Pyroclasts (or “ tephra“) are any volcanic fragment that was hurled through the air by volcanic activity. A pyroclastic eruption is one in which the great majority of activity involves fountaining or explosions. A pyroclastic deposit is the resulting layer or pile of material that has fallen to the ground by one or many pyroclastic eruptions. A pyroclastic rock is the hardened, solidified, or compressed version of an originally loose pyroclastic deposit.

Spatter (a type of pyroclast) is globs of lava thrown through the air that land while still molten. Photograph by T.T. English, U.S. Geological Survey, April 30, 1982

 The word is a combination of “pyro” or fire, and “clastic” or made up of many pieces.

Click here for more information on volcanic products!

Those definitions are not set in stone, and they mean different things to different people and to different volcanoes. One of the simpler ways to answer is that an active volcano is one that has erupted since the last ice age (i.e., in the past ~10,000 years). That is the definition of active used by the Smithsonian in their catalogs. A dormant volcano would then be one that hasn’t erupted in the past 10,000 years, but which is expected to erupt again. An extinct volcano would be one that nobody expects to ever erupt again. These are human definitions of natural things – there have been a number of eruptions from “extinct” volcanoes!

View from old carabinero's station near the road to Laguna Verde. Elevation here is approximately 4500 meters as shown on the sign. Photo by Robert Runyard, January, 1998.

The highest volcano is Ojos del Salado in Chile. It is 22,589 feet (6,887 m) tall.

However, from base to summit, the tallest would be Mauna Kea, which, when measured from its base on the ocean floor, is more than 30,000 feet high!

Photograph by J.D. Griggs, U.S. Geological Survey, January 10, 1985.

The largest volcano in the world is Mauna Loa in Hawaii.   Only part of the volcano is above the water, however, using high resolution bathymetry of the underwater slopes of the volcano, data from seismic profiling and gravity studies, and the subsidence rate to define the shape and density of the buried part of the volcano, an estimate can be calculated.   The current volume estimate is around 80,000 cubic kilometers!

The biggest eruption was at Yellowstone about 2.2 million years ago. An explosive eruption produced 2,500 cubic kilometers of ash!   (That’s about 2,500 times more ash than Mount St. Helens erupted!)  

Yellowstone has had three very large eruptions in the last 2 million years. These eruptions occurred 2.2, 1.2, and 0.6 million years ago.

The biggest volcanic eruptions…last 10,000 years

Left Panorama of Crater Lake caldera. Photograph by Steve Mattox, August 1987

Left Panorama of Crater Lake caldera. Photograph by Steve Mattox, August 1987.

Only four eruptions in the last 10,000 have been assigned a Volcanic Explosivity Index of 7. They are:

• Tambora, Indonesia 1815
• Baitoushan, China-Korea border, about 1050
• Kikai, Japan, about 4350 B.C.
• Crater Lake, Oregon, USA, about 4895 B.C.

A Big Ten list of eruptions based on explosive force and destruction in historical time would include:

1. Tambora, Indonesia 1815: VEI=7, 92,000 casualties
2. Santorini, Greece 1628 B.C.: VEI=6, unknown casualties
3. Krakatau, Indonesia 1883: VEI=6, 36,400 casualties
4. Santa Maria, Guatemala 1902: VEI=6, 6,000 casualties
5. Mount St. Helens, USA 1980: VEI=5, 57 casualties
6. Vesuvius, Italy 79: VEI=5, 3,360 casualties
7. Pinatubo, Philippines 1991: VEI=5, 932 casualties
8. Mount Pelee, Martinique 1902: VEI=4, 29,000 casualties
9. Nevado del Ruiz, Columbia 1985: VEI=3, 23,000 casualties
10. Unzen, Japan 1792: VEI=2, 15,000 casualties.

Sources of information:

Decker, R., and Decker, B., 1989, Volcanoes: W.H. Freeman, New York, 285 p.

Nuhfer, E.B., Proctor, R.J., and Moser, P.H., 1993, The citizen’s guide to geologic hazards: American Institute of Professional Geologists, Arvada, CO, 134 p.

Simkin, T., and Fiske, R.S., Krakatau 1883: The volcanic eruption and its effects: Smithsonian Institution Press: Washington, D.C., 464 p.

Simkin, T., and Siebert, L., 1994, Volcanoes of the world: Geoscience Press, Tucson, Arizona, 349 p.

