Geology of the National Parks

Textbook 3.3: More Plate Tectonics, Mt. St. Helens


Mt. St. Helens & Volcanic Hazards; More to Worry About

Map of US with Mt. St. Helens, Washington highlighted
Credit: R.B. Alley

Mt. St. Helens, in southwestern Washington, was in some ways the queen of the Cascades Range. Beautifully symmetric, snow-capped, it had been called the Fujiyama of the Pacific Northwest. Scores of people flocked to St. Helens’ flanks to hike, camp, ski, and generally enjoy the environment. And all that changed in 1980.

Mt. St. Helens has also been the most active of the Cascades volcanoes over the most recent centuries. In early 1980, the volcano clearly was “waking up”. Earthquakes shook it almost continuously, including special “harmonic tremors”, similar to those sometimes caused by fluid flow in pipes, which showed that liquid rock was moving up from below. Small eruptions occurred, and hot springs and fumaroles (steam or gas vents) became increasingly active. The north side of the mountain was bulging, blowing up like a balloon as the magma moved into it. Scientists were scrambling to study the volcano, and predict its course. They recommended evacuation for safety, and most people (but not all, including some scientists) were moved out of the way. Penn State professor Barry Voight warned that the huge bulge on the north side of the volcano would fail, unleashing a giant landslide and a devastating eruption.

Eruption of Mt. St. Helens in 1980.
Eruption of Mt. St. Helens in 1980.
Credit: USGS

On the morning of May 18, 1980, Professor Voight’s prediction came frighteningly, awesomely true. The bulge failed. A large earthquake either caused, or was caused by, failure of the north side of the mountain in a giant landslide. Like pulling the cap off a hot, well-shaken soda bottle, the liquid beneath flashed into froth, driving an eruption 12 miles (20 km) high. A shock wave knocked over full-grown trees in an area 20 x 10 miles (32 x 16 km). The landslide eventually poured more than 100 million cubic yards of rock material down the Toutle and Cowlitz Rivers, raising the floor of the North Fork of the Toutle as much as 600 feet (200 m), and sweeping roads and houses downstream, with the debris reaching and clogging the shipping channels of the Columbia River. The Toutle floor now sat higher than the smaller streams that fed it, and lakes began to form; only quick work by the Army Corps of Engineers prevented those lakes from overtopping the mud damming them, cutting quickly down through it and releasing further floods.

All told, the Corps of Engineers spent $250 million clearing shipping channels and doing other critical work. Approximately 60 people were killed in the blast and landslide; some were buried under hundreds of feet of steaming mud and their bodies were never recovered.

President Jimmy Carter scowled at the disaster from a helicopter. Disaster planners pontificated. And in the shadows of the other Cascades volcanoes, people continued building houses in regions of known volcanic hazard.

The Mt. St. Helens Volcanic Memorial today has little in common with conditions pre-1980. The center of the volcano was more than 1/2 mile (nearly 1 km) lower after the eruption than before, with the missing rock spread over the surrounding countryside, forming a visible layer as far as 900 miles (1500 km) away. (Professor Alley and his wife Cindy were driving in Alberta, Canada during the summer of 1980, on a great, seven-week, see-the-national-parks-in-a-Chevette-with-a-tent honeymoon. A secondary eruption of Mt. St. Helens put enough ash in the air to halt traffic because of reduced visibility, hundreds of miles from the volcano.) Many of the trees knocked over by the blast still lie there—hundred-foot-long toothpicks pointing in the direction of the searing winds of the blast. Among these dead trees, however, salmonberry and fireweed and young firs are pushing skyward, elk are grazing, and coyotes search for rodents. In some places, salvage-logging of the downed trees was allowed. In some of those places, it appears that erosion has run amok, large gullies have developed, and the return of vegetation has been greatly slowed. In the crater of the volcano, a new lava dome is forming, squeezing slowly upwards like toothpaste from a very hot tube, and amazingly, a glacier sits behind it, fed by the great snowfall and the avalanches down into the crater. At night, the dome glows dull red. (It may seem weird that we focus on an event from before most of you were born, from 1980, when larger eruptions have happened more recently. But, St. Helens is the largest recent eruption in the lower-48 of the US, the easiest eruption site to get to and observe, and it really is awesome. The elder Alley daughter, Janet, was a ranger there one summer, and recommends that you take in Ape Cave if you visit, but the real goal is to see just how immense the eruption's effects really were.)

