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According to the Smithonian Institute, the world counts are some 1500 volcanoes of recent eruption history, and some twenty volcanoes are in activity every moment. Studies suggest that vulcanicity is a rather constant activity, but centered in a number of volcanic belts only. It can be argued, though, that several periods of our planet's geological history have seen a higher level of activity compared to today.
During all eruptions, volcanoes vent gasses (among these: H20, C02, H2S, F) that are either released into the atmosphere, or remain dissolved in the ocean, this being the case of many submarine eruptions.
If we consider volcanic eruptions on land, there are only a handful of known eruptions during the last couple of hundreds of years that temporarily affected world climate. The amount of solid material (ash), and gasses blown into the atmosphere, as well as gas composition are important parameters. Dust blown up in Plinian clouds is dense enough to stall a jet plane’s engines in altitudes of 30000 feet and above. Both fine volcanic ash, and sulfurdioxide (S02) act as sun-ray absorbers and can cause a cooling of the atmosphere.
But only a few events are powerful enough to cause changes in climate recordable on a global scale. This article summarizes events (sorted in chronological order) related to the eruptions of the volcanoes: Laki (Iceland), Tabora, Krakatau ( both Indonesia), Katmai (Alaska), Mt St. Helens, and Mt Pinatubo (Luzon, Philipines).
This article is a selective compilation of data from the following sources:
- Smithsonian Institute : http://www.volcano.si.edu
- Wikipedia: http://en.wikipedia.org/wiki
- Testimony of the Mt. Pinatubo ashfall in Malaysian Borneo
This volcano in Iceland erupted in 1783 with 130 craters opened with phreatomagmatic explosions because of the groundwater interacting with the rising basalt magma. The eight month emission of sulphuric aerosols resulted in one of the most important climatic and socially repercussive events of the last millennium.
The eruption, also known as the Skaftáreldar ("Skaftá river fires") or Síðueldur, produced an estimated 14 km³ (3.4 cu mi) of basalt lava, and the total volume of tephra emitted was 0.91 km³ (0.2 cu mi). Lava fountains were estimated to have reached heights of 800-1400 m (~2,600-4,600 ft).
In Great Britain, the summer of 1783 was known as the "sand-summer" due to ash fallout. The gases were carried by the convective eruption column to altitudes of about 15 kilometres (10 mi). The aerosols built up caused a cooling effect in the Northern Hemisphere.
The eruption continued until 7 February 1784, but most of the lava was erupted in the first five months. Grímsvötn volcano, from which the Laki fissure extends, was also erupting at the time from 1783 until 1785. The outpouring of gases, including an estimated 8 million tons of fluorine and estimated 120 million tons of sulphur dioxide gave rise to what has since become known as the "Laki haze" across Europe.
Consequences in Iceland
The consequences for Iceland were catastrophic. An estimated 25% of the population died in the famine and fluorine poisoning after the fissure eruptions ceased. Around 80% of sheep, 50% of cattle and 50% of horses died because of dental and skeletal fluorosis from the 8 million tons of fluorine that were released.
Gilbert White recorded his perceptions of the event at Selborne:
“The summer of the year 1783 was an amazing and portentous one, and full of horrible phaenomena; for besides the alarming meteors and tremendous thunder-storms that affrighted and distressed the different counties of this kingdom, the peculiar haze, or smokey fog, that prevailed for many weeks in this island, and in every part of Europe, and even beyond its limits, was a most extraordinary appearance, unlike anything known within the memory of man. By my journal I find that I had noticed this strange occurrence from June 23 to July 20 inclusive, during which period the wind varied to every quarter without making any alteration in the air. The sun, at noon, looked as blank as a clouded moon, and shed a rust- coloured ferruginous light on the ground, and floors of rooms; but was particularly lurid and blood-coloured at rising and setting. All the time the heat was so intense that butchers' meat could hardly be eaten on the day after it was killed; and the flies swarmed so in the lanes and hedges that they rendered the horses half frantic, and riding irksome. The country people began to look with a superstitious awe, at the red, louring aspect of the sun.”
