Volcanoes of the United States

by Steven R. Brantley

Cascade volcanoes

Volcanoes of the Cascade Range erupt far less frequently than Kilauea and Mauna Loa, but they are more dangerous because of their violently explosive behavior and their proximity to populated and cultivated areas in Washington, Oregon, and California. The 1980 eruption of Mount St. Helens in southwest Washington dramatically illustrated the type of volcanic activity and destruction these volcanoes can produce. Scientific studies of the eruption of Mount St. Helens and the eruptive histories of other Cascade volcanoes continue to improve public awareness and understanding of these potentially dangerous peaks.

In contrast to Kilauea, Cascade volcanoes erupt a variety of magma types that generate a wide range of eruptive behavior and build steep-sided cones known as composite volcanoes. In addition to basalt, andesite and dacite magmas are common.

Cascade Volcanoes.

EXPLANATION Volcano active during past 2,000 years Potentially active volcano Area of potential volcanic activity Population centers 40,000 to 100,000 100,000 to 350,000 350,000 to 1,000,000 Greater than 1,000,000 PACIFIC OCEAN WASHINGTON Mount Baker Glacier Park Seattle Spokane Mount Rainier Mount St. Helens Mount Adams MONTANA Great Falls Billings IDAHO Boise Craters of the Moon OREGON Portland Mount Hood Mount Jefferson Three Sisters Eugene Newberry Crater Crater Lake WYOMING Yellowstone Casper Cheyenne CALIFORNIA Medicine Lake Mount Shasta Lassen Peak Clear Lake Sacramento San Francisco Long Valley Caldera Coso Los Angeles San Diego NEVADA Reno Las Vegas UTAH Salt Lake City COLORADO Denver ARIZONA San Francisco Field Phoenix Tucson NEW MEXICO Albuquerque Bandera Field

These magmas are so highly viscous, or sticky, that expanding volcanic gases cannot easily escape from them. This causes a tremendous build-up in pressure, often leading to extremely explosive eruptions. During such eruptions, magma is shattered into tiny fragments (chiefly ash and pumice) and ejected thousands of meters into the atmosphere or even the stratosphere. Under the force of gravity, sometimes these fragments sweep down a volcano’s flanks at speeds of more than 100 kilometers per hour, mixing with air and volcanic gases to form pyroclastic flows. Rock fragments can also mix with water in river valleys to form lahars (volcanic debris flows and mudflows) that destroy everything in their paths.

Andesite and dacite magmas also erupt to form lava flows. Because these lavas are more viscous (“stickier”) than basalt, they tend to form thicker flows that travel shorter distances from the vent; consequently, andesite and dacite lavas typically build tall cones with steep slopes of more than 20 degrees.

Mt. Hood, Oregon. Eruptions from the volcano about 1,800 and 200 years ago from the Crater Rock lava dome formed a broad apron of rock debris on the volcano’s south side. (Photograph by Lyn Topinka.)

Mount Baker, Washington.

Eyewitness reports of small ashy plumes and active steam vents on Mount Baker dating as far back as the mid-1800’s were clear evidence that the ice-covered volcano had one of the most active geothermal systems among Cascade volcanoes. When new fumaroles and unusually dark vapor plumes appeared abruptly in March 1975, however, people in the Northwest became concerned about an impending eruption and possible avalanches and lahars from Sherman Crater, a vent just south of Mount Baker’s summit. Despite a tenfold increase in the release of heat by the volcano during the next 12 months, which resulted in extensive changes to the ice cover in Sherman Crater and produced minor releases of ash, no eruption occurred. The thermal activity was not accompanied by earthquakes, which generally precede most eruptions, and since 1976, the volcano has not showed additional signs of activity.

Mount Baker viewed to the west. Increased fumarolic activity occurred in Sherman Crater (left of summit) during the mid-1970’s.

The increased thermal activity between 1975 and 1976 prompted public officials and Puget Power to temporarily close public access to the popular Baker Lake recreation area and to lower the reservoir’s water level by 10 meters. Significant avalanches of debris from the Sherman Crater area could have swept directly into the reservoir, triggering a disastrous wave that would have caused loss of life and damage to the reservoir.

Mount Rainier towers 3,000 meters above the surrounding valleys, all of which have been swept by lahars during the past 10,000 years. Future eruptions will probably trigger similar lahars. (Photograph by David Wieprecht.)

Mount Rainier, Washington.