The absolute number of volcanoes that exists depends on your definition: active only, active, dormant plus extinct volcanoes? And even if we decide on a definition, nobody has really counted all of the volcanoes, especially the tens on thousands on the sea floor.

The best guess is 1511 volcanoes have erupted in the last 10,000 years and should be considered active. This number is from the Smithsonian Institution book, “Volcanoes of the World: Second Edition” compiled by Tom Simkin and Lee Siebert.

The eruption of Tambora in Indonesia in 1815 killed the most people. It was a huge eruption that sent ash into the stratosphere that then spread around the world. World climate was noticeably cooler the following year, and in places it was called “the year without a summer”. Closer to the eruption itself thousands of people were killed, and due to the destruction of crops, disease, contamination of water, etc., tens of thousands more died in the years that followed. Overall about 92,000 people died as a result of the eruption. Back in 1815 there wasn’t much news coming out of Indonesia to the western world but if it were to happen today it would definitely have been a big deal.

Click here for a list of the deadliest eruptions.

Mt. Fuji is a beautiful example of a stratovolcano, and is almost a perfect symmetric cone (at least when viewed from far away). It is mostly basalt, which is a little bit unusual for stratovolcanoes. Most stratovolcanoes are constructed of andesite or dacite compositions. The fact that it is a stratovolcano means that it is composed of layers of both lava and ash. The fact that it is such a beautiful cone probably indicates that it hasn’t recently suffered a big eruption.

Mount Fuji is famous for it's perfectly symmetrical cone. This photograph shows the shadow of Fuji projected of the adjacent countryside at sunrise. This photograph, taken in May of 1962, is copyrighted by Robert Decker.

“Volcanoes of the World” by Tom Simkin and Lee Seibert lists 63 eruptions of Mt. Fuji since about 9000 years ago. Obviously most of these have been determined by using carbon-14 dating rather than accounts by witnesses. However, the most recent 22 eruptions are listed as having been recorded by people. The most famous eruption was probably the one in 1707.

Anak Krakatau. Photograph courtesy of and copyrighted by Robert Decker.

Krakatau erupted in 1883, in one of the largest eruptions in recent time. Krakatau is an island volcano along the Indonesian arc, between the much larger islands of Sumatra and Java (each of which has many volcanoes also along the arc). There is a very fine book about the Krakatau eruption by Tom Simkin and Richard Fiske (Simkin, T., and Fiske, R.S., Krakatau 1883: The volcanic eruption and its effects: Smithsonian Institution Press: Washington, D.C., 464 p.), so if you really want to know about the eruption you should go to the nearest bookstore or library to find that.

Here are some highlights from their summary of effects:

1. The explosions were heard on Rodriguez Island, 4653 km distant across the Indian Ocean, and over 1/13th of the earth’s surface.
2. Ash fell on Singapore 840 km to the north, Cocos (Keeling) Island 1155 km to the SW, and ships as far as 6076 km west-northwest. Darkness covered the Sunda Straits from 11 a.m. on the 27th until dawn the next day.
3. Giant waves reached heights of 40 m above sea level, devastating everything in their path and hurling ashore coral blocks weighing as much as 600 tons.
4. At least 36,417 people were killed, most by the giant sea waves, and 165 coastal villages were destroyed.
5. When the eruption ended only 1/3 of Krakatau, formerly 5×9 km, remained above sea level, and new islands of steaming pumice and ash lay to the north where the sea had been 36 m deep.
6. Every recording barograph in the world documented the passage of the atmospheric pressure wave, some as many as 7 times as the wave bounced back and forth between the eruption site and its antipodes for 5 days after the explosion.
7. Tide gauges also recorded the sea wave’s passage far from Krakatau. The wave “reached Aden in 12 hours, a distance of 3800 nautical miles, usually traversed by a good steamer in 12 days”.
8. Blue and green suns were observed as fine ash and aerosol, erupted perhaps 50 km into the stratosphere, circled the equator in 13 days.
9. Three months after the eruption these products had spread to higher latitudes causing such vivid red sunset afterglow that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration. Unusual sunsets continued for 3 years.
10. Rafts of floating pumice-locally thick enough to support men, trees, and no doubt other biological passengers-crossed the Indian Ocean in 10 months. Others reached Melanesia, and were still afloat two years after the eruption.
11. The volcanic dust veil that created such spectacular atmospheric effects also acted as a solar radiation filter, lowering global temperatures as much as 1.2 degree C in the year after the eruption. Temperatures did not return to normal until 1888. The book is full of many more amazing bits of information. Hopefully these small excerpts will be useful to you.

