HISTORY AND HAZARDS OF MOUNT RAINIER, WASHINGTON 

Mount Rainier is an active volcano 

that first erupted about half a million 

years ago. Because of Rainier's great 

height (14,41 0 feet above sea level) 

and northerly location, glaciers have 

cut deeply into its lavas, making it 

appear deceptively older than it 

actually is. Mount Rainier is known 

to have erupted as recently as in the 

1840s, and large eruptions took place 

as recently as about 1,000 and 2,300 

years ago. 

Mount Rainier and other similar 

volcanoes in the Cascade Range, such 

as Mount Adams and Mount Baker, 

erupt much less frequently than the 

more familiar Hawaiian volcanoes, 

but their eruptions are vastly more 

destructive. Hot lava and rock debris 

from Rainier's eruptions have melted 

snow and glacier ice and triggered 

debris flows (mudflows) - with a 

consistency of churning wet concrete 

-that have swept down all of the river 

valleys that head on the volcano. 

Debris flows have also formed by 

collapse of unstable parts of the 

volcano without accompanying 

eruptions. Some debris flows have 

traveled as far as the present margin 

of Puget Sound, and much of the 

lowland to the east of Tacoma and the 

south of Seattle is formed of prehistoric debris from Mount Rainier 

(Figure 1 ). 

The northeast part of Mount 

Rainier slid away about 5,600 years 

ago as part of a catastrophic collapse 

similar to, but much larger than, that 

of May 18, 1980 at Mount St. Helens. 

Debris from this collapse created the 

Osceola and Paradise mudflows that 

traveled down the White and 

Nisqually Rivers, reaching Puget 

Sound and pushing out the shoreline 

by as much as 10 miles. The scar 

from this collapse was a horseshoeshaped crater, about 1 .25 miles wide, 

open to the northeast. Since the 

collapse, lava flows and avalanches 

of hot lava fragments have erupted 

from the crater and largely filled it, 

forming the present summit cone of 

Mount Rainier. 

Figure 1. Map showing areas innundated 

by mudflows (black)from Mount Rainier 

in the last 5, 600 years. 

Very little of the young lava can 

actually be seen, because it is almost 

completely concealed by snow and 

ice. The summit lava cone is most 

clearly recognized from the northeast 

(Sunrise), where it floors the large 

Emmons and Winthrop Glaciers that 

slope smoothly up to Rainier's summit 

(Figure 2). Broad lobes on the 

glaciers' surfaces show the locations 

of the youngest lavas from Rainier's 

summit, now buried under hundreds 

of feet of ice. The summit itself is 

formed of two small overlapping 

craters, each about a quarter mile in 

diameter; the younger of these forms 

a nearly perfect circle of radiallyoutward-sloping lavas. The shallow 

floors of these craters are fi lied with 

snow and ice, but the raised rims are snow-free year-round because of high 

winds and because much of the 

ground is still hot. Steam or warm 

mist, at or ju st below boiling 

temperature, rises from the crater rims 

in many areas and has melted an 

intricate system of caves into the base 

of the crater-filling ice. On calm days, 

a faint odor of sulfur can also be 

smelled. The hot ground, steam, and sulfur smell , as well 

as the littleeroded shape of the summit craters 

attest to Rainier's recent activity. 

Volcanic rocks older than the 

summit cone form all but the highest 

slopes visible from other directions 

and can be recognized by their craggy 

appearance, caused by deep glacial 

erosion. The stair-step or banded look 

of these older rocks is caused by ice 

and landslides having cut into the 

lavas and exposed their dense 

interiors, that form cliffs, and their 

rubbly tops that form the intervening 

ledges, often covered with snow. 

Many of the lavas that make up upper 

Mount Rainier overlie, and are thus 

younger than, a lava that erupted 

40,000 years ago (as measured by 

radiometric techniques), but are older 

than the 5,600-year-old collapse 

event. Many of these lavas were 

erupted during the last great ice age. 
summit and in the 

lower foreground). 

Lavas on Rainier's lower slopes 

formed flows to several hundred feet 

thick that now hold up most of the 

ridges radiating from Mount Rainier. 

Many of the measured eruption ages 

cluster in the interval 130,000 to 

90,000 years ago, perhaps indicating 

a period of particularly voluminous 

volcanic activity. 

There is nothing to suggest that 

volcanic activity has ended at Mount 

Rainier. Mount Rainier will surely 

erupt again, and this will affect people 

who live in the surrounding areas or 

who visit Mount Rainier National 

Park. Experience at other volcanoes 

indicates that renewed eruptions will 

likely be preceded by weeks or 

months of small earthquakes centered 

beneath the volcano. These 

earthquakes can be accompanied by 

swelling or other changes in the shape 

of the volcano, as well as changes in 

ground temperatures and the amount 

and type of gas released from the 

volcano. Earthquakes at Mount 

Rainier and other Cascade volcanoes 

are monitored by the University of 

Washington and the U.S. Geological 

Survey (USGS), and the volcanoes' 

shapes are measured regularly by staff 

of the USGS's Cascades Volcano 

Observatory, located in Vancouver, 

Washington. 

An eruption would probably begin 

with small steam blasts located at the 

November 1995 

summit, but could escalate in size and 

intensity, perhaps leading to a release 

of new magma (hot, molten rock). 

Depending on the amount of magma 

released, the eruptions could have 

relatively minor effect on the 

surrounding area or could produce 

large, destructive floods and debris 

flows, affecting areas far from the 

volcano. The shaking by earthquakes 

or explosions wi II also dislodge 

masses of unstable rock; the resulting 

rocks I ides could damage Park 

facilities. Particularly large landslides 

could also create destructive, fartraveling debris flows. 

There is no immediate indication 

of renewed activity at Mount Rainier. 

However, the rapidly increasing 

population in the southern Puget 

Sound region, and the expansion of 

communities near Mount Rainier, has 

led to an increased overall risk to 

people and property once activity resumes. Scientists from universities and 

the govenment have responded by 

increasing the level of monitoring at 

Mount Rainier. New studies have 

been started aimed at learning the 

style and size of Rainier's past eruptions to predict better the nature of 

future activity, mapping the locations 

of particularly weak hydrothermallyaltered rocks that would be more 

prone to collapse, and determining 

the structure of the rocks below the 

gency management to reduce the severity of natural disasters. The Decade Volcanoes are the focus of coordinated earth-science studies and 

land-use planning to learn the best 

ways to reduce the risks to life and 

property from volcano-related hazards. Products from Decade Volcano 

studies at Mount Rainier will include 

updated maps showing the areas and 

levels of hazards, maps showing the 

locations and ages of Rainier's lavas 

and debris flow deposits, and reports 

on the style and size of selected eruptions and on the structure and makeup 

of the rocks forming Mount Rainier 

and its underpinnings. 

-Thomas W Sisson 

For additional information about 

volcano hazards at Mt. Rainier or 

other volcanoes, contact: 

U.S. Geological Survey 

Cascades Volcano Observatory 

5400 MacArthur Blvd. 

Vancouver, Washington 98661 

Telephone: (360) 696-7693 

Fax: (360) 696-7866 

E-mail: cvo@pwavan.wr.usgs.gov 

lnternet:http://vulcan. wr. usgs.gov/ 

Open-File Report 95-642