Benjamin Franklin recorded his observations in a 1784 lecture:
“During several of the summer months of the year 1783, when the effect of the sun's rays to heat the earth in these northern regions should have been greater, there existed a constant fog over all Europe, and a great part of North America. This fog was of a permanent nature; it was dry, and the rays of the sun seemed to have little effect towards dissipating it, as they easily do a moist fog, arising from water. They were indeed rendered so faint in passing through it, that when collected in the focus of a burning glass they would scarce kindle brown paper. Of course, their summer effect in heating the Earth was exceedingly diminished. Hence the surface was early frozen. Hence the first snows remained on it unmelted, and received continual additions. Hence the air was more chilled, and the winds more severely cold. Hence perhaps the winter of 1783-4 was more severe than any that had happened for many years.”
2. Mount Tambora (or Tomboro) is an active stratovolcano, also known as a composite volcano, on Sumbawa island, Indonesia. Sumbawa is flanked both to the north and south by oceanic crust, and Tambora was formed by the active subduction zones beneath it. This raised Mount Tambora as high as 4,300 m (14,000 ft), making it one of the tallest peaks in the Indonesian archipelago, and drained off a large magma chamber inside the mountain. It took centuries to refill the magma chamber, its volcanic activity reaching its peak in April 1815.
Tambora erupted in 1815 with a rating of seven on the Volcanic Explosivity Index, making it the largest eruption since the Lake Taupo eruption in AD 181. The explosion was heard on Sumatra Island (more than 2,000 km or 1,200 mi away). Heavy volcanic ash falls were observed as far away as Borneo, Sulawesi, Java and Maluku islands. The death toll was at least 71,000 people (perhaps the most deadly eruption in history), of who 11,000–12,000 were killed directly by the eruption; the often-cited figure of 92,000 people killed is believed to be an overestimate. The eruption created global climate anomalies; 1816 became known as the Year Without a Summer because of the effect on North American and European weather. Agricultural crops failed and livestock died in much of the Northern Hemisphere, resulting in the worst famine of the 19th century.
During an excavation in 2004, a team of archaeologists discovered cultural remains buried by the 1815 eruption. They were kept intact beneath the 3 m (10 ft) deep pyroclastic deposits. At the site, dubbed the Pompeii of the East, the artifacts were preserved in the positions they had occupied in 1815.
The 1815 eruption released sulfur into the stratosphere, causing a global climate anomaly. Different methods have estimated the ejected sulfur mass during the eruption: the petrological method; an optical depth measurement based on anatomical observations; and the polar ice core sulfate concentration method, using cores from Greenland and Antarctica. The figures vary depending on the method, ranging from 10 Tg S to 120 Tg S.
In the spring and summer of 1816, a persistent dry fog was observed in the northeastern U.S. The fog reddened and dimmed the sunlight, such that sunspots were visible to the naked eye. Neither wind nor rainfall dispersed the "fog". It was identified as a stratospheric sulfate aerosol veil. In summer 1816, countries in the Northern Hemisphere suffered extreme weather conditions, dubbed the Year Without a Summer. Average global temperatures decreased about 0.4–0.7 °C (0.7–1.3 °F), enough to cause significant agricultural problems around the globe. On 4 June 1816, frosts were reported in Connecticut, and by the following day, most of New England was gripped by the cold front. On 6 June 1816, snow fell in Albany, New York, and Dennysville, Maine. Such conditions occurred for at least three months and ruined most agricultural crops in North America. Canada experienced extreme cold during that summer. Snow 30 cm (12 in) deep accumulated near Quebec City from 6 to 10 June 1816.
1816 was the second coldest year in the northern hemisphere since AD 1400, after 1601 following the 1600 Huaynaputina eruption in Peru. The 1810s are the coldest decade on record, a result of Tambora's 1815 eruption and other suspected eruptions somewhere between 1809 and 1810 (see sulfate concentration figure from ice core data). The surface temperature anomalies during the summer of 1816, 1817 and 1818 were −0.51, −0.44 and −0.29 °C, respectively. As well as a cooler summer, parts of Europe experienced a stormier winter.
