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volcano. Other hazards may affect areas 10 or 20 miles away from the volcano, whi~e ash falls may affect areas many miles downwind of the eruption site. Lava flows are eruptions of molten rock. Lava flows for the major Cascades volcanoes tend to be thick and viscous, forming cones and thus typically affecting areas only very near the eruption vent. However, flows from the smaller mafic volcanoes may be less viscous flows that spread out over wider areas. Lava flows obviously destroy everything in their path. Blast effects may occur with violent eruptions, such as Mount St. Helens in 1980. Most volcanic blasts are largely upwards. However, the Mount St. Heiens blast was lateral, with impacts 17 miles from the volcano. Similar or larger blast zones are possible in future eruptions of any of the major Cascades volcanoes. Pyroclastic flows are high-speed avalanches of hot ash, rock fragments and gases. Pyroclastic flows can be as hot as 1500 °F and move downslope at 100 to 150 miles per hour. Pyroclastic flows are extremely deadly for anyone caught in their path. Ash falls result when explosive eruptions blast rock fragments into the air. Such blasts may include tephra (solid and molten rock fragments). The largest rock fragments (sometimes called "bombs") generally fa~l within two miles of the eruption vent. Smaller ash fragments (less than about 0.1") typically rise into the area forming a huge eruption column. In very large eruptions, ash falls may total many feet in depth near the vent and extent for hundreds or even thousands of miles downwind. Lahars or mudflows are common during eruptions of volcanoes with heavy loading of ice and snow. These flows of mud, rock and water can rush down channels at 20 to 40 miles an hour and can extend for more than 50 miles. For some volcanoes, lahars are a major hazard because highly populated areas are built on lahar flows from previous eruptions. Landslides or debris flows are the rapid downslope movement of rocky material, snow and/or ice. Volcano landslides can range from small movements of loose debris to massive collapses of the entire summit or sides of a volcano. Landslides on volcanic slopes may be triggered be eruptions or by earthquakes or simply by heavy rainfall. 11.3 Volcanic Hazards for the Eugene/Springfield IVletro Area Severa~ of the 20 active volcanoes in Oregon (See Table 11.1 ) are located along the crest of the Cascades near the eastern boundary of Lane County. These volcanoes include the Three Sisters, Mt. Bachelor and the Davis Lake volcanic field. Other relatively nearby active volcanoes include several near the eastern boundary of Linn County, including: Mt. Jefferson, Blue Lake Crater, Mt. Washington, the Be~knap Crater field, and the Sand Mountain field. Other relatively nearby volcanoes include Public Review Draft: October 11, 2004 11-3