Krakatau is still active. The presently-active vent has formed a small island in the middle of the ocean-filled caldera that developed during the famous big eruption of 1883. The island is called Anak Krakatau, which means child-of-Krakatau. It is pretty much erupting all the time at a low level, but once or twice a year it has slightly larger eruptions that people notice and sometimes report in the news. Of course none of these are anywhere near the size of the famous 1883 eruption.

Krakatau is following a pattern that is pretty common for volcanoes. This pattern involves hundreds to thousands of years of small eruptions to build up the volcano followed by 1 or more huge eruptions that causes the volcano to collapse into a caldera, and then the cycle starts over again.

The chances of a huge 1883-style eruption are very small for the time being.   However, it is certainly dangerous to go onto Anak Krakatau, especially if it is one of its more agitated moods. It is probably not even very smart to spend too much time on the small islands that form the remnants of what was once the main Krakatau island. This is because even a small collapse of Anak Krakatau could generate a small tsunami that could sweep towards these islands. Since they are so close to Anak Krakatau there wouldn’t be very much time for a warning.

There has not been any eruptions in Australia in this century. The most recent eruption in Australia was at Mt. Gambier, a shield volcano in the Newer Volcanic Province, Victoria. The Newer Volcanics Province in Victoria Australia is made of four shield volcanoes and associated vents: Red Rock, Mt. Napier, Mt. Schank, and Mt. Gambier. They last erupted between 5850 and 2900 B.C. The eruptions were explosive and some generated lava flows. It is impossible to say if the volcanoes will erupt again. However, there have been rare earthquakes in the area, most recently in 1976. There are numerous volcanic islands north and east of Australia including North Island, New Zealand, the islands of Vanuatu, the Solomon Islands, New Britian, and Indonesia. There are numerous interesting volcanic provinces in Australia. There are flood basalts of Cambrian age (about 650 million years old) northeast of Halls Crossing in northern Australia. Volcanism commenced about 70 million years ago at volcanic centers in southeast Queensland and northeast South Wales. Compositions range from basalt to rhyolite and includes shields, plugs, and domes. In north Queensland there are some very long basaltic lava flows. For example, at Undara a flow is 100 miles (160 km) long. You might have a look at Johnson and others (1989).

Sources of Information:
Johnson, R.W., 1976, Volcanism in Australia: Elsevier, New York, 405 p.

Johnson, R.W., Knutson, J., and Taylor, S.R., 1989, Intraplate volcanism in eastern Australia and New Zealand: Cambridge University Press, Cambridge, England, 408 p.

Simkin, T., and Siebert, L., 1994, Volcanoes of the world: Geoscience Press, Tucson, Arizona, 349 p.

The most recent eruption of Mount Erebus began in 1972 and stopped in 1992. It shares some similarities with both Kilauea and Mount St. Helens but also has some significant differences. Like Kilauea current eruption, the vent was a lava lake that produced some lava flows. Unlike Kilauea, Mount Erebus is a stratovolcano, the same type of volcano as Mount St. Helens. Eruptive activity at Mount Erebus tends to be strombolian , a little more explosive than Kilauea but far less than Mount St. Helens. The composition of the volcanic products at each volcano is different. The silica content of lava from Kilauea and Mount Erebus are about 50 weight percent. However, Mount Erebus rocks have greater amounts of alkali elements (sodium and potassium), about 9-10 weight percent compared to the 3 weight percent for rocks from Kilauea. Ash and lava from Mount St. Helens are called dacite because they contain about 64 weight percent silica, much more than the other two volcanoes.

The tectonics around Erebus are not too clearly understood, but as summarized by Tom Simkin and Lee Seibert in “Volcanoes of the World”, there is a large continental rift that is cutting through the W part of Antarctica (I guess it is this splitting-apart that has formed the Ross Sea). The most famous continental rift is the E. African Rift, and it too is associated with volcanism.

There are chapters on Mt. Erebus in “Volcanoes of the Antarctic Plate and Southern Oceans” by WE LeMasurier and JW Thomson, editors. In the summary chapter on the Mt. Erebus volcanic province, PR Kyle writes “Mt Erebus is an active volcano…and contains a persistent convecting lava lake of anorthoclase phonolite magma. Small Strombolian eruptions occur on a daily basis…often ejecting anorthoclase phonolite bombs onto the crater rim. From September to December 1984, larger Strombolian eruptions occurred more frequently, ejecting bombs up to 2 km from the crater and sending small eruption columns to over 2 km high.,,”

The lava lake in the summit crater counts as an ongoing eruption but it is not a particularly active eruption. The fact that the volcano is composed of layers of lava and ash erupted mainly from the summit means that it does erupt in a bigger way, it is just that no humans have seen it happen.

