VOLCANO HAZARDS HISTORY AND HAZARDS OF MOUNT RAINIER, WASHINGTON Mount Rainier is an active volcano Osceola and Paradise mudflows that of feet of ice. The summit itself is that first erupted about half a million traveled down the White and formed of two small overlapping years ago. Because of Rainier's great Nisqually Rivers, reaching Puget craters, each about a quarter mile in height (14,41 0 feet above sea level) Sound and pushing out the shoreline diameter; the younger of these forms and northerly location, glaciers have by as much as 10 miles. The scar a nearly perfect circle of radially­ cut deeply into its lavas, making it from this collapse was a horseshoe­ outward-sloping lavas. The shallow appear deceptively older than it shaped crater, about 1 .25 miles wide, floors of these craters are fi lied with actually is. Mount Rainier is known open to the northeast. Since the snow and ice, but the raised rims are to have erupted as recently as in the collapse, lava flows and avalanches snow-free year-round because of high 1840s, and large eruptions took place of hot lava fragments have erupted winds and because much of the as recently as about 1,000 and 2,300 from the crater and largely filled it, ground is still hot. Steam or warm years ago. forming the present summit cone of mist, at or just below boiling Mount Rainier and other similar Mount Rainier. temperature, ri ses from the crater rims volcanoes in the Cascade Range, such in many areas and has melted an intricate system of caves into the base as Mount Adams and Mount Baker, erupt much less frequently than the of the crater-filling ice. On calm days, more familiar Hawaiian volcanoes, a faint odor of sulfur can also be but their eruptions are vastly more smelled. The hot ground, steam, and destructive. Hot lava and rock debris sulfur smell, as well as the little­ from Rainier's eruptions have melted eroded shape of the summit craters snow and glacier ice and triggered attest to Rainier's recent activity. debris flows (mudflows) - with a Volcanic rocks older than the consistency of churning wet concrete summit cone form all but the highest -that have swept down all of the river slopes visible from other directions valleys that head on the volcano. and can be recognized by their craggy Debris flows have also formed by appearance, caused by deep glacial Figure 1. Map showing areas innundated collapse of unstable parts of the by mudflows (black)from Mount Rainier erosion. The stair-step or banded look volcano without accompanying in the last 5, 600 years. of these older rocks is caused by ice eruptions. Some debris flows have and landslides having cut into the traveled as far as the present margin Very little of the young lava can lavas and exposed their dense of Puget Sound, and much of the actually be seen, because it is almost interiors, that form cliffs, and their lowland to the east of Tacoma and the completely concealed by snow and rubbly tops that form the intervening south of Seattle is formed of pre­ ice. The summit lava cone is most ledges, often covered with snow. historic debris from Mount Rainier clearly recognized from the northeast Many of the lavas that make up upper (Figure 1). (Sunrise), where it floors the large Mount Rainier overlie, and are thus The northeast part of Mount Emmons and Winthrop Glaciers that younger than, a lava that erupted Rainier slid away about 5,600 years slope smoothly up to Rainier's summit 40,000 years ago (as measured by ago as part of a catastrophic collapse (Figure 2). Broad lobes on the radiometric techniques), but are older similar to, but much larger than, that glaciers' surfaces show the locations than the 5,600-year-old collapse of May 18, 1980 at Mount St. Helens. of the youngest lavas from Rainier's event. Many of these lavas were Debris from this collapse created the summit, now buried under hundreds erupted during the last great ice age. volcano so that any new earthquake 1t-5,600-year-old Java cone and .. - swarms can be interpreted quickly and Po crater~ ~--~ Figure 2. View ofMount accurately. Burled edge of 5,600-year-old collapse Rainier from the northeast showing The potential hazards posed by post-5,600-year-old Mount Rainier led to its inclusion as lava cone and crater; one of sixteen volcanoes worldwide buried edge of to be designated Decade Volcanoes. collapse crater The Decade Volcano initiative is part (hachured line), and of a United Nations program aimed older volcanic rocks at better utilizing science and emer­ (left and right of the summit and in the gency management to reduce the se­ lower foreground). verity of natural disasters. The De­ cade Volcanoes are the focus of coor­ dinated earth-science studies and land-use planning to learn the best ways to reduce the risks to life and Lavas on Rainier's lower slopes summit, but could escalate in size and property from volcano-related haz­ formed flows to several hundred feet intensity, perhaps leading to a release ards. Products from Decade Volcano thick that now hold up most of the of new magma (hot, molten rock). studies at Mount Rainier will include ridges radiating from Mount Rainier. Depending on the amount of magma updated maps showing the areas and Many of the measured eruption ages released, the eruptions could have levels of hazards, maps showing the cluster in the interval 130,000 to relatively minor effect on the locations and ages of Rainier's lavas 90,000 years ago, perhaps indicating surrounding area or could produce and debris flow deposits, and reports a period of particularly voluminous large, destructive floods and debris on the style and size of selected erup­ volcanic activity. flows, affecting areas far from the tions and on the structure and makeup volcano. The shaking by earthquakes There is nothing to suggest that of the rocks forming Mount Rainier or explosions wi II also dislodge volcanic activity has ended at Mount and its underpinnings. Rainier. Mount Rainier will surely masses of unstable rock; the resulting erupt again, and this will affect people rocks I ides could damage Park -Thomas W Sisson who live in the surrounding areas or facilities. Particularly large landslides who visit Mount Rainier National could also create destructive, far­ Park. Experience at other volcanoes traveling debris flows. indicates that renewed eruptions will There is no immediate indication likely be preceded by weeks or ofrenewed activity at Mount Rainier. months of small earthquakes centered However, the rapidly increasing beneath the volcano. These population in the southern Puget earthquakes can be accompanied by Sound region, and the expansion of swelling or other changes in the shape communities near Mount Rainier, has of the volcano, as well as changes in led to an increased overall risk to ground temperatures and the amount people and property once activity re­ and type of gas released from the sumes. Scientists from universities and For additional information about volcano. Earthquakes at Mount the govenment have responded by volcano hazards at Mt. Rainier or Rainier and other Cascade volcanoes increasing the level of monitoring at other volcanoes, contact: are monitored by the University of Mount Rainier. New studies have Washington and the U.S. Geological U.S. Geological Survey been started aimed at learning the Cascades Volcano Observatory Survey (USGS), and the volcanoes' style and size of Rainier's past erup­ 5400 MacArthur Blvd. shapes are measured regularly by staff tions to predict better the nature of Vancouver, Washington 98661 of the USGS's Cascades Volcano future activity, mapping the locations Observatory, located in Vancouver, of particularly weak hydrothermally­ Telephone: (360) 696-7693 Fax: (360) 696-7866 Washington. altered rocks that would be more E-mail: [email protected] An eruption would probably begin prone to collapse, and determining lnternet:http://vulcan. wr. usgs.gov/ with small steam blasts located at the the structure of the rocks below the November 1995 Open-File Report 95-642 .