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On a longer geological time scale, volcanic activity in the Cascades has been very widespread. A DOGAMI report on prehistoric and historic volcanic eruptions in Oregon (see website below) notes that in the Cascades as a whole, over 3000 large and small volcanoes have erupted over the past five million years. Within histodca~ times, between 1843 and 1860 there were a series of 21 eruptions in the Cascades and there is some scientific speculation that the Northwest may be entering another period of volcanic activity. A great dea~ of general background information on Oregon volcanoes and on volcanoes in general is available on several websites, including the following. Table 11.2 Volcano Websites institution Website Smithsonian institution www.volcano.si.edu (Global Volcanism Project) United States Geological Survey www usgs gov (USGS) - general site USGS Cascades Volcano Observatory (Vancouver, WA) http://vulcan'wr'usqs'g°v DOGAMI www.oreqonc~eoloqy com The numerous volcanoes of the Cascades differ markedly in their geological characteristics. The largest volcanoes are generally what geologists call composite or stratovolcanoes. These volcanoes may be active for tens of thousands of years to hundreds of thousands of years. In some cases, these large volcanoes may have explosive eruptions such as Mt. St. Heiens in 1980 or Crater Lake about 7,700 years ago. The much more numerous sites of volcanic activity are generally what geologists call mafic volcanoes. This type of volcano is typically active for much shorter time periods, up to a few hundred years, and generally forms small craters or cones. Mafic volcanoes are not subject to large explosive events. 11.2 Volcanic Hazard Types in Oregon, awareness of the potential for volcanic eruptions was greatly increased by the May 18, 1980 eruption of nearby Mount. St. Heiens in Washington which killed 57 people. In this eruption, lateral blast effects covered 230 square miles and reached 17 miles northwest of the crater, pyroclastic flows covered six square mi~es and reached 5 miles north of the crater, and landslides covered 23 square mi~es. Ash accumulations were about t0 inches at 10 miles downwind, I inch at 60 mi~es downwind, and ½ inch at 300 miles downwind. Lahars (mudflows) affected the North and South Forks of the Toutle River, the Green River, and ultimately the Cok~mbia River as far as 70 miles from the volcano. Volcanic eruptions often involve several distinct types of hazards to people and property, as well evidenced by the Mount St. Helens eruption. Major volcanic hazards include: ~ava flows, blast effects, pyroclastic flows, ash flows, lahars, and landslides or debris flows. Some of these hazards (e.g., lava flows) only affect areas very near the Public Review Draft: October 11, 2004 11-2