page_title: 1980 eruption of Mount St. Helens
section_title: Lateral blast result
hierarchical_section_title: 1980 eruption of Mount St. Helens / Pyroclastic flows / Lateral blast result
caption_attribution_description: English: Car after Mount St. Helens 1980 Eruption. Reid Blackburn's (photographer, National Geographic, Vancouver Columbian) car, about 10 miles from Mount St. Helens.
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 huge ensuing ash cloud sent skyward from St. Helens' northern foot was visible throughout the quiet zone. The near-supersonic lateral blast, loaded with volcanic debris, caused devastation as far as 19 miles (31 km) from the volcano. The area affected by the blast can be subdivided into three roughly concentric zones:
  Direct blast zone, the innermost zone, averaged about eight miles (13 km) in radius, an area in which virtually everything, natural or artificial, was obliterated or carried away. For this reason, this zone also has been called the "tree-removal zone". The flow of the material carried by the blast was not deflected by topographic features in this zone. The blast released energy equal to 24 megatons of TNT.
  Channelized blast zone, an intermediate zone, extended out to distances as far as 19 miles (31 km) from the volcano, an area in which the flow flattened everything in its path and was channeled to some extent by topography. In this zone, the force and direction of the blast are strikingly demonstrated by the parallel alignment of toppled large trees, broken off at the base of the trunk as if they were blades of grass mown by a scythe. This zone was also known as the "tree-down zone". Channeling and deflection of the blast caused strikingly varied local effects that still remained conspicuous after some decades. Where the blast struck open land directly, it scoured it, breaking trees off short and stripping vegetation and even topsoil, thereby delaying revegetation for many years. Where the blast was deflected so as to pass overhead by several metres, it left the topsoil and the seeds it contained, permitting faster revegetation with scrub and herbaceous plants, and later with saplings. Trees in the path of such higher-level blasts were broken off wholesale at various heights, whereas nearby stands in more sheltered positions recovered comparatively rapidly without conspicuous long-term harm.
  Seared zone, also called the "standing dead" zone, the outermost fringe of the impacted area, a zone in which trees remained standing but were singed brown by the hot gases of the blast.
  By the time this pyroclastic flow hit its first human victims, it was still as hot as 680 °F (360 °C) and filled with suffocating gas and flying debris. Most of the 57 people known to have died in that day's eruption succumbed to asphyxiation, while several died from burns. Lodge owner Harry R. Truman was buried under hundreds of feet of avalanche material. Volcanologist David A. Johnston was one of those killed, as was Reid Blackburn, a National Geographic photographer. Robert Landsburg, another photographer, was killed by the ash cloud. He was able to protect his film with his body, and the surviving photos provided geologists with valuable documentation of the historic eruption.
caption: Photographer Reid Blackburn's Volvo after the eruption
exif: {}