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2025 Hazard Mitigation Plan <br />Kittitas County, Washington <br /> <br /> <br />Chapter 4: Hazard Identification and Risk Assessment 175 <br />Volcanoes can display a range of eruptive phenomena. Some consist of quiet emissions of fluid lava <br />flows while others have cataclysmic eruptions. Table 4-127 lists the three (3) fundamental ways of <br />characterizing a volcanic eruption.220F <br />220 <br />Table 4-127. Volcanic Eruption Types <br />Type Definition <br />Magmatic Eruptions <br />Include the ejection of lava or tephra from a magma source within the earth. There is <br />a significant range in the intensity, magnitude, explosivity, eruption rate, and amount <br />of magma erupted. Magmatic eruptions are generally described as being effusive <br />(dominated by passive emissions of lava) or explosive (dominated by eruption of <br />fragmental (pyroclastic) material). <br />Phreatomagmatic <br />(Hydrovolcanic) <br />Eruption <br />Violent explosive eruptions driven by steam explosions produced by the interaction <br />of hot magma with surface water or shallow groundwater. Hydrovolcanic eruptions <br />include tephra derived from juvenile magma. These eruptions produce tephra and <br />steam. <br />Phreatic Eruption <br />Eruptions driven by steam explosions due to the superheating of groundwater by a <br />magmatic source. Phreatic eruptions may include volcanic ash made of shattered <br />rock material, but incorporate no magmatic (e.g., juvenile) material. Phreatic <br />eruptions may be precursors to magmatic eruptions. <br /> <br />Volcanic eruptions can last days, weeks, or years, and although they are typically focused around a single <br />vent area, volcanoes vary significantly in explosivity. Therefore, volcanic hazards, listed on Table 4-128, <br />can have extensive consequences that can affect a small area or the whole planet.221F <br />221 Furthermore, <br />volcanic eruptions can dramatically change an area’s landscape. <br />Table 4-128. Volcano Related Hazards <br />Type Definition <br />Lava Flows <br />Moving lava and the resulting solidified deposit are referred to as lava flows. Because <br />of the wide range in viscosity of the different lava types (i.e., basalt, andesite, dacite, <br />and rhyolite), lava discharge during eruptions, and characteristics of the erupting vent <br />and topography over which lava travels, lava flows come in a great variety of shapes <br />and sizes. <br />Pyroclastic Flows and <br />Surges <br />Pyroclastic flows are hot (greater than 1,475°F), chaotic mixture of rock fragments, <br />gas, and ash that travels rapidly (up to 150 mph) away from a volcanic vent or <br />collapsing flow front. <br /> <br />Pyroclastic surges are ground-hugging clouds of ash, rock, and volcanic gas that <br />move at hurricane velocities and have temperatures of several hundred degrees <br />Fahrenheit. <br />Lahars <br />A mixture of water and volcanic debris that moves rapidly downstream. Consistency <br />can range from muddy water to that of wet cement, depending on the ratio of water <br />to debris. They form in a variety of ways, mainly as a result of the rapid melting of <br />snow and ice due to pyroclastic flows, intense rainfall on loose volcanic rock deposits, <br />breakout of a lake dammed by volcanic deposits, and as a consequence of debris <br />avalanches. <br />Landslides The downslope movement of rock, soil, or related debris. Landslides are addressed <br />separately in this Plan. <br /> <br />220 National Park Service. (2022). Volcanoes, Craters, & Lava Flows: Eruption Classifications. Retrieved from <br />https://www.nps.gov/subjects/volcanoes/eruption-classifications.htm. <br />221 United States Geological Survey. (2016). Volcano Hazards Program Glossary. Retrieved from <br />https://volcanoes.usgs.gov/vsc/glossary/.