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Eugene -Springfield Area Natural Hazards Mitigation Plan 2. Hazard Descriptions Cascade volcanoes have erupted, including Mt. Baker, Glacier Peak, Mt. Rainier, Mount St. Helens, Mt. Hood, Mt. Shasta, and Mt. Lassen. Over the past 4,000 years, Oregon has experienced three eruptions of Mt. Hood, four eruptions in the Sisters area, and two eruptions in the Newberry Volcano area. Minor eruptions have taken place near Mt. Jefferson, at Blue Lake Crater in the Sand Mountain Field (Santiam Pass) near Mt. Washington, and near Belknap Crater. During this period, the most active volcano in the Cascades has been Mount St. Helens with over 14 eruptions. Volcanic eruptions often involve several distinct types of hazards to people and property, as evidenced by the Mount St. Helens eruption in 1980. Major volcanic hazards include lava flows, blast effects, pyroclastic flows, ash falls, lahars, landslides, and debris flows. Some of these hazards (e.g., lava flows) only affect areas very near to the volcano. Other hazards may affect areas 10 to 20 miles away. Ash falls may affect areas hundreds of miles downwind of the eruption site. The primary volcanic hazards of concern for Eugene -Springfield are: 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 fall within two miles of the eruption vent. Smaller ash fragments (less than about 0.1 inch) 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 extend for hundreds or even thousands of miles downwind. Lahars are common during eruptions of volcanoes with heavy loading of ice and snow or glaciers. 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 located on lahar flows from previous eruptions. 2.8.2 Climate Change Climate change may impact the effect of a volcanic eruption in many ways. The most plausible, and perhaps most significant, is the severity of a lahar. Warming temperatures are causing a steady decline in mountain snowpack. This directly correlates to the amount of snow and ice available to form a lahar during an eruption. Significant long-term climate change implications for volcanic eruptions will, more than likely, not be known for some time. 2.8.3 History of the Hazard The history of volcanic activity in the Cascades is contained in its geologic record and the age of the volcanoes vary considerably. Figure 2-9 shows the history of volcanic incidents in the Cascades. Scientists utilize a range of techniques to 2-52 January 2020