslide. At the southern edge of this ancient crater is a younger, more active cone known as Crater Peak. Scientists have determined that Crater Peak is the source for at least 35 ash layers found in the Cook Inlet area, all of which were erupted in the past 6,000 years. Until recently, a warm turquoise-colored lake partially filled its crater. A series of explosive eruptions from Crater Peak on June 27, 1992, generated ash plumes as high as about 14 kilometers, small pyroclastic flows that swept down the south and east sides of the cone, and small lahars. The reawakening of Crater Peak followed nearly a year of increased earthquake activity, which escalated further on June 26, less than 1 day before its first eruption. Not all of the explosive episodes were preceded by a change in seismicity beneath the volcano, a condition that required scientists to maintain a 24-hour watch for extended periods of time in order to issue sudden reports and warnings of eruptive activity. The west side of Cook Inlet received a light to moderate ashfall during the largest explosive episode on August 18; Anchorage was blanketed with about 3 millimeters of ash, causing the Anchorage International Airport to close for a few hours. During an eruption at night on September 17, a spectacular display of lightning and incandescent ballistics and pyroclastic flows were witnessed by hunters who camped about 18 kilometers to the southeast; a faint glow above Crater Peak was also visible from as far away as Anchorage. [Illustration: Steaming Crater Peak, a satellite vent on the south side of Mount Spurr volcano, produced three explosive eruptions in 1992. (_Photograph by Cynthia Gardner._)] [Illustration: Aerial view of the Mono-Inyo Craters Volcanic Chain, California. Several eruptions occurred along both chains as recently as about 550 to 600 years ago. (_Photograph by C. Dan Miller._)] Mono Craters Wilson Butte Northern part of Inyo Volcanic Chain Obsidian Dome Glass Creek Dome Restless calderas The largest and most explosive volcanic eruptions eject tens to hundreds of cubic kilometers of magma onto the Earth’s surface. When such a large volume of magma is removed from beneath a volcano, the ground subsides or collapses into the emptied space, to form a huge depression called a caldera. Some calderas are more than 25 kilometers in diameter and several kilometers deep.