Mount Rainier: the Magnificent Volcano of the Cascades archival/download.php... · Hill, R. L. 2004...

Open Access e-Journal Earth Science India- www.earthscienceindia.info Popular Issue, IV (IV), October, 2011, p. 1-8

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Mount Rainier: the Magnificent Volcano of the Cascades

Arun Kumar

Mount Rainier is a good example of a stratovolcanoe. These are composite volcanoes, typically steep-sided, symmetrical cones of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs………….Evolution of the Cascades begins ~50 Ma when the oceanic Juan de Fuca plate began to subduct under the North American plate, scraping the upper layer of the oceanic crust thus uplifting the coastal mountains

The 700 mile long Cascade mountain range commonly known as the Cascades

extend from southern British Columbia, Canada in the north through Washington and Oregon

to northern California in the south (Figure 1). This range runs ~100 to150 miles inland

almost parallel to the Pacific coast and comprises the only chain of active volcanoes in the

continental USA. Among these volcanoes seven have erupted during the past 250 years and

enormous explosions occurred at Lassen Peak and Mount St. Helens during past few decades

(Hill, 2004). This region is comprised of alpine landscape (Figure 2) dotted with a series of

snow-capped volcanoes in north-south direction, ranging from Mount Baker in Washington

in the north to Mount Lassen in California in the south. Other significant geological features

of the Cascades include glaciers (Figure 3; 8), rivers and streams, lakes (Figure 4), valleys,

canyons and gorges, craters, caves, hot springs, waterfalls, cinder-cone buttes, and mountains

of basalts and other volcanic rocks (Figures 2 and 5) etc.

According to the United States Geological Survey (USGS) Cascades Volcano

Observatory website volcanoes are mountains built by the accumulation of their own eruptive

products like lava, bombs (crusted over lava blobs), ash flows, and tephra (airborne ash and

dust). Most commonly a volcano is a conical hill or mountain built around a vent that

connects with reservoirs of molten rock below the surface of the Earth. The process of

volcano formation is described in the website as, “Driven by buoyancy and gas pressure the

molten rock, which is lighter than the surrounding solid rock, forces its way upward and my

ultimately break through zones of weaknesses in the Earth's crust. If so, an eruption begins,

and the molten rock may pour from the vent as non-explosive lava flows, or it may shoot

violently into the air as dense clouds of lava fragments. Larger fragments fall back around

the vent, and accumulations of fallback fragments may move downslope as ash flows under

the force of gravity. Some of the finer ejected materials may be carried by the wind only to

fall to the ground many miles away. The finest ash particles may be injected miles into the

atmosphere and carried many times around the world by stratospheric winds before settling

out.”


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Figure 1: Locations of the volcanoes of the Cascade Range with their photographs (Lyn Topinka, 1996)

(http://vulcan.wr.usgs.gov/Volcanoes/Cascades/ImageMaps/CascadeRange/cascade_range.html)

Mount Rainier (14,410 feet high and 7000 feet above its surroundings) is the highest

volcano of the Cascades, is central attraction in the Mount Rainier National Park spread over

368 square miles (Figures 6). Its height may have reached 16,000 feet, but a violent event

5,700 years ago destroyed about 2000 feet off its summit. Hill (2004) explained this event as

“A huge landslide - perhaps triggered by magma forcing its way into the volcano – swept

down the mountain’s northeast side creating a deep horseshoe – shaped cater.” During the

past 4000 years Mount Rainier has erupted four times (see Figure 4 in Kumar 2010) and it

last erupted ~ 150 years ago. It has been ‘sleeping’ since then.


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Figure 2: Alpine landscape with mountain tops of basalt flows surrounding Mount Rainier (Photo: Anita Maisuria)

Figure 3: Glaciers on top and on the slopes of Mount Rainier (Photo: Paresh Maisuria)

Figure 4: Lake Sunrise located on the northeastern slope of Mount Rainier (Photo: Paresh Maisuria)

Figure 5: Outcrop of columnar basalt on the road cut (Highway 410) near Mount Rainier (Photo: Anita Maisuria)

Due to very high temperatures deep within the Earth, subducted rocks gradually melt

and become magma. Since magma is lighter than the surrounding solid rock it rises and

collects in magma chambers and subsequently pushes through vents and fissures in the

Earth's surface leading to volcanic eruptions (Figure 7). Erupted magma is called lava.

Generally volcanic eruptions are explosive but all volcanic eruptions are not. The explosive

nature of an eruption depends on the viscosity of magma. In low viscosity magma gases

escape easily and when this type of magma erupts, it flows out of the volcano. Sometimes

pressure beneath the Earth’s surface builds up until the gases escape as violent explosions

causing the magma blasts into the air along with clouds of hot tephra from the side or top of a

volcano. The powdered rock called ash also erupts into the sky and falls back on earth to

form blankets of ash that potentially can suffocate plants, animals, and humans.