Source of information:
Simkin, T., and Siebert, L., 1994, Volcanoes of the world: Geoscience Press, Tucson, Arizona, 349 p.

Click here for more information on Mount Erebus on VW.

Volcanoes in Cameroon are part of the Cameroon line, a chain of volcanoes extending from Annobon Island in the Atlantic Ocean northeastward through Cameroon. The oldest rocks have been dated at 70 million years old. Nine volcanoes along the line are active. A fissure eruption occurred at Mt. Cameroon in 1982.

Volcanism along the Cameroon line is related to rifting – where a continent breaks into two pieces. About 110 million years ago a giant rift broke apart what became Africa and South America and the South Atlantic Ocean began to form. A smaller rift formed within the African continent. This older rift, called the Benue Trough, is north of and parallel to the Cameroon line. About 80 million years ago, during a reorganization of plate boundaries, the African plate rotated counter-clockwise. Then a new rift formed that failed to split Africa but apparently did form conduits that allowed magma to ultimately reach the surface and form the volcanoes of the Cameroon line.

In 1986, a cloud of gas burst out of Lake Nyos because a landslide disturbed the stratification with the lake. The stratification (layering) is caused by different amounts of CO2 dissolved in the water. The layers are stable (they don’t mix or changes position). The alternative mechanism, that the burst was caused by a phreatic eruption (involving lava into the lake), is not generally accepted.

Dr. Niels Oskarsson proposed reducing the amount of CO2 in the lake by continuously draining the bottom waters from the lake. Water from rainfall would displace an equal volume of water from the bottom of the lake. Over time this would establish low CO2 levels in the lake.

The 1987 conference recommended continuous telemetered monitoring of the deep water temperature, pH, conductivity, and alkalinity of the lake. Unfortunately, no funding or plans for implementation were developed (in 1987).  

There is a pretty good consensus as to what happened at Lake Nyos (in 1986, by the way). A few folks disagree on the details, but the main event was the sudden release of a large cloud of carbon dioxide gas from the lake. Lake Nyos is a water-filled throat of an old volcano and it is deep and funnel-shaped. Although no longer erupting, there is still gas being released by the old plumbing system under the lake. Carbon dioxide gas was released directly into the deepest waters of the lake, where it could remain in solution (the way that carbon dioxide stays in solution in an un-opened soda or beer). In this situation the lake could build up a large amount of carbon dioxide dissolved in the deeper water. This was a stable situation. The carbon-dioxide charged water was slightly denser than the normal water in the upper levels of the lake, and the weight of the overlying water kept the carbon dioxide in solution in the deeper parts of the lake.

However, nature decided to unbalance the situation. This is where the disagreement among volcanologists comes in. It is agreed that somehow some of that carbon dioxide-rich water was displaced upward into shallower depths to the point where the overlying water pressure was lower and carbon dioxide bubbles could start to form (like when you lower the pressure on a soda by opening the bottle and suddenly bubbles start to form). At Lake Nyos, once \these bubbles started to form they wanted to rise to the top, this brought up more carbon dioxide-rich water which then also started to develop bubbles, and pretty soon there was a big rush of carbon dioxide bubbles to the surface. What people don’t agree on is what the trigger for this unbalancing event was. Most people, I think, feel that there was some sort of landslide into the lake that stirred up the water. There are a few volcanologists who think there was some type of eruption in the deeper part of the lake, but they are in the minority.

Once all this carbon dioxide reached the surface, it splashed some lake water out of the lake, like a big bubble bursting. Carbon dioxide is denser than air, so it hugged the ground and flowed down the stream valley that leads away from the lake. Unfortunately many homes and at least one town are also along this valley and the inhabitants were caught by this cloud of ground-hugging gas. Carbon dioxide usually kills people by displacing the air that they need to breathe, but in high-enough concentrations it is poisonous as well.

