2019_KittitasCountyHMP_Volume 1_For Adoption

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14-1 CHAPTER 14. VOLCANO 14.1. GENERAL BACKGROUND Hazards related to volcanic eruptions are distinguished by the different ways in which volcanic materials and other debris are emitted from the volcano. The molten rock that erupts from a volcano (lava) forms a hill or mountain around the vent. The lava may flow out as a viscous liquid, or it may explode from the vent as solid or liquid particles. Ash and fragmented rock material can become airborne and travel far from the erupting volcano to affect distant areas. Washington State has five active volcanoes: Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens, and Mount Adams. These volcanoes are all capable of generating destructive lahars, ash fall, lava, pyroclastic flows, and debris avalanches. The phenomena that pose the greatest threat are ash fall and lahars. Mount Hood in Oregon also poses a threat to communities along the Washington side of the Columbia River. All of these volcanoes pose a high to very high threat to life, property, the environment, and civil and military aviation in areas more than a few miles from the mountains’ slopes. 14.2. HAZARD PROFILE 14.2.1 Past Events All five of Washington’s volcanoes have been active in the last 4,000 years, with Mount St. Helens (more than a dozen eruptive events) and Glacier Peak (at least six eruptions) the most active. Mount St. Helens has been the most active in the past 40 years, with a massive eruption in 1980, followed by dome building eruptions in the 1980-1986 and 2004-present periods. In the 1980 Mount St. Helens eruption, 23 square miles of volcanic material buried the North Fork of the Toutle River and there were 57 human fatalities. All Washington volcanoes have had eruptions in the past 300 years that generated ash fall and/or lahars. Figure 14-1 and Table 14-1 summarize past eruptions in the Cascades. 14.2.2 Location Figure 14-1 shows the location of the Cascade Range volcanoes, most of which have the potential to produce a significant eruption. The Cascade Range extends more than 1,000 miles from southern British Columbia into northern California and includes 13 potentially active volcanic peaks in the U.S. •DEFINITIONS •Lahar—A rapidly flowing mixture of water and rock debris that originates from a volcano. While lahars are most commonly associated with eruptions, heavy rains, and debris accumulation, earthquakes may also trigger them. •Lava Flow—The least hazardous threat posed by volcanoes. Cascades volcanoes are normally associated with slow moving andesite or dacite lava. •Stratovolcano—Typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs, rising as much as 8,000 feet above their bases. The volcanoes in the Cascade Range are all stratovolcanoes. •Tephra—Ash and fragmented rock material ejected by a volcanic explosion •Volcano—A vent in the planetary crust from which magma (molten or hot rock) and gas from the earth’s core erupts.