page_title: 1980 eruption of Mount St. Helens
section_title: Initial lateral blast
hierarchical_section_title: 1980 eruption of Mount St. Helens / Pyroclastic flows / Initial lateral blast
caption_attribution_description: Computer animation showing May 18 1980 Mount St. Helens landslide
context_page_description: On March 27, 1980, a series of volcanic explosions and pyroclastic flows began at Mount St. Helens in Skamania County, Washington, United States. It initiated as a series of phreatic blasts from the summit then escalated on May 18, 1980, as a major explosive eruption. The eruption, which had a Volcanic Explosivity Index of 5, was the most significant to occur in the contiguous 48 U.S. states since the much smaller 1915 eruption of Lassen Peak in California. It has often been declared the most disastrous volcanic eruption in U.S. history. The eruption was preceded by a two-month series of earthquakes and steam-venting episodes, caused by an injection of magma at shallow depth below the volcano that created a large bulge and a fracture system on the mountain's north slope.
  An earthquake at 8:32:11 a.m. PDT on Sunday, May 18, 1980, caused the entire weakened north face to slide away, creating the largest landslide recorded. This allowed the partly molten, high-pressure gas- and steam-rich rock in the volcano to suddenly explode northwards toward Spirit Lake in a hot mix of lava and pulverized older rock, overtaking the avalanching face.
context_section_description: The landslide exposed the dacite magma in St. Helens' neck to much lower pressure, causing the gas-charged, partially molten rock and high-pressure steam above it to explode a few seconds after the landslide started. Explosions burst through the trailing part of the landslide, blasting rock debris northward. The resulting blast directed the pyroclastic flow laterally. It consisted of very hot volcanic gases, ash and pumice formed from new lava, as well as pulverized old rock, which hugged the ground. Initially moving at approximately 220 miles per hour (350 km/h), the blast quickly accelerated to around 670 mph (1,080 km/h), and it may have briefly passed the speed of sound.
  Pyroclastic flow material passed over the moving avalanche and spread outward, devastating a fan-shaped area 23 miles across by 19 miles long (37 km × 31 km). In total about 230 square miles (600 km²) of forest was knocked down, and extreme heat killed trees miles beyond the blow-down zone. At its vent the lateral blast probably did not last longer than about 30 seconds, but the northward-radiating and expanding blast cloud continued for about another minute.
  Superheated flow material flashed water in Spirit Lake and North Fork Toutle River to steam, creating a larger, secondary explosion that was heard as far away as British Columbia, Montana, Idaho, and Northern California. Yet many areas closer to the eruption (Portland, Oregon, for example) did not hear the blast. This so-called "quiet zone" extended radially a few tens of miles from the volcano and was created by the complex response of the eruption's sound waves to differences in temperature and air motion of the atmospheric layers and, to a lesser extent, local topography.
  Later studies indicated that one-third of the 0.045 cubic miles (0.19 km³) of material in the flow was new lava, and the rest was fragmented, older rock.
caption: Computer graphics showing the May 18 landslide (green) being overtaken by the initial pyroclastic flow (red)
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