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2025 Hazard Mitigation Plan Kittitas Gounty, Washington ffiL^%#] time.) or that has the potential to erupt again in the future.-22e USGS classifies the five (5) volcanoes in the State of Washington as potentially active - Glacier Peak, Mount Adams, Mount Baker, Mount St. Helens, and Mount Rainier. Mount St. Helens is the most active volcano in the Cascades, with four (4) major eruptions in the last 500 years. The volcano activity annualized frequency value represents the average number of recorded volcano activity hazard occurrences, in events, per year over the period of record (11,332 years). Table 4-133 outlines the annualized frequency for volcano activity, based on FEMA NRI data, for Kittitas County. Per the FEMA NRl, volcanic activity occurs via vents that function as a conduit between the Earth's surface and inner layers, and erupt gas, molten rock, and volcanic ash when gas pressure and buoyancy drive molten rock upward and through zones of weakness in the Earth's crust. Table 4-133. Volcano Activity Annualized Frequency (FEMA National Risk lndex) 4.6.8.5. Past Events Since the Mount St. Helen eruption in 2008, there has not been significant volcanic activity in the Kittitas County region. However, all five (5) 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 (6) eruptions) being the most active. AllWashington volcanoes have had eruptions in the past 300 years that generated ash falland/or lahars. ln the past 40 years, Mount St. Helens has been the most active with dome building eruptions between 1980and 1986, and 2004and 2008. The mostsignificanteruption occurred in 1980 when Mount St. Helens erupted resulting in a towering plume of ash for more than nine (9) hours, and winds carried the ash hundreds of miles away. Lahars (i.e., volcanic mudflows) carried large boulders and logs, which destroyed forests, bridges, roads, and buildings. The event resulted in 57 deaths, and it is the worst volcanic disaster in the United States recorded history.-23o Table 4-134 outlines the eruption history for each of the volcanoes in the State of Washington..231 Table 4-134. Past Volcano Eruptions 22e United States GeologicalSurvey. (2022). Active, Dormant, and Extinct: Clarifying Confusing Classifications Retrieved from https://www.usqs.qov/observatories/vvo/news/active-dormant-and-extinct-clarifvinq-confusinq- classifications. 230 United States Geological Survey. (2020). Mount St. Helens' 1980 Eruption. Retrieved from https://www.usqs.qov/news/featured-storv/mount-st-helens-1980-eruption-chanqed-future-volcanoloqy. zsr Washington State Department of Natural Resources. (n.d.) Volcanoes and Lahars. Retrieved from https://www.dnr.wa.qov/proqrams-and-services/qeoloqv/qeoloqic-hazards/volcanoes-and-lahars. 0.0 events per yearKittitas County 2 Annualized frequency is defined as the expected frequency or probability of a hazard occurrence per year. Location Annualized FrequencyEvents on Record lOBC_ Mount Adams There have been no historical eruptions in the Mount Adams volcanic field. The volcanic center first erupted between 520,000 and 500,000 years ago and continued up to about 1,000 years ago. However, there were a series of debris avalanches and lahars between approximatelv 600 and 300 vears ago. Mount Baker Mount Baker is one of the youngest Cascade volcanoes and erupts infrequently. lts last major eruptive period occurred about 6,600 years ago, where large portions of the flank repeatedly collapsed generating massive lahars. There are additional reports of eruptions and lahars from the 19th century. ln 1975, fumarole activity and snow melt ramped up dramaticallv for several vears. Volcano Eruption History Chapter 4: Hazard ldentification and Risk Assessment 181