Types of Volcanoes

Volcanoes occur where melted rock rises to the Earth’s surface. Almost all volcanoes are associated with one of three settings—pull-apart margins (spreading ridges), push-together subduction zones, and hot spots. We’ve already met the volcanoes at spreading ridges, where low-silica basalt is erupted, and those producing higher-silica andesite at subduction zones.

Hot spots are creatures of another type in the zoo of Earth’s features, and especially interesting creatures at that. Deep in the warm, soft, convecting mantle of the planet, in some places a rising tower of hot rock forms and then lasts for quite a while. Some of these rising towers may come all the way from the base of the mantle where it meets the core; others may start shallower. (To see something that looks vaguely like the formation of such a hot spot, go back and view again the “lava lamp” film of Dr. Anandakrishnan in the introductory material to this unit.)

As the lithosphere drifts overhead, the hot spot may “punch through” to make a volcano. Then as the lithosphere carries that volcano away, the hot spot punches through a new place to make a new volcano. Rising melt behaves a little bit like people driving cars, who use one road or the other but not the lawn in-between; hot spots often make a string of separate volcanoes rather than a continuous line or ridge, by coming up through one hole in the lithosphere for a while and then switching to a different one. Hot spots bring melt from deep in the mantle, and so normally make basaltic volcanoes (it takes fairly subtle and sophisticated chemistry to tell the difference between hot-spot basalt and sea-floor basalt from spreading ridges). However, where a hot spot pokes through a continent rather than through sea floor, silica from the continental rocks may mix with the melt to increase its silica content, as at Yellowstone.

The Hawaiian hot spot has produced a string of volcanoes, shown here, as the Pacific sea floor has moved over the rising hot rock of the hot spot. Diagram described thoroughly in the text.
The Hawaiian hot spot has produced a string of volcanoes, shown here, as the Pacific sea floor has moved over the rising hot rock of the hot spot.
Credit: R.B. Alley

When a new hot-spot first rises from below, the top must push through the mantle and crust, and the resistance of the stuff in the way of the rising column causes its top to spread out like the head of a thunderhead, or of a mushroom cloud from an atomic bomb, or of a blob in a lava lamp, and for the same reasons. When that wide head reaches the surface, immense lava flows can be produced that spread across state-sized areas and bury them hundreds of feet deep. Much of central and eastern Washington and Oregon is buried by the “flood basalts” from the head of the Yellowstone hotspot. As the continent has moved across the hot spot after it reached the surface, a string of volcanoes erupted including Craters of the Moon National Monument in Idaho. The hot spot now fuels Yellowstone (which is why it is called the Yellowstone hot spot…a lot of this stuff isn’t that difficult!).

The flood basalts from the Hawaiian hot spot have been subducted beneath the Aleutians, but the string of volcanoes that has formed since from the hot spot can be traced from the Aleutians south all the way to the volcanically active big island of Hawaii. A new volcano, Loihi Seamount, is growing undersea just off Hawaii as the big island slowly drifts away, and Loihi will someday (not in our lifetimes!) be the new, volcanically active “big island” while the volcanoes of the current big island die and the island slowly erodes away.

So, melt can leak up from below to feed volcanoes at spreading ridges, at hot spots, and above subduction zones. But very different volcanoes develop: sea floor from spreading ridges; flood basalts and then wide, not-very-steep Hawaii-shaped volcanoes from hot spots; and, steep Mt. St. Helens-type volcanoes above subduction zones. The type of volcano that develops at a place depends on a host of factors: temperature, composition, supply of melt, duration of supply, and several others. We will focus on two: composition (how much silica) and volatile content (mostly how much water, although carbon dioxide, hydrogen sulfide, and other compounds that are gaseous under earth-surface conditions may be present and classified with volatiles). Silicon and oxygen get together in melt to form the material we call silica. Left to itself, each silicon atom will be surrounded by four oxygen atoms, which form a tetrahedron (a little pyramid). But, give them a little time, and the tetrahedra will start sticking together, or polymerizing, into chains and sheets and bigger clumps, with some oxygens serving in more than one tetrahedron. If these lumps get big enough, they are minerals and the melt has solidified.

