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2025 Hazard Mitigation Plan <br />Kittitas County, Washington <br /> <br /> <br />Chapter 4: Hazard Identification and Risk Assessment 176 <br />Type Definition <br />Volcanic Gases <br />Magma contains dissolved gases that provide the driving force causing most volcanic <br />eruptions. As magma rises towards the surface and pressure decreases, gases are <br />released from the liquid portion of the magma. These gases continue to travel upward <br />and are eventually released into the atmosphere, during and between eruptions. The <br />majority of the gas emitted by volcanoes is water vapor (steam), derived from recent <br />precipitation and groundwater. However, toxic gases including carbon dioxide, sulfur <br />dioxide, hydrogen sulfide, hydrogen halides, and fluorine may also be released. <br />Depending on their concentration, toxic gases can have both short and long term <br />effects on human and animal lives, property, agriculture, and the natural <br />environment. <br />Tephra Any type and size of rock fragment that is forcibly ejected from the volcano and travels <br />an airborne path during an eruption (i.e., ash, bombs, and scoria). <br />Eruption Columns and <br />Clouds <br />The ascending, vertical part of the mass of erupting debris and volcanic gas that <br />rises directly above a volcanic vent. Higher in the atmosphere, columns usually <br />spread laterally into plumes or umbrella clouds. <br /> <br />A cloud of tephra and gases that forms downwind of an erupting volcano is called an <br />eruption cloud. The vertical pillar of tephra and gases rising directly above a vent is <br />an eruption column. Eruption clouds may drift for thousands of kilometers downwind <br />and often become increasingly spread out over a larger area with increasing distance <br />from an erupting vent. Large eruption clouds can encircle Earth within days. <br />Ashfall <br />Dust-sized ash particles are the byproducts of many volcanic eruptions. Ash, when <br />blown into the air, can travel large distances causing significant problems for distal <br />hazard zones. Areas impacted by volcanic ash largely depend upon the prevailing <br />wind direction during eruptions. <br /> <br />A one (1) inch layer of ash weighs an average of five (5) to 10 pounds per square <br />foot when dry (10 to 15 pounds when wet), causing danger of structural collapse. Ash <br />is gritty, abrasive, sometimes corrosive, and it has a sulfuric odor. Additionally, when <br />an ash cloud combines with rain, sulfur dioxide in the cloud combines with the <br />rainwater to form acid rain which may cause minor, but painful burns to the skin, eyes, <br />nose, and throat. <br /> <br />Volcanologists classify volcanoes as active, dormant, or extinct. Active volcanoes have a recent history <br />of eruptions (about 10,000 years ago) and are likely to erupt again. Volcanoes that have not erupted for <br />an extended period of time but are expected to erupt in the future are classified as dormant. Extinct <br />volcanoes are not expected to erupt in the future.222F <br />222 <br />4.6.8.2. Hazard Location <br />The Cascade Range extends more than 1,000 miles from southern British Columbia into northern <br />California and includes 13 potentially active volcanic peaks in the United States, listed in Table 4-129, <br />most of which have the potential to produce a significant eruption.223F <br />223 <br /> <br /> <br />222 National Geographic. (n.d.). Volcanoes. Retrieve from <br />https://education.nationalgeographic.org/resource/volcanoes/. <br />223 United States Geological Survey. (n.d.). Cascades Volcano Observatory. Retrieved from <br />https://www.usgs.gov/observatories/cvo.