This pattern of climate anomaly has been blamed for the severity of typhus epidemic in southeast Europe and the eastern Mediterranean between 1816 and 1819. Much livestock died in New England during the winter of 1816–1817. Cool temperatures and heavy rains resulted in failed harvests in the United Kingdom of Great Britain and Ireland. Families in Wales traveled long distances as refugees, begging for food. Famine was prevalent in north and southwest Ireland, following the failure of wheat, oat and potato harvests. The crisis was severe in Germany, where food prices rose sharply. Due to the unknown cause of the problems, demonstrations in front of grain markets and bakeries, followed by riots, arson and looting, took place in many European cities. It was the worst famine of the 19th century.
3. Krakatoa (Indonesian: Krakatau), also spelled Krakatao or Krakatowa, is a volcanic island in the Sunda Strait between Java and Sumatra in Indonesia. The name is used for the island group, the main island (also called Rakata), and the volcano as a whole. It has erupted repeatedly, massively, and with disastrous consequences throughout recorded history. The best known eruption culminated in a series of massive explosions on August 26 - 27, 1883, which was among the most violent volcanic events in modern times. With a Volcanic Explosivity Index of 6, it was equivalent to 200 megatons of TNT — about 13,000 times the yield of the Little Boy bomb (13 to 16 KT) that devastated Hiroshima, Japan.
The 1883 eruption ejected more than 25 cubic kilometres of rock, ash, and pumice, and generated the loudest sound historically reported: the cataclysmic explosion was distinctly heard as far away as Perth in Australia approx. 1,930 miles (3,110 km), and the island of Rodrigues near Mauritius approx. 3,000 miles (5,000 km). Near Krakatoa, according to official records, 165 villages and towns were destroyed and 132 seriously damaged, at least 36,417 (official toll) people died, and many thousands were injured by the eruption, mostly from the tsunamis that followed the explosion.
In the years before the 1883 eruption, seismic activity around the volcano was intense, with some earthquakes felt as far distant as Australia. Beginning 20 May 1883, three months before the final explosion, steam venting began to occur regularly from Perboewatan, the northernmost of the island's three cones. Eruptions of ash reached an altitude of 6 km (20,000 ft) and explosions could be heard in Batavia (Jakarta) 160 km (100 miles) away. Activity died down by the end of May.
The volcano began erupting again around 20 July. The seat of the eruption is believed to have been a new vent or vents which formed between Perboewatan and Danan, more or less where the current volcanic cone of Anak Krakatau is. The violence of the eruption caused tides in the vicinity to be unusually high, and ships at anchor had to be moored with chains as a result. On 11 August larger eruptions began, with ashy plumes being emitted from at least eleven vents. On 24 August, eruptions further intensified. At about 1pm (local time) on 26 August, the volcano went into its paroxysmal phase, and by 2pm observers could see a black cloud of ash 27 km (17 miles) high. At this point, the eruption was virtually continuous and explosions could be heard every ten minutes or so. Ships within 20 km (11 nautical miles) of the volcano reported heavy ash fall, with pieces of hot pumice up to 10 cm in diameter landing on their decks. A small tsunami hit the shores of Java and Sumatra some 40 km (28 miles) away between 6pm and 7pm.
On August 27, the volcano entered the final cataclysmic stage of its eruption. Four enormous explosions took place at 05:30 hrs, 06:42 hrs, 08:20 hrs, and 10:02 hrs, local time. The last explosion was the loudest. Each was accompanied by very large tsunamis, which are believed to have been over 30 meters (100 ft) high in places. A large area of the Sunda Strait and a number of places on the Sumatran coast were affected by pyroclastic flows from the volcano. The explosions were so violent that they were heard 3,500 km away in Perth, Western Australia and the island of Rodrigues near Mauritius, 4,800 km away; the sound of Krakatoa's destruction is the loudest sound in recorded history, reaching levels of 180 dBSPL 100 miles (160 km) away. The pressure wave from the final explosion was recorded on barographs around the world, which continued to register it up to 5 days after the explosion. The recordings show that the shockwave from the final explosion reverberated around the globe 7 times in total. Ash was propelled to a height of 80 km. The eruptions diminished rapidly after that point, and by the morning of August 28 Krakatoa was quiet.
