Mt. St. Helens Serves as Lab for Understanding Volcanoes
From the south, snow-covered Mt. St. Helens looms proudly under a fleecy halo of clouds, rivaling the majestic beauty of neighboring Mt. Rainier, Mt. Hood and Mt. Adams. Salmon fishermen dot the shores of lakes and streams in the mountain’s shadow, trucks loaded with fresh-cut timber barrel down back roads and deer peer out from stands of tall fir trees.
But, from the north, the mountain mutates into a hollow, steep-sided basin, its contents spewed across the landscape by the collapse of one side. A huge ash-gray mound rests at the crater’s bottom, surrounded by barren cliffs. This is the other, darker face of Mt. St. Helens.
Experts do not know exactly why volcanoes erupt and just what causes molten rock deep in the Earth to begin rising after hundreds or even thousands of years. However, Mt. St. Helens’ eruption has taught geologists invaluable lessons about how volcanoes work. Such information will be crucial in saving lives and property when other dormant volcanoes in the northwestern United States--and around the world--re-awaken, as geologists predict they someday will.
Dormant Volcanoes Monitored
Since 1912, scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory have pioneered the study of volcanoes through work on Mauna Loa and Kilauea volcanoes on the island of Hawaii. In Vancouver, Wash., scientists at the Cascades Volcano Observatory are studying the after-effects of Mt. St. Helens’ cataclysmic eruption as well as monitoring a number of other now-dormant volcanoes in the western United States.
A new traveling exhibition, “Inside Active Volcanoes: Kilauea and Mt. St. Helens,” at the Smithsonian’s National Museum of Natural History in Washington, D.C., examines what scientists know about these awesome natural forces. The exhibition, organized by the Smithsonian Institution Traveling Exhibition Service and the Museum of Natural History in cooperation with the Geological Survey, reflects research from the last 75 years.
“Volcanic eruptions are much more complicated events than most people realize,” said Steven Brantley, a geologist at the Cascades Volcano Observatory and co-curator of “Inside Active Volcanoes.”
“Most people assume that Mt. St. Helens merely erupted, when, in fact, the main eruption was preceded by an earthquake which triggered a massive landslide. Lava flows, lateral blasts, avalanches, debris flows and floods are only a few events associated with active volcanoes which need to be studied,” he said.
Volcanic Rock Collection
A primary goal of such study is to establish hazard assessments for individual volcanoes, Brantley said. By examining deposits from eruptions, geologists can determine the frequency of a volcano’s eruptions over thousands of years, how long the eruptions lasted and the extent of their power. At the Smithsonian Museum of Natural History, a large collection of volcanic rocks from around the world serves as a valuable resource for studying volcano characteristics.
“Hazard assessments are used in long-range land use and evacuation planning,” said Dan Miller, a geologist at Cascades Observatory, which was established in 1981 after the eruption of Mt. St. Helens. “You obviously don’t want to build a nuclear power plant or city in a volcano hazard zone.”
Emergency evacuations and roadblocks set up by officials soon after Mt. St. Helens became active were instrumental in saving several hundred lives, he noted.
Although both are volcanoes, Mt. St. Helens and Kilauea have very different shapes because of the chemical composition and temperature of their magmas--the mixture of molten rock, tiny crystals and gas inside a volcano. (Magma is called lava once it breaks through the Earth’s surface.)
Kilauea’s magma contains less silica, giving it a higher temperature and causing its lava to flow like molasses.
Magma High in Silica
Mt. St. Helens’ magma is high in silica, making it cooler and too thick to flow and giving the volcano high, steep sides. “Composite” volcanoes, as Mt. St. Helens and others like it are called, are much more likely to explode because their sticky lava traps volatile gases, building up enormous pressure.
