U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM VOLCANO HAZARDS Living with Volcanic Risk in the Cascades he Cascade Range of the Pacific TNorthwest has more than a dozen potentially active volcanoes. Cascade volcanoes tend to erupt explosively, and on average two eruptions occur per century—the most recent were at Mount St. Helens, Washington (1980–86), and Lassen Peak, Cali- fornia (1914–17). To help protect the Pacific Northwest’s rapidly expand- ing population, USGS scientists at the Cascades Volcano Observatory in Vancouver, Washington, monitor and assess the hazards posed by the region’s volcanoes. On May 18, 1980, after 2 months of earthquakes and minor eruptions, Mount St. The more than 1 million residents of the Seattle-Tacoma, Washing- Helens, Washington, exploded in one of the ton, area live in the shadow of 14,411-foot-high Mount Rainier, the most devastating volcanic eruptions of the tallest volcano in the United States outside of Alaska. Several communities near the volcano, built on the 20th century. Although less than 0.1 cubic deposits of giant lahars of volcanic ash and debris that are less than 1,200 years old, are at risk from similar future lahars. Inset photograph shows a U.S. Geological Survey (USGS) scientist monitoring the volcano for mile of molten rock (magma) was erupted, 57 signs of subtle ground movement that might lead to an eruption or landslide. Photos by Lyn Topinka, USGS. people died, and damage exceeded $1 billion. Fortunately, most people in the area were able (pyroclastic flows), lava flows, and landslides years ago, an especially powerful series of to evacuate safely before the eruption because can devastate areas 10 or more miles away; eruptions deposited volcanic ash at least as far public officials had been alerted to the danger and huge mudflows of volcanic ash and debris, away as Wyoming. by U.S. Geological Survey (USGS) and called lahars, can inundate valleys more than 50 Mount Rainier has produced at least four other scientists. As early as 1975, USGS miles downstream. Falling ash from explosive eruptions and numerous lahars in the past 4,000 researchers had warned that Mount St. Helens eruptions can disrupt human activities hundreds years. It is capped by more glacier ice than might soon erupt. Coming more than 60 of miles downwind, and drifting clouds of fine the rest of the Cascade volcanoes combined, years after the last eruption in the Cascades ash can cause severe damage to jet aircraft even and parts of Rainier’s steep slopes have (Lassen Peak), the explosion of St. Helens thousands of miles away. been weakened by hot, acidic volcanic gases was a spectacular reminder that the millions Because the population of the Pacific and water. These factors make this volcano of residents of the Pacific Northwest share the Northwest is rapidly expanding, the volcanoes especially prone to landslides and lahars. region with live volcanoes. of the Cascade Range in Washington, Oregon, Mount St. Helens is the most frequently and northern California are some of the most active volcano in the Cascades. During the past Volcanoes of the Cascades dangerous in the United States. 4,000 years, it has produced many lahars and a The volcanoes of the Cascade Range, wide variety of eruptive activity, from relatively which stretches from northern California into Washington quiet outflows of lava to explosive eruptions British Columbia, have produced more than Mount Baker erupted in the mid-1800s much larger than that of May 18, 1980. 100 eruptions, most of them explosive, in for the first time in several thousand years. Mount Adams has produced few eruptions just the past few thousand years. However, Activity at steam vents (fumaroles) in during the past several thousand years. This individual Cascade volcanoes can lie dormant Sherman Crater, near the volcano’s summit, volcano’s most recent activity was a series of for many centuries between eruptions, and increased in 1975 and is still vigorous, but small eruptions about 1,000 years ago. the great risk posed by volcanic activity in the there is no evidence that an eruption region is therefore not always apparent. is imminent. Oregon When Cascade volcanoes do erupt, Glacier Peak has erupted at least six Mount Hood last erupted about 200 years high-speed avalanches of hot ash and rock times in the past 4,000 years. About 13,000 ago, producing pyroclastic flows, lahars, and U.S. Department of the Interior USGS Fact Sheet 165–97 U.S. Geological Survey Revised March 2003 ERUPTIONS OF THE PAST 4000 YEARS a prominent lava dome (Crater Rock) near the Of the 13 po- Mount Baker volcano’s summit. Most recently, a series of tentially active volcanoes in the Glacier Peak steam blasts occurred between 1856 and 1865. Cascade Range Mount Rainier Mount Jefferson last erupted more than of the Pacific n WASH Mount St. Helens 20,000 years ago. However, eruptions