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2025 Hazard Mitigation Plan <br />Kittitas County, Washington <br /> <br /> <br />Chapter 4: Hazard Identification and Risk Assessment 178 <br />Figure 4-22. Volcanoes Within 100 Miles of Kittitas County <br /> <br />4.6.8.3. Hazard Extent/Intensity <br />Volcanic eruptions can range from emissions of gases to quiet eruptions of lava flows that can be safely <br />observed to powerful eruptions that can blow apart mountains and devastate many square miles. For <br />example, the explosive disintegration of Mount St. Helens’ north flank in 1980 vividly demonstrated the <br />power of Cascade volcanoes. However, the 1980 eruption of Mount St. Helens, which covered over <br />22,000 square miles with ash is considered a relatively small volcanic eruption. Larger eruptions, such <br />as the 1883 eruption of Krakatoa in the south Pacific, produced enough ash to slightly lower the <br />temperature of the entire earth for several years.224F <br />224 <br /> <br />USGS utilizes the Volcanic Explosivity Index (VEI), outlined in Table 4-130, as a numeric scale that <br />measures the relative explosivity of historic eruptions. The explosivity value is determined by the volume <br />of products, eruption cloud height, and qualitative observations (by using terms like gentle to mega - <br />colossal). The VEI is open-ended with the largest volcanic eruptions in history (i.e., supereruptions) given <br />a magnitude of eight (8), and a value of zero (0) for non-explosive eruptions. The scale is logarithmic, <br /> <br />224 Washington State Department of Natural Resources. (n.d.). Volcanoes and Lahars. Retrieved from <br />https://www.dnr.wa.gov/programs-and-services/geology/geologic-hazards/volcanoes-and-lahars.