Mount Rainier’s potential for generating destructive mudflows is enhanced by its great height above surrounding valleys and its “soft” interior. The volcano stands about 3,000 meters above river valleys leading from its base. Volcanic heat and ground water have turned some of the volcano’s originally hard lava into soft clay minerals, thereby weakening its internal structure. These conditions make Mount Rainier extremely susceptible to large landslides. Several have occurred in the past few thousand years, one as recently as about 600 years ago. These landslides, apparently containing great volumes of water, quickly turned into lahars as they rushed down river valleys. Mount St. Helens, Washington. The catastrophic eruption on May 18, 1980, was preceded by 2 months of intense activity that included more than 10,000 earthquakes, hundreds of small _phreatic_ (steam-blast) explosions, and the outward growth of the volcano’s entire north flank by more than 80 meters. A magnitude 5.1 earthquake struck beneath the volcano at 08:32 on May 18, setting in motion the devastating eruption. [Illustration: Mount St. Helens crater and lava dome viewed from the north, 1990. Inset: Close view of lava dome with new lava extrusion on top (snow-free part) from the south, 1986. (_Photographs by Lyn Topinka._)] Within seconds of the earthquake, the volcano’s bulging north flank slid away in the largest landslide in recorded history, triggering a destructive, lethal lateral blast of hot gas, steam, and rock debris that swept across the landscape as fast as 1,100 kilometers per hour. Temperatures within the blast reached as high as 300 degrees Celsius. Snow and ice on the volcano melted, forming torrents of water and rock debris that swept down river valleys leading from the volcano. Within minutes, a massive plume of ash thrust 19 kilometers into the sky, where the prevailing wind carried about 520 million tons of ash across 57,000 square kilometers of the Western United States. [Illustration: Stand of timber in the process of being harvested was instead knocked over by the lateral blast. An estimated 4 million board feet of timber was destroyed. (_Photograph by H.H. Kieffer._)] [Illustration: Small eruption of gas and ash from the lava dome caused by violent release of volcanic gas or the geyser-like flashing of superhot ground water to steam. (_Photograph by Dan Dzurisin._)] [Illustration: Mount St. Helens towers above the chaotic landslide deposit that fills a former valley to a depth of as much as 195 meters. Note many small hills atop the landslide, called “hummocks” by geologists. (_Photograph by Lyn Topinka._)]