nearby Northwest, 11 a have erupted in e Mount Adams have produced several lava flows and small c the past 4,000 O Mount Hood volcanic cones in the past 10,000 years. years, and 7 in c i Mount Jefferson just the past 200 f Three Sisters Volcanic Center in central i years. More than c Three Sisters Oregon includes five large volcanoes—North a ORE 100 eruptions, P Newberry Volcano Sister, Middle Sister, South Sister, Broken Top, most of which Crater Lake and Mount Bachelor. About 2,000 years ago, were explosive, have occurred Medicine Lake Volcano eruptions occurred on South Sister, as well as in the past 4,000 Mount Shasta from several small volcanoes north of North years, making the volcanoes of the Lassen Peak Sister. Since 1997, a broad area centered 3 CALIF Cascade Range 4000 2000 present miles west of South Sister has domed upward 200 some of the most YEARS BEFORE PRESENT by more than 5 inches. Scientists think that hazardous in this doming reflects the ongoing accumulation the United States. Each eruption symbol in the diagram represents from one to several dozen eruptions closely spaced in time. Eruptions have also occurred from other vents (not shown) scattered throughout the Cascade of magma at a depth of 3 to 4 miles. The Range, especially in central Oregon and southwestern Washington. outcome of this activity is uncertain, but there is no evidence that an eruption is imminent. 22, 1915, produced a large pyroclastic flow, at Mount St. Helens. Observatory scientists The USGS and its partners have increased lahars, and ash that fell as far away as Elko, also have conducted many new studies of the monitoring efforts in the area to detect any Nevada, 300 miles to the east. past geologic behavior of Cascade volcanoes. changes that might warrant more concern. These studies allow assessments of long-term Newberry Volcano, a broad shield Reducing the Risk hazards (including lahars and landslides, covering more than 500 square miles, is After the 1980 eruption of Mount St. which can occur even when a volcano is not capped by Newberry Crater, a large volcanic Helens, Congress provided increased funding erupting) and provide the data for hazard- depression (caldera) 5 miles across. Its most that enabled the USGS to establish a volcano zone maps that enable communities to make recent eruption was about 1,300 years ago. observatory for the Cascade Range. Located in wise land-use-planning decisions and to Crater Lake occupies a 6-mile-wide caldera Vancouver, Washington, the David A. Johnston prepare emergency-response plans. formed 7,700 years ago when the summit of an Cascades Volcano Observatory (CVO) was The work of CVO scientists is helping ancient volcano (referred to as Mount Mazama) named for a USGS scientist killed at a forward residents of the Pacific Northwest live with collapsed during a huge explosive eruption. More observation post by the May 18, 1980, eruption. volcanic risk and plan for volcano emergencies. than 10 cubic miles of magma was erupted, 10 Scientists at CVO quickly recognized that it This work is only part of the USGS Volcano times as much as in any other eruption in the was not economically feasible to fully monitor Hazards Program’s ongoing efforts to protect Cascades during the past 10,000 years. Smaller all potentially active Cascade volcanoes. To people’s lives and property in all of the eruptions ending about 5,000 years ago formed address this and similar problems elsewhere volcanic regions of the United States, including Wizard Island and several submerged cones and in the United States and abroad, the USGS the Pacific Northwest, eastern California, lava domes on the lake floor. developed a suite of portable volcano- Wyoming, Alaska, and Hawaii. monitoring instruments—essentially, a portable Northern California Dan Dzurisin, Peter H. Stauffer, and volcano observatory. In the Pacific Northwest, James W. Hendley II Medicine Lake Volcano, a broad shield when regional networks of earthquake sensors, Graphics and layout by capped by a 4- by 7-mile-wide caldera, has operated in cooperation with the University of Sara Boore, Bobbie Myers, and Susan Mayfield erupted at least eight times in the past 4,000 Washington’s Pacific Northwest Seismograph COOPERATING ORGANIZATIONS years, most recently about 900 years ago. With Network, detect unusual seismic activity at a Federal Aviation Administration National Oceanic and Atmospheric Administration a volume of more than 130 cubic miles, it is the volcano, CVO staff will rapidly deploy this National Park Service largest volcano in the Cascades. portable equipment to evaluate the hazard and, State geological surveys and emergency-management agencies of Washington, Oregon, and California Mount Shasta has been the most active if needed, provide timely warnings to local University of Washington volcano in California during the past 4,000 officials and the public.