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CHAPTER 14. VOLCANO <br />14-3 <br />– Mount St Helens approximately 95 miles southwest of Ellensburg <br />– Mount Adams approximately 70 miles southwest of Ellensburg. <br />Mount Hood constitutes a low hazard because of distance, direction of prevailing winds, and evidence that <br />its previous ash eruptions were confined to its immediate vicinity. <br />Ash Falls <br />Ash falls, also called “tephra,” are from explosive eruptions that blast fragments of rock and ash into the <br />air. Large fragments fall to the ground close to the volcano. Small fragments and ash can travel thousands <br />of miles downwind and rise thousands of feet into the air. In some cases, ash can harm the human respiratory <br />system. Heavy ash fall can create darkness. Ash can clog waterways and machinery, cause electrical short <br />circuits, and drift into roadways, railways and runways. Ash harms mechanical and electronic equipment <br />and can cause jet engines on aircraft to stall. T he weight of ash, particularly when it becomes water <br />saturated, can cause structural collapse. Ash carried by winds can be a hazard to machinery and <br />transportation systems for months after an eruption. <br />The most serious tephra hazard in the region is from Mount St. Helens, the most prolific producer of tephra <br />in the Cascades during the past few thousand years. Figure 14-2 provides estimates of the annual probability <br />of tephra fall of 10 centimeters (about 4 inches) or greater affecting the region from all volcanoes. <br />Probability zones extend farther to the east of the range than to the west because prevailing winds are from <br />the west most of the time. <br /> <br />Figure 14-2. Probability of Tephra Accumulation in Pacific Northwest