Kilauea’s lava flows readily, spreading away from the volcano in broad, thin layers, forming low “shield” mountains. Surprisingly, most of the lava from Kilauea comes not from the volcano’s summit but from fissures stretching down the volcano’s flanks, called rift zones. These lava flows rarely threaten lives because they move slowly, allowing people to get out of the way as they destroy everything in their path.
“Both volcanoes are believed to contain shallow reservoirs of magma inside,” Richard S. Fiske, a Smithsonian volcanologist and exhibition co-curator, said. “When magma moves, it fractures rock inside the volcano, causing small earthquakes.”
In the weeks before Mt. St. Helens erupted, seismographs recorded thousands of minor earthquakes inside the volcano. As a result, geologists monitoring the area were able to issue a warning that the mountain could soon erupt.
Mountains Change Shape
Filling and emptying of the volcano’s magma reservoir can cause it to swell or shrink, Fiske said. By using electronic tiltmeters and surveying equipment, geologists can measure even the smallest change in a mountain’s shape. Before its explosion, Mt. St. Helens’ northern flank was moving outward at a rate of five feet a day.
As a pocket of magma is emptied inside Kilauea, the upper surface of the ground can collapse, forming a crater. Lava lakes are created in Hawaii when such a crater fills with molten lava.
Kilauea’s magma originates deep beneath the Pacific Plate of the Earth’s mantle at a “hot spot” and rises buoyantly toward the surface, Fiske said. Over millions of years, as the plate drifted, a chain of underwater mountains hundreds of miles long was created. The Hawaiian Islands are the largest and most recent additions to this chain. Old volcanoes die as the slow moving plate carries them away from the hot spot.
Mt. St. Helens is above a seam where two plates meet. Magma is generated in the region where the plates meets.
Volcanologists typically learn about volcanic processes by studying deposits often thousands of years old, but the eruption of Mt. St. Helens provided a rare chance to observe how these deposits were created.
Tons of Magma Ejected
Geological Survey volcanologist Richard Hoblitt used photographs of the eruption to study the mechanics of “pyroclastic flows,” caused when a blast of volcanic gas ejects tons of new magma and broken fragments of older rock down a volcano’s flank.
Mixed with hot gases, pyroclastic currents hug the ground like a fluid and can move as fast as 450 m.p.h., burying everything in their path. Steam-blast explosions occur when these super-heated flows meet bodies of water. By studying new flow deposits, geologists are better able to judge how these deposits change over hundreds and thousands of years and recognize them more easily at other, older sites.
“Lahars” are also under study at Mt. St. Helens. These are mud flows of water, rock and loose volcanic fragments that move swiftly, causing massive destruction and erosion. Snow and ice at the top of a volcano, melted by an eruption, can set off a lahar.
“What begins as a flood can quickly become a devastating mud flow as it picks up more and more debris,” Geological Survey volcanologist Thomas Pierson said. “Lahars contain more than 50% (debris)--sand, trees, large boulders, even cars and houses--and have greater momentum than a flood.”
Lateral Blast a Rarity
Despite a hazard assessment finished in 1978 and a volcano emergency plan set up well in advance of its eruption on May 18, 1980, Mt. St. Helens still took most people by surprise. No one could have predicted the massive slump of the volcano’s northern flank and subsequent lateral blast, one of only two such known events in the last 40,000 years.
The blast killed dozens of people and sent tons upon tons of volcanic debris down the north fork of the Toutle River, which flows into the Columbia River.
“It was the largest debris avalanche in recorded history,” Brantley said. “A $58-million dam is now under construction to trap volcanic sediment from the resulting avalanche debris and prevent (it) from clogging the Toutle River channel downstream.”
Despite precautions and extensive planning, volcanoes will always hold a few surprises, which scientists are committed to reducing to a bare minimum. In this quest, researchers have a vested interest.
One of the victims of Mt. St. Helens was David A. Johnston, a Geological Survey scientist monitoring the north flank of the volcano at a point more than five miles away. In a fitting tribute, the Cascades Volcano Observatory has been dedicated to his memory.