Mount Rainier has not produced a significant eruption in the past 500 years, but scientists consider it to be one of the most hazardous volcanoes in the Cascades. Mount Rainier has 26 glaciers containing more than five times as much snow and ice as all the other Cascade volcanoes combined. If only a small part of this ice were melted by volcanic activity, it would yield enough water to trigger enormous lahars.

Mount Rainier’s potential for generating destructive mudflows is enhanced by its great height above surrounding valleys and its “soft” interior. The volcano stands about 3,000 meters above river valleys leading from its base. Volcanic heat and ground water have turned some of the volcano’s originally hard lava into soft clay minerals, thereby weakening its internal structure. These conditions make Mount Rainier extremely susceptible to large landslides. Several have occurred in the past few thousand years, one as recently as about 600 years ago. These landslides, apparently containing great volumes of water, quickly turned into lahars as they rushed down river valleys.

Mount St. Helens, Washington.

The catastrophic eruption on May 18, 1980, was preceded by 2 months of intense activity that included more than 10,000 earthquakes, hundreds of small phreatic (steam-blast) explosions, and the outward growth of the volcano’s entire north flank by more than 80 meters. A magnitude 5.1 earthquake struck beneath the volcano at 08:32 on May 18, setting in motion the devastating eruption.

Mount St. Helens crater and lava dome viewed from the north, 1990. Inset: Close view of lava dome with new lava extrusion on top (snow-free part) from the south, 1986. (Photographs by Lyn Topinka.)

Within seconds of the earthquake, the volcano’s bulging north flank slid away in the largest landslide in recorded history, triggering a destructive, lethal lateral blast of hot gas, steam, and rock debris that swept across the landscape as fast as 1,100 kilometers per hour. Temperatures within the blast reached as high as 300 degrees Celsius. Snow and ice on the volcano melted, forming torrents of water and rock debris that swept down river valleys leading from the volcano. Within minutes, a massive plume of ash thrust 19 kilometers into the sky, where the prevailing wind carried about 520 million tons of ash across 57,000 square kilometers of the Western United States.

Stand of timber in the process of being harvested was instead knocked over by the lateral blast. An estimated 4 million board feet of timber was destroyed. (Photograph by H.H. Kieffer.)

Small eruption of gas and ash from the lava dome caused by violent release of volcanic gas or the geyser-like flashing of superhot ground water to steam. (Photograph by Dan Dzurisin.)

Mount St. Helens towers above the chaotic landslide deposit that fills a former valley to a depth of as much as 195 meters. Note many small hills atop the landslide, called “hummocks” by geologists. (Photograph by Lyn Topinka.)

The well-documented landslide at Mount St. Helens has helped geologists to recognize more than 200 similar deposits at other volcanoes in the world, including several other Cascade peaks. Geologists now realize that large landslides from volcanoes are far more common than previously thought—seventeen such volcanic landslides have occurred worldwide in the past 400 years. Consequently, when scientists evaluate the types of volcanic activity that may endanger people, giant landslides are now included, in addition to other types of volcanic activity such as lava flows, pyroclastic flows, lahars, and falling ash.

Following the 1980 explosive eruption, more than a dozen extrusions of thick, pasty lava built a mound-shaped lava dome in the new crater. The dome is about 1,100 meters in diameter and 250 meters tall.

Giant mushroom-shaped ash cloud of May 22, 1915, viewed from 80 kilometers west of Lassen Peak. (Photograph provided by National Park Service.)

Lassen Peak, California.

Long before the recent activity of Mount St. Helens, a series of spectacular eruptions from Lassen Peak between 1914 and 1917 demonstrated the explosive potential of Cascade volcanoes. Small phreatic explosions began on May 30, 1914, and were followed during the next 12 months by more than 150 explosions that sent clouds of ash as high as 3 kilometers above the peak. The activity changed character in May 1915, when a lava flow was observed in the summit crater. A deep red glow from the hot lava was visible at night 34 kilometers away. On May 19, an avalanche of hot rocks from the lava spilled onto snow and triggered a lahar that extended more than 15 kilometers from the volcano.

The most destructive explosion occurred on May 21, when a pyroclastic flow devastated forests as far as 6.5 kilometers northeast of the summit and lahars swept down several valleys radiating from the volcano. An enormous ash plume rose more than 9 kilometers above the peak, and the prevailing winds scattered the ash across Nevada as far as 500 kilometers to the east. Lassen Peak continued to produce smaller eruptions until about the middle of 1917.