When the lumps are present but not too big, the melt is like lumpy oatmeal—it doesn’t flow very well. There are three ways to get rid of the lumps: make the melt really hot; fill the melt with iron, magnesium and other elements that interfere with the tetrahedra polymerizing; or, fill the melt with volatiles that interfere with the tetrahedra polymerizing. When polymerization is low, the melt flows easily. Lava comes out of the volcano quietly, without making big explosions, and flows easily and far from the mouth of the volcano. In extreme cases, flows may be nearly horizontal and cover much of a state, as in the flood basalts. If the melt spreads almost as easily as flood basalts, the lava will have very slight slopes of only a few degrees, forming shield volcanoes (they look like a warrior’s shield lying on its side) such as Hawaii. Hawaiian lavas and flood basalts flow easily because they are hot and are high in iron and magnesium.

When volatiles remove the lumps, a different situation develops. This is because the volatiles will only stay in the melt under high pressure. Just as a bottling company can force CO2 into the water of a soft drink under high pressure, but the CO2 escapes as the pressure falls when you open the can, the water and CO2 and other volatiles stay in the melt under high pressure down in the Earth but escape when the melt gets close to the surface.

Silica-rich melts usually form with many volatiles. Remember that in subduction zones, wet sediment dragged down the trench releases water (and carbon dioxide and others) that promote melting. When the melt (called magma when it is in the Earth and lava when it reaches the surface) nears the surface, the lower pressure allows the volatiles to bubble off and escape into hot springs, geysers, etc. (Note that most of the fluids that come out of such hot springs are rainwater that has circulated down into the earth, but some of the fluids may be “juvenile” waters from the magma below.) Silica-rich, relatively cool lava that has lost its volatiles flows only with great difficulty. It may emerge from the volcano and flow a little ways as a very thick, slow-moving, steep flow. It may not even flow, but simply form a dome directly over the volcanic vent. And, it may “plug the system” when it solidifies. Then the stage may be set for a big explosion.

The next melt that rises in the volcano cannot follow the same path, because hardened lava above prevents escape. The gases are trapped, and pressure builds up. The volcano is like a hot, shaken pop bottle. If the top is removed, either by an opener (say, a landslide as in the case of Mt. St. Helens, or a crack opened by an earthquake) or just because the pressure becomes great enough to blow the top off, the sudden release allows the soda, or the magma, to come foaming out. A good champagne may fountain to many times the bottle’s height, and blast the cork across the room. A powerful volcano may blast ash higher than jet flight paths. The melt really does get foamy, and that foam hardens into little glass shards. The ash layer deposited by Mt. St. Helens, which stopped drivers hundreds of miles away, was mostly of such little glass shards, although torn-up bits of the former volcano were also included.

The andesitic volcanoes of the Ring of Fire are typically stratovolcanoes, formed of alternating layers of thick lava flows and of pyroclastics—things thrown through the air by the volcano. The steepness comes from the flows, which cannot get far from the vent. Some of the andesitic volcanoes, including the rebuilding Mt. St. Helens, include plug-dome elements, the oozing lava staying right above the vent.

So, the major volcanoes for our purposes are the quiet, basaltic shield volcanoes of hot spots, the quiet basaltic rift volcanoes of spreading ridges, and the steep, scenic, explosive, andesitic volcanoes of the Ring of Fire. Other types exist, notably cinder cones thrown up by typically minor eruptions tossing pyroclastics short distances. Also, hot spots or rifts trying to poke through continental rather than oceanic crust may produce explosive silica-rich volcanoes. But if you understand shields and stratovolcanoes, you will be a long way toward understanding volcanism.

Volcanic Hazards and Prediction

People who live near volcanoes should be worried about them. Volcanoes can do much damage. The volcanic-triggered landslide that buried Armero, Colombia in 1985, and the eruption of Mt. Pelee on the island of Martinique in the Caribbean in 1902, each killed about 30,000 people. Other volcanic disasters bring the human death toll to perhaps 200,000 over the last few centuries. Compared to war, disease, or even automobile accidents, this is not a terribly high toll; however, the 200,000 people directly involved almost certainly would have appreciated enough warning to get out of harm’s way. One of the goals of modern geology is to predict volcanic hazards and to save lives and property by doing so.