So you see that although volcanologists might disagree on the tectonic details underlying Cameroon and might even disagree on the triggering mechanism for the 1986 disaster, but the danger is pretty well understood. Obviously one way to minimize the chances of this happening again is to prevent the deep lake waters from becoming gas-charged. A program was started to have a pump running that brought up the deep water (in a small controlled way), where it was pumped into the air like a fountain. This allowed smaller amounts of deep water to lose their carbon dioxide gradually rather than having the potential of a big bubble occurring again. Another worry is that the lake walls themselves are not very strong (they are constructed of tuff, partially solidified ash). The problem is that there is this lake at an elevation higher than the main towns nearby, and if for any reason the walls of the lake were breached there would be a flood of water that could be just as dangerous as a flood of carbon dioxide. I’m not sure but I think there were plans to pump water out of the lake to try and keep the level and pressure down.

Photograph of the lava lake by Jack Lockwood, U.S. Geological Survey, August 24, 1994.

Nyiragongo (elevation: 11,365 feet, 3,465 m) is a stratovolcano. A crater at the summit of Nyiragongo contained a lava lake from 1894 to 1977. On January 10, 1977, the lava lake drained in less than one hour. The lava erupted from fissures on the flank of the volcano and moved at speeds up to 40 miles per hour (60 km/hr). About 70 people were killed. The fluid lava reached within 2,000 feet (600 m) of the Goma airport. From June 1982 to early 1982 the volcano was active with a lava lake in the crater and phreatic explosions and lava fountaining.

The most recent activity at Nyiragongo began in June of 1994. A lava lake once again filled part of the crater. The lava lake was approximately 130 feet (40 m) in diameter and sent lava flows onto the floor of the 2,600 foot (800 m) diameter crater. The surface of the lava lake was about 500 feet (150 m) below the level of the lake when it drained in 1977.

The lava at Nyiragongo is a nephelinite. Nephelinite is a type of alkaline (high concentration of alkali elements, Na and K) lava. It tends to be aphanitic (no crystals visible to the unaided eye). With a microscope crystals of the following minerals might be seen: nepheline, clinopyroxene, olivine, iron-titanium oxides, feldspar and possibly leucite.

Sources of Information:

Best, M.G., 1982, Igneous and Metamorphic Petrology; W.H. Freeman Co., New York, 630 p.

Bulletin of the Global Volcanism Network, 1995, Smithsonian Institution, Washington, D.C., v. 20, no. 1, p. 11-12.

McClelland, L., Simkin, T., Summers, M., Nielson, E., Stein, T.C., 1989, Global volcanism 1975-1985: Prentice Hall, Englewood Cliffs, New Jersey, and American Geophysical Union, Washington DC, 655 p.

Sahama, T.G., The main Nyiragongo cone; Musee Royal de L’Afrique Centrale – Tervuren, Belgique Annales – Serie IN-8 – Sciences Geologiques – n 81, 1978.

Simkin, T., and Siebert, L., 1994, Volcanoes of the world: Geoscience Press, Tucson, Arizona, 349 p.

Smithsonian Institution’s Global Volcanism Network, 1994, Summary of Recent Activity: Bulletin of Volcanology, v. 56, p. 414.

The eruption killed 57 people, in the lateral blast, ashfall, and lahars. The causes to death included asphyxiation, thermal injuries, and trauma. Four indirect death were caused by a cropduster hitting powerlines during the ashfall, a traffic accident during poor visibilty, and two heart attacks from shoveling ash.

The Cost of Volcanic Eruptions details the economic impacts of the eruption.

The Toutle River was flooded by melting snow and ice from the mountain. About 12 million board feet of stockpiled lumber were sweep in the river. Eight bridges were destroyed. 200 homes were destroyed or damaged. Debris dams were added to help control sediment in the rivers.

Thirty logging trucks, 22 transport vehicles, and 39 railcars were damaged or destroyed along with 4.7 billion board feet of timber.

Shipping was stopped on the Columbia River and some vessels were stranded. In eastern Washington, falling ash stranded 5,000 motorist. Ash had to be cleared from runways and highways.

For a limited time, some people living near the eruption suffered from post traumatic stress syndrome: depression, troubled sleep, irritability, ans a sense of powerlessness.

From 1980-1990, 74 research projects were funded by the National Science Foundation at a total cost of just less than $5 million. The Mount St. Helens Visitors Center at Castle Rock cost $5.5 million to construct. Trails, roads in the park, and interpretive centers cost another $42.3 million. New highway and bridges from the Toutle River to Johnston Ridge cost $145 million. facilities along this road will cost another $25 million.