Mount Rainier is a good example of a stratovolcanoe. These are composite volcanoes,

typically steep-sided, symmetrical cones of alternating layers of lava flows, volcanic ash,

cinders, blocks, and bombs. They have a crater at their summit which contains a central vent

or group of vents. Composite volcanoes are characterized by a conduit system through which


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Figure 6: A panoramic view of downtown Seattle with Mount Rainier in background. (http://www.google.com/imgres?imgurl=http://www.atmos.washington.edu/academic/ug/images/image004.jpg&imgrefurl)

Figure 7: Diagrammatic representation of a subduction zone showing relationship of continental and oceanic lithosphere and formation of magma which eventually erupts as

volcanoes (http://myweb.cwpost.liu.edu/vdivener/notes/subd_zone.htm)


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magma from a reservoir deep in the Earth's crust rises to the surface. Such volcanoes tend to

erupt explosively. Shield volcanoes are another type of volcanoes that are built almost

entirely of a large numbers of fluid lava flows, they have a central summit vent, or group of

vents that builds a broad, gently sloping cone of flat, generally of domical shape. Some of the

largest volcanoes in the world are shield volcanoes. Such volcanoes also occur in the Cascade

Range where many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to

2,000 feet.

Geological History of the Cascades

Evolution of the Cascades begins ~50 Ma when the oceanic Juan de Fuca plate began

to subduct under the North American plate, scraping the upper layer of the oceanic crust thus

uplifting the coastal mountains. Continued subduction pushed the Juan de Fuca plate below

the Earth’s crust which melted under high temperature and pressure forming magma deep

beneath the Earth’s surface. Between 40 and 5 Ma boiling steam forced the magma to either

rise to the surface in the form of volcanoes or formed massive plutons underground that gave

rise to the formation of Western Cascades. Volcanoes of the Western Cascades are dormant

and severely eroded, and Iron Mountain the highest peak is almost half the size of the tallest

volcano of the High Cascades. During the past ~4 Ma several new volcanoes erupted in a

north-south series of higher mountains called High Cascades located several miles west of

Western Cascades. Mount Rainier (14,410 feet) located in Washington is the highest

mountain of this range.

Volcanic Hazards

The absolute serenity and unmatched splendor of the Cascades hides the region’s

extremely hazardous past. This was evident as late as on May 18, 1980 when Mount St.

Helens erupted causing more than $ 1 billion in damages and killing fifty-seven people

(Kumar, 2010). Lava flows, ash falls, mud flows, landslides and avalanches are potential

hazards related volcanoes of the region. Toxic gases (Sulfur dioxide, carbon dioxide,

hydrogen sulfide, hydrogen chloride, hydrogen fluoride etc.) emitted by volcanoes along with

steam and heated groundwater and associated acid rain can kill various forms of life

including human beings. Debris flows also known as ‘lahars’ have swept the valleys around

this volcano several times during the past 10 ka. Lahars usually move between 25 to 50 miles

per hour down the mountain valley slopes and can form without a volcanic eruption (Figure

9). They can be triggered by an earthquake or due to over saturation of rain water or from a

high rate of glacial melt in hot weather. Large landslide triggered lahars are known to occur

in the region every 500 to 1000 years, about a dozen lahars during the past 6,800 years have

filled the mountain slope valleys with extensive debris-flow deposits (Hill, 2004). Smaller

flows are quite common on Mount Rainier, during the past fifty years about 24 debris flows


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and glacial outburst flows have occurred (Hill, 2004). In addition to possible volcanic

eruption and high magnitude earthquakes debris flows also are potential hazards for the

Seattle/Tacoma region of the Washington State.

Figure 8: Major Glaciers of Mount Rainier (Modified from: Driedger, 1992, USGS Open-File

Report 92-474:

(http://vulcan.wr.usgs.gov/Volcanoes/Rainier/Maps/map_rainier_glaciers.html)


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Figure 9: Map of Lahar Hazards in the area around Mount Rainier. (http://vulcan.wr.usgs.gov/Imgs/Jpg/Rainier/Outreach/screen_poster_lahar_haz.jpg)


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Acknowledgements: I thank Paresh and Anita, my son-in-law and daughter, who drove me from Seattle to

Mount Rainier National Park on September 10, 2011, which was a beautiful sunny and cool day. We enjoyed

the splendor of Mount Rainier with its stunning natural surroundings. My eight months old grand-daughter Diya

made this a trip memorable one.

Further Readings:

Alt, D. D. and Hyndman, D. W. 2010 Roadside Geology of Washington.Roadside Geology

Series.Mountain Press, Missoula, MT, 19th Print, 288 P.

Hill, R. L. 2004 Volcanoes of the Cascades: their rise and their risks. Falcon, Guilford,

Connecticut, 1st Edition, 90 P.

Kumar, A. 2010. May 18, 1980 the day Mount St. Helens exploded: an amazing story of

destruction and recovery process of the environment. e-Journal Earth Science India.

(www.earthscienceindia.info)

United States Geological Survey (USGS) Cascades Volcano Observatory, 2011.

(vulcan.wr.usgs.gov)

About the Author

Dr. Arun Kumar is Adjunct Professor, Department of Earth Science, Carleton University 1125 Colonel By drive, Ottawa, ON K1S 5B6, Canada Email: [email protected]

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