There are many hazards to worry about. These include:

  • Pyroclastic flows. Often, a volcano will produce a dense mixture of ash and hot gases (up to 1500oF or 800oC, and including poisons such as hydrogen sulfide). This potentially deadly mix is either forced away from the volcano (the lateral blast released by the landslide on Mt. St. Helens) or forced upward to then collapse and flow under gravity, at speeds up to hundreds of miles (hundreds of km) per hour. The deaths on Martinique were caused by such a “glowing cloud” (nuee ardente in French, where the only survivor in the whole city of St. Pierre was a lone man locked in a heavily built prison.)
  • Pyroclastics. If the heat and gases don’t get you, the rocks might. People have been killed by rocks up to car-sized or bigger, called bombs, that were thrown from volcanoes. Having a car-sized rock fall on your head from a great height is not recommended. Even fine-grained ash deposits may bury and kill nearby crops. Jet aircraft are endangered by flying into ash at high speeds. From 1980-1995, ash caused an estimated $200 million in damage to the 80 aircraft that flew into eruption clouds, mostly over the Pacific. Of those, seven lost engine power and came close to crashing. Improved monitoring of eruption clouds, to provide warnings and steer the airplanes into safer air, has greatly reduced damages since then. However, to avoid crashes from the 2010 eruption of Eyjafjallajökull in Iceland, literally millions of passengers were stranded in Europe and elsewhere as flights were suspended over weeks.
  • Poisonous gases. Sometimes, a volcano will smother or poison victims without burning them up. Pompeii and Herculaneum, the cities entombed by the eruption of Vesuvius in the year 79, have proven to be archaeological treasures, but certainly would be considered tragedies by the many people killed there and by their relatives. The people apparently were killed before they were buried, and they were not “fried” by intense heat; they probably were killed by poisonous gases and then buried by falling ash. Lake Nyos in Cameroon rests in a volcanic crater. Volcanic CO2 feeds into the bottom of the lake, but the lake typically remains stratified and does not mix. The CO2 thus builds up in the deep waters. In 1986, the lake overturned, perhaps because a landslide from the crater wall temporarily mixed the water at one end. The escaping CO2 made a great fountain, filled the crater, and then flowed down outside the crater, killing about 1700 people through some combination of suffocation and poisoning. The lake now is re-stratified and building up CO2 again; schemes to vent the CO2 safely through a huge pipe are working, but additional work may be needed to prevent a repetition.
  • Landslides and mudflows. These are less dramatic, but more dangerous cumulatively, than the explosive events. Most of the andesitic volcanoes are steep, and many are capped by very large glaciers. Mt. Rainier, for example, has 25 times as much glacier ice as Mt. St. Helens had, ready to melt and trigger mudflows after even a minor eruption. The tragedy at Armero arose from a minor eruption that triggered a big landslide. It is worth noting that Armero was built on a known, older debris-flow deposit.
  • Tsunamis. A large undersea eruption may move a lot of water. This water movement may form into a tsunami, a long, low wave that moves very rapidly. When a tsunami nears a shore, the water “piles up” into a short, steep wave that may be 100 feet high. Such waves, which also can be caused by landslides or earthquakes, may affect coasts hundreds of miles (or kilometers) from the source. The largest eruption of historical times, that of Krakatau in Indonesia in 1883, killed thousands of people on neighboring islands in this way. The great Tohoku earthquake of 2011 in Japan made a tsunami that was over 130 feet high (40 m) at its worst, where it came ashore where people lived. We’ll look more at tsunamis soon.
  • Climate change. A large volcanic eruption puts a lot of sulfur gases into the stratosphere, together with ash and other materials. The sulfur eventually forms sulfuric-acid droplets, which typically remain aloft for one to a few years before falling out across much or all of the planet. While they are aloft, the sulfuric-acid droplets block some of the sunlight, cooling the planet a little. This can produce killing frosts during normal growing seasons, leading to widespread starvation in sensitive regions. The Tambora eruption of 1815 is associated with the starvation “year without a summer” of 1816. Ice cores from Greenland, Antarctica and elsewhere record volcanic fallout (the ash and sulfuric acid are preserved in the ice) and the temperature (from certain indicators including the isotopic composition of the ice), and show that big eruptions typically are accompanied by a cooling of a good chunk of a degree for a year or two, with more cooling in some places and seasons, and less in others. This isn’t a huge change, but when one killing frost can cause starvation, it may be too much. If many volcanic eruptions occurred in a short period of time, it might produce major climate changes; however, volcanism doesn’t seem to get organized—there is no way for a volcano in Alaska to tell a volcano in Indonesia that it is time to erupt. (Volcanoes also release carbon dioxide, which tends to warm the climate, as we will see later in the course. However, not a lot of carbon dioxide comes out in one volcanic eruption. If all the world’s volcanoes started erupting a lot faster, maybe twice as fast as normal, enough carbon dioxide would be released in “only” a few hundred thousand years to start warming the world notably. Over really short time scales of years to centuries, more volcanism would cause more cool years, because the sun-blocking effect would be much bigger than the warming-from-carbon-dioxide effect. If you turned up volcanoes, you would get first cooling and then warming.)