Sources of Information

Carson, R., 1990, Mount St. Helens: The Eruption and Recovery of a Volcano: Sasquatch Books, Seattle Washington, 160 p.

Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, Tucson, Arizona, 349 p.

A volcanic dome growing in the Mount St. Helens crater. Photo courtesy of the U.S. Geological Survey

It is likely that Mt. St. Helens will have one or more small eruptions during your lifetime, but probably unlikely that it will have another big one. At the time of the 1980 eruption there was the weight of the whole volcanic cone available to keep the magma pressure from erupting. This allowed a large amount of pressure to build up and consequently the eruption was large. Now the whole top of the mountain is gone so there is a good deal less weight available as a counter balance to the pressure. This means that eruptions occur after lesser amounts of pressure have built up and the eruptions are therefore smaller. Eventually, of course, Mt. St. Helens will recover its cone-shaped shape by filling in the amphitheater with lava domes and ash. Once this occurs (probably over a period of a few hundred years) it’ll be ready to suffer another large 1980-size eruption.

Since the famous 1980 eruption there has been a lava dome growing within the caldera, and every once in a while part of the dome either collapses or is blasted away by either gas or steam explosions. These eruptions seem to be getting less and less frequent but have probably not stopped.

Mount St. Helens erupted at least 10 times in the 200 years before the 1980 eruption. It is considered to be the most active volcano in the Cascades. Volcanologists that study Mount St. Helens believe it is likely to erupt again within a few decades or a century at most.

It can be difficult to know when a volcano is dead. Many cinder cones, are monogenetic, erupting only once before the volcano is dead. Stratovolcanoes can erupt many times in only a few centuries, like Llaima in Chile, which has erupted at least 40 times in the last 350 years. Stratovolcanoes can also go many centuries without an eruption, like Newberry volcano in Oregon, which has not erupted in 1,300 years but will probably erupt again. Large volcanic systems, like Yellowstone, can go hundreds of thousands of years between eruptions.

Chronologies and summaries of the May 18, 1980 eruption For more information on what happen during the May 18, 1980 eruption of Mount St. Helens click here and here. The USGS’s Cascade Volcano Observatory has a very detailed account of the eruption, with a list of references.

Time
The climatic eruption began at 08:32 PDT on May 18, 1980.

Deaths
57 people were killed directly by the eruption. There was also a plane crash, a traffic accident, and shoveling ash which killed a total of 7 more.

Height of Mount St. Helens
The summit elevation was 9,760 feet (2,975 m) before the eruption. After the eruption the elevation of the new summit was about 8,525 feet (2,600 m).

Volume
About 0.25 cubic kilometers of new volcanic rock was erupted on May 18, 1980. This would make a cube about 600 meters (~2000 ft) on a side! But much more material was moved by the eruption: the entire northern side of the volcano collapsed and flowed down hill. The volume of this collapse was about 2.5 cubic kilometers – ten times bigger than the new lava.

Ash
The eruption began at 8:45 a.m. At noon, the ash plume (in the upper troposphere and lower stratosphere) had reached Moscow, Idaho. By about 3 p.m. it was near Missoula, Montana and starting to spread south. By 6 pm it was eas! of Pocatello, Idaho. At the end of the day, about 16 hours after the eruption started, the ash plume was near central Colorado.

A huge volume of ash was created by the various 1980 eruptions of Mount St. Helens. Every community affected had its own ways of dealing with the ash. Tons of ash was probably washed down storm drains and into sewer systems as people cleaned roofs and sidewalks. Local landfills received ash. Many tons of ash came down rivers and streams into the Columbia River. The river had to be dredged to allow shipping to pass, and the sand dredged from the bottom was deposited in large dikes along the Columbia. These dikes are now covered in grass and trees, but if a person was to dig down a few feet they would find the ash.

Ash can still be found on the floor of forested areas all around the mountain, and the blast zone is still heavily covered by ash. Souvenir “ash” trays and mugs made from Mount St. Helens ash can be purchased at gift shops near the mountain.

Sources of Information:
Christiansen, R.L., and Peterson, D.W., 1981, Chronology of the 1980 eruptive activity, in Lipman, P.W., and Mullineaux, D.R., eds., The 1980 eruptions of Mount St. Helens, Washington: U.S. Geological Survey Professional Paper 1250, p. 17-30.

Danielsen, E.F., 1981, Trajectories of the Mount St. Helens eruption plume: Science, v. 211, p. 819-821.