"The Burning Ashes of Ketimbang"
Around noon on August 27, a rain of hot ash fell around Ketimbang in Sumatra. Around a thousand people were killed, the only large number of victims killed by Krakatoa itself, and not the waves or after-effects. Verbeek and later writers believe this unique event was a lateral blast or pyroclastic flow, similar to what happened in 1980 at Mt. St. Helens, which crossed the water. The region of the ashfall ended to the northwest of Ketimbang, where the bulk of Sebesi Island offered protection from any horizontal surges.
The combined effects of pyroclastic flows, volcanic ashes and tsunamis had disastrous results in the region. There were no survivors from 3,000 people located at the island of Sebesi, about 13 km from Krakatoa. Pyroclastic flows killed around 1,000 people at Ketimbang on the coast of Sumatra some 40 km north from Krakatoa. The official death toll recorded by the Dutch authorities was 36,417, although some sources put the estimate at 120,000 or more. Many settlements were destroyed, including Teluk Betung and Ketimbang in Sumatra, and Sirik and Semarang in Java. The areas of Banten on Java and the Lampung on Sumatra were devastated. There are numerous documented reports of groups of human skeletons floating across the Indian Ocean on rafts of volcanic pumice and washing up on the east coast of Africa, up to a year after the eruption. Some land on Java was never repopulated; it reverted to jungle and is now the Ujung Kulon National Park.
Tsunamis and distant effects
Ships as far away as South Africa rocked as tsunamis hit them, and the bodies of victims were found floating in the ocean for weeks after the event. The tsunamis which accompanied the eruption are believed to have been caused by gigantic pyroclastic flows entering the sea; each of the five great explosions was accompanied by a massive pyroclastic flow resulting from the gravitational collapse of the eruption column. This caused several cubic kilometers of material to enter the sea, displacing an equally huge volume of seawater. The town Merak was destroyed by a 46 metre-high tsunami. Some of the pyroclastic flows reached the Sumatran coast as much as 25 miles (40 km) away, having apparently moved across the water on a "cushion" of superheated steam. There are also indications of submarine pyroclastic flows reaching 10 miles (15 km) from the volcano.
Smaller waves were recorded on tidal gauges as far away as the English Channel. These occurred too soon to be remnants of the initial tsunamis, and may have been caused by concussive air waves from the eruption. These air waves circled the globe several times and were still detectable using barographs five days later.
As a result of the huge amount of material deposited by the volcano, the surrounding ocean floor was drastically altered. It is estimated that as much as 18-21 km³ of ignimbrite was deposited over an area of 1.1 million km², largely filling the 30-40 m deep basin around Krakatoa. The land masses of Verlaten and Lang were increased, and volcanic ash continues to be a significant part of the geological composition of these islands. Poolsche Hoed ("Polish Hat") disappeared. A new rock islet called Bootsmansrots ('Bosun's Rock', a fragment of Danan) was left.
Two nearby sandbanks (called Steers and Calmeyer after the two naval officers who investigated them) were built up into islands by ashfall, but the sea later washed them away. Seawater on hot volcanic deposits on Steers and Calmeyer caused steam which some people mistook for continued eruption.
The fate of Krakatoa itself has been the subject of some dispute among geologists. It was originally proposed that the island had been blown apart by the force of the eruption. However, most of the material deposited by the volcano is clearly magmatic in origin and the caldera formed by the eruption is not extensively filled with deposits from the 1883 eruption. This indicates that the island subsided into an empty magma chamber at the end of the eruption sequence, rather than having been destroyed during the eruptions.