Predicting Eruptions

So, we wish to predict when and where volcanoes will endanger people. Various things can be done. For problems such as climate change, the best we can do is to know that every few years or decades some region is likely to experience difficulties with crop production because of eruptions. The solutions are either to maintain a little excess food to feed those endangered people, or to ignore them and figure that some will starve to death. (Many other climate changes, including droughts, give us the same choice. Despite the apparent silliness—either we stockpile food and figure out how to distribute it to the needy, or we let people starve to death—it is surprising how often starving to death is the outcome.)

For tsunamis, an operational warning system now exists around the Pacific and in some other places. If a tsunami is detected, or if seismographs detect the shaking caused by a large earthquake, landslide, or volcanic eruption, warnings are relayed to coastal regions likely to be affected, in time to allow evacuation. Such a system is being assembled for the Indian Ocean following the tragic 2004 earthquake-generated tsunami that killed approximately 250,000 people.

One way to avoid volcanic hazards is to stay out of harm’s way. Geologists can map regions where large pyroclastic chunks have fallen, or where landslides have occurred, with great confidence. Using carbon dating of logs caught in debris flows, or tree-ring dating of those growing on landslides (just hang on; explanation of such things will come later), we can determine the recurrence interval—how often do such disasters happen? Today, whole subdivisions are being built around Mt. Rainier National Park in the growing Seattle-Tacoma region that have a danger of destruction by landslide many, many times higher than their danger of destruction by fire. Roughly 200,000 people work, and more than 100,000 people live, on debris-flow deposits less than 10,000 years old, with more people coming. The homeowners will all carry fire insurance, but few if any are insured against the volcano; presumably, if they survive the next volcanic disaster, these people are counting on disaster aid from the rest of the country to bail them out.

(Much argument is attached to sending disaster aid for predictable events even if they are not very common. Should those who wish to live in beautiful but risky areas carry insurance to pay for their gambles? Increasingly, planners are saying “yes,” and much effort is being devoted to quantifying the hazards so that insurance rates can be set wisely. This applies to such things as hurricanes along coasts, earthquakes along faults, and floods along rivers. Geologists have an important role to play in learning hazards and thus setting rates.)

With sufficient care, volcanic eruptions can be predicted with some confidence. Volcanoes usually give off many signals before an eruption: the ground swells as magma moves up; the moving magma and the swelling ground create earthquakes and especially the distinctive harmonic tremors of fluid flowing in a pipe; small eruptions occur; gaseous emissions increase as the magma nears the surface and then cease if the system becomes plugged and builds up pressure for an explosion. A monitoring program of seismographs to detect earthquakes, repeat surveying of laser reflectors set on the mountain to watch for deformation patterns, gas sampling, and perhaps photographic or other sensors to watch for landslides, can track a volcano’s behavior and allow timely warning. Monitoring of ground shape from space can even see the changes in volcanoes as magma moves under them. The eruption of Mt. St. Helens was predicted well enough to save hundreds of people including the residents of a YMCA camp. The eruption of Mt. Pinatubo in the Philippines in 1991, which heavily damaged the U.S. military bases there, was predicted accurately, allowing timely evacuation and saving tens or hundreds of thousands of lives of residents and military personnel.

The burden of predicting eruptions is very high. Imagine telling an Air Force general to abandon his or her assigned duty post, spend a few hundred thousand dollars to move tens of thousands of people, and then having nothing happen—the general, and all of those people, would be very unhappy. Imagine instead deciding to wait another day to be sure, and having all of those people (possibly including you) killed. Important as this is, predicting disasters is not for the faint of heart.

The Mt. St. Helen’s eruption was a small one compared to many others. Each of the major eruptions of Yellowstone moved about 1000 times more material than Mt. St. Helens did, and Yellowstone’s eruptions were not the largest known. Small eruptions are more common than large ones. But, eruptions ten times as big as Mt. St. Helens are perhaps five times as rare, but not ten times as rare. This means that, as for earthquakes, most of the “work” done by volcanoes is achieved by the few big ones, not the many little ones.