McClelland, L., Simkin, T., Summers, M., Nielson, E., Stein, T.C., 1989, Global volcanism 1975-1985: Prentice Hall, Englewood Cliffs, New Jersey, and American Geophysical Union, Washington DC, 655 p.

Sarna-Wojcicki, A.M., Shipley, S., Waitt, Jr., R.B., Dzurisin, D., and Wood, S.H., 1981, Areal distribution, thickness, mass, volume, and grain size of air-fall from the six major eruptions of 1980, in Lipman, P.W. and Mullineaux, D.R., (eds.),The 1980 eruptions of Mount St. Helens, Washington: U.S. Geological Survey Professional Paper 1250, p. 577-600.

Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, P.O. Box 42948, Tucson, AZ 85733-2948)

A view to the north of the "two tone" mountain - an appearance produced by prevailing easterly winds during the initial activity of Mount St. Helens. Mount Rainier is visible in background. Photo by C. Dan Miller, USGS

Eruptions began at Mount St. Helens about 40,000 years ago. The deposits are air-fall tephra and pyroclastic flows, the type of material produced by explosive eruptions. One pumiceous tephra deposit produced during this episode had a volume as great as any subsequent tephra eruption at Mount Saint Helens. I think it would qualify as a big eruption.

A recent paper suggests that eruptions at Mount Saint Helens began as long ago as 80,000 years. Berger and Busacca (1995) dated a loess (wind-blown slit-sized material) deposit just below a tephra layer in eastern Washington that is known to be from Mount St. Helens. The loess is about 80,000 years old. The tephra is thought to be slightly younger.

According to Volcanoes of the World, by Simkin and Siebert (1994, Geoscience Press, P.O. Box 42948, Tucson, AZ 85733-2948), Mt. St. Helens erupted 23 times prior to 1831, based on charcoal dates. After that (and before the 1980 eruption) it erupted in 1831, 1835, 1842, 1847, 1848, 1849, 1853, 1854, and 1857.

There is a good summary in Volcanoes of North America: United States and Canada by Charles Wood and Jurgen Kienle (1990) (Cambridge University Press, New York, NY, (800) 221-4512).

For more details about the eruptive history of Mount St. Helens visit the Mount St. Helens page in VolcanoWorld and USGS’ Cascade Volcano Observatory home page

Additional Sources of Information:
Berger, G., and Busacca, AJ., 1995, Thermoluminescence dating of late Pleistocene loess and tephra from eastern Washington and southern Oregon and implications for the eruptive history of Mount St. Helens: Journal of Geophysical Research, v. 100, p. 22,361-22,374.

Mullineaux, D.R., and Crandell, D.R., 1981, The eruptive history of Mount St. Helens, in Lipman, P.W., and Mullineaux, D.R., (eds.), The 1980 eruptions of Mount St. Helens, Washington, U.S. Geological Survey Professional Paper 1250, p. 3-15.

Hawaiian Volcanoes will not explode like Mount St. Helens. Mount St. Helens magma is more viscous. Therefore gas cannot escape as readily, resulting in explosive eruptions. One index of explosivity is volume of eruption. Since the start of the current Kilauea eruption more than 1,400 million cubic meters of lava have been erupted. Mount St. Helens erupted 1 cubic kilometer of ash (about 10 times greater than the current Kilauea eruption).

About 200 years ago, as Kilauea caldera formed, large volumes of ash erupted at the summit and lava erupted on the lower east rift zone. The ash blankets the summit of Kilauea volcano. Photograph by J.D. Griggs, U.S. Geological Survey, August 14, 1986.

There have been explosive eruptions at Kilauea. The Uwekahuna Ash was erupted about 1,500 years ago and may be related to an older caldera that filled with lava prior to the development of the present caldera. The Keanakakoi Ash was erupted during weeks or months of activity in 1790. The early activity was driven by degassing magma that interacted with groundwater. Later eruptions were driven by steam explosions. Hawaiian warriors were killed near the end of the eruption by what geologists call a base surge. Surges are gas-rich and have little volcanic material. This “steam blast” was not enough to burn their skin but did cause them to suffocate. A 1924 eruption was caused by a 660 feet (200 m) drop in the lava lake that allowed groundwater to enter the conduit. The resulting steam-driven explosions shot rocks weighing several tons as far as 2,600 feet (800 m). At the end of the eruption Halemaumau was 1,320 feet (400 m deep) (deep enough that the Empire State building would fit inside) and had doubled in width from 1,400 to 3,000 feet (430 to 920 m). Note that these explosive eruptions are infrequent.