In the year following the eruption, average global temperatures fell by as much as 1.2 degrees Celsius. Weather patterns continued to be chaotic for years, and temperatures did not return to normal until 1888. The eruption injected an unusually large amount of sulfur dioxide (SO2) gas high into the stratosphere which was subsequently transported by high-level winds all over the planet. This led to a global increase in sulfurous acid (H2SO3) concentration in high-level cirrus clouds. The resulting increase in cloud reflectivity (or albedo) would reflect more incoming light from the sun than usual, and cool the entire planet until the suspended sulfur fell to the ground as acid precipitation.
Global optical effects
The dramatic skyline in Edvard Munch's The Scream (1893) is thought to be based on the global optical effects caused by the eruption and seen over Oslofjord, Norway. The eruption darkened the sky for days afterwards, and produced spectacular sunsets throughout the world for many months. British artist William Ashcroft made thousands of colour sketches of the red sunsets half-way around the world from Krakatoa in the years after the eruption. In 2004, researchers proposed the idea that the blood-red sky shown in Edvard Munch's famous 1893 painting The Scream is also an accurate depiction of the sky over Norway after the eruption. Munch said: "suddenly the sky turned blood red ... I stood there shaking with fear and felt an endless scream passing through nature." Also, a so called blue moon had been seen for two years as a result of the eruption.
4. Mount Katmai is a large stratovolcano (composite volcano) on the Alaska Peninsula in southern Alaska, located within Katmai National Park and Preserve. It is about 6 miles (10 km) in diameter with a central lake-filled caldera about 3 by 2 mi (4.5 by 3 km) in area. The caldera rim reaches a maximum elevation of 6,716 feet (2,047 m). In 1975 the surface of the crater lake was at an elevation of about 4,220 feet (1,286 m), and the estimated elevation of the caldera floor is about 3,400 ft (1,040 m).
On June 6-9, 1912, the most spectacular Alaskan eruption in recorded history and one of the two largest eruptions in the world in the twentieth century (the other being Mt. Pinatubo in 1991) resulted in the formation of a large summit caldera at Katmai volcano. The over 60-hour-long eruption actually took place at a vent about 6 mi (10 km) to the west of Mt. Katmai (now marked by Novarupta dome) from which an estimated 30-35 km³ of ash flows and tephra were ejected rather than at Mt. Katmai itself. Based on geochemical and structural relationships, it has been suggested that magma drained from beneath Katmai Volcano to Novarupta via the plumbing system beneath Trident Volcano. The withdrawal of magma beneath Katmai resulted in the collapse of the summit area, forming the caldera. Following the subsidence, a small dacitic cinder cone was emplaced on the floor of the caldera; this is the only juvenile material erupted from Katmai caldera during the historical eruption.
Approximately 12-15 km³ of magma was vented during the 1912 eruption producing about 35 km³ of tephra. An estimated 11-15 km³ of ash flow tuff traveled 12 miles (20 km) northwest covering an area of about 120 km² in what subsequently came to be known as the Valley of Ten Thousand Smokes. The ash flow tuff produced in the 1912 eruption is made up of a silica-rich volcanic rock called rhyolite. In fact, this is the only major Quaternary eruption of rhyolite to have occurred in Alaska. Maximum thickness of the ashflow is estimated to be about 800 feet (240 m). About 20 km³ of airfall tephra was carried east and southeast with a minor lobe to the north covering 77,000 km² with more than 1 in (2.5 cm) of ash. Light ash fall was reported as far away as the Puget Sound region 1,500 mi (2,400 km) away. Extremely fine ash blown into the stratosphere remained in suspension as aerosols for months and caused spectacular red sunsets in many parts of the world.
In 1919, geologists noted a lake covering a large part of the caldera floor, but by 1923 the lake was gone and numerous fumaroles, mud pots, and a large mud geyser had replaced it. The lake has since refilled to a depth of over 800 ft (240 m). Small glaciers have also formed on a bench within the caldera beside the lake.
5. Surtsey (Icelandic: "Surtur's