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CHAPTER 14. VOLCANO <br />14-4 <br />14.2.3 Frequency <br />Many Cascade volcanoes have erupted in the recent past and will be active again in the foreseeable future. <br />Given an average rate of one or two eruptions per century during the past 12,000 years, these disasters are <br />not part of our everyday experience; however, in the past hundred years, California’s Lassen Peak and <br />Washington’s Mount St. Helens have erupted with terrifying results. The U.S. Geological Survey classifies <br />Glacier Peak, Mt. Adams, Mt. Baker, Mt. Hood, Mt. St. Helens, and Mt. Rainier as potentially active <br />volcanoes in Washington State. Mt. St. Helens is by far the most active volcano in the Cascades, with four <br />major explosive eruptions in the last 515 years. <br />14.2.4 Severity <br />The explosive disintegration of Mount St. Helens’ north flank in 1980 vividly demonstrated the power that <br />Cascade volcanoes can unleash. A 1-inch deep layer of ash weighs an average of 10 pounds per square foot, <br />causing danger of structural collapse. Ash is harsh, acidic and gritty, and it has a sulfuric odor. Ash may <br />also carry a high static charge for up to two days after being ejected from a volcano. When an ash cloud <br />combines with rain, sulfur dioxide in the cloud combines with the rain water to form diluted sulfuric acid <br />that may cause minor, but painful burns to the skin, eyes, nose, and throat. <br />In an assessment published in April 2005, the U.S. Geological Survey rated the threat to civil and military <br />aviation, life, and property posed by Mount St. Helens, Mount Rainier, Mount Baker and Glacier Peak to <br />be “very high,” the highest classification. The report rated the threat posed by Mount Adams as “high.” <br />14.2.5 Warning Time <br />Constant monitoring of all active volcanoes means that there will be more than adequate time for evacuation <br />before an event. Since 1980, Mount St. Helens has settled into a pattern of intermittent, moderate and <br />generally non-explosive activity, and the severity of tephra, explosions, and lava flows have diminished. <br />All episodes, except for one very small event in 1984, have been successfully predicted several days to <br />three weeks in advance. However, scientists remain uncertain as to whether the volcano’s current cycle of <br />explosivity ended with the 1980 explosion. The possibility of further large-scale events continues for the <br />foreseeable future. <br />14.3. SECONDARY HAZARDS <br />The secondary hazards associated with volcanic eruptions are mud flows and landslides. <br />14.4. CLIMATE CHANGE IMPACTS <br />Large-scale volcanic eruptions can reduce the amount of solar radiation reaching the Earth’s surface, <br />lowering temperatures in the lower atmosphere and changing atmospheric circulation patterns. The massive <br />outpouring of gases and ash can influence climate patterns for years. Sulfuric gases convert to sub-micron <br />droplets containing about 75 percent sulfuric acid. These particles can linger three to four years in the <br />stratosphere. Volcanic clouds absorb terrestrial radiation and scatter a significant amount of incoming solar <br />radiation, an effect that can last from two to three years following a volcanic eruption. <br />14.5. EXPOSURE AND VULNERABILITY <br />According to the 2018 Washington State Enhanced Hazard Mitigation Plan, Kittitas County is not at risk <br />of exposure to lahar or lava flow. As ash fall can travel much further, all of Kittitas County has exposure to <br />ash fall from any of the active volcanos in the region. The plan estimates that Kittitas County has a 1 in <br />1,000 chance of receiving 10 centimeters (4 inches) of ash fall each year.