Account of the 1790 eruption

The footprints of Hawaiian warriors are preserved in a layer of ash and accretionary lapilli in the Kau Desert southwest of the summit of Kilauea. Photo by Steve Mattox, July 1995.

“The company in advance had not preceded far before the ground began to shake and rock beneath their feet and it became impossible to stand. Soon a dense cloud of darkness was seen to rise out of the crater, and almost at the same instant the electrical effect upon the air was so great that the thunder began to roar in the heavens and the lightening to flash. It continued to ascend and spread abroad until the whole region was enveloped and the light of day was entirely excluded. The darkness was the more terrific, being made visible by an awful glare from streams of red and blue light variously combined that issued from the pit below, and being lit up at intervals by the intense flashes of lightening from above. Soon followed an immense volume of sand and cinders which were thrown in high heaven and came down in a destructive shower for many miles around. Some few persons of the forward company were burned to death by the sand and cinders and others were seriously injured. All experienced a suffocating sensation upon the lungs and hastened on with all possible speed.”

Written by Kamakau and reported in Westervelt’s Hawaiian Legends of Volcanoes.

The eruption also killed eighty Hawaiian warriors about six miles (10 km) southwest of the summit. The warriors were returning to the Kau district to defend it from an attack by Kamehameha. Some viewed the eruption, which reduced the strength of a rival chiefs army, as a demonstration of Pele’s favor of Kamehameha. Kamehameha ultimately united and ruled all of the Hawaiian Islands.

From Eruptions of Hawaiian Volcanoes: Past, Present, and Future: U.S. Geological Survey General Interest Publication.

In the mantle, hot, solid rock rises to the hot spot from greater depths. Due to the lower pressure at the shallower depth, the rock begins to melt, forming magma. The magma rises through the Pacific Plate to supply the active volcanoes. The older islands were once located above the stationary hot spot but were carried away as the Pacific Plate drifted to the northwest.

About 95% of the world’s volcanoes are located near the boundaries of tectonic plates. The other 5% are thought to be associated with mantle plumes and hot spots. Mantle plumes are areas where heat and/or rocks in the mantle are rising towards the surface. A hot spot is the surface expression of the mantle plume. Geologists think the hot spots are stationary and the tectonic plates are mobile. The hot spot provides magma which supplies volcanoes. The movement of the plate carries the volcano off the hot spot and it gradually becomes extinct and usually subsides below sea level. A new volcano begins to form on the sea floor and grow towards the surface. Some of these volcanoes rise above sea level to make islands. The classic example is the Hawaii-Emperor volcanic chain, a line of volcanoes and seamounts that extends from Hawaii to Daikakuji, 2,200 miles (3,500 km) to the northwest. Suiko seamount is 65 million years old, the oldest seamount associated with the Hawaiian hot spot. The hot spot is probably older but any volcanoes it produced have been destroyed by subduction.

At first glance the shape and topography of the Hawaii-Emperor volcanic chain might remind you of an island arc. However, volcanoes of the Hawaii-Emperor volcanic chain are progressively younger towards Hawaii and made of basalt. Island arcs are produced at convergent plate boundaries where an ocean plate is subducted beneath an adjacent ocean plate. The Aleutians of Alaska is an example. There is no age progression and andesite is the most common rock.

Click here for more information about the evolution of Hawaiian Volcanoes.

Before the cataclysmic eruption on May 18, 1980, humans likely had a small but noticeable effect on erosion and deposition on Mt. St. Helens.  Prior to that large eruption, humans unitized the mountain (or volcano) for recreation and logging.  These uses, especially the logging and road building, increased erosion in some places and this resulted in increased deposition in other places.  Once the earth materials were disturbed by humans, natural forces such as wind and rain could more easily pick up and transport (erode) this material and deposit it in other places. 

Since the 1980 eruption, access to the volcano has been extremely limited and thus humans have had no impact on erosion or deposition.  Also, the large eruption in 1980 added huge amounts of ash, lahars (mud flows) and pyroclastic (“fire broken”) materials to the landscape.  These materials are very easily eroded by natural processes so erosion and deposition rates have been naturally very high on Mt. St. Helens since 1980 and any erosion or deposition caused by human activities would be infinitesimal by comparison.    

For more information on Mt. St. Helens, check out the Cascades Volcano Observatory (CVO) website at:  http://vulcan.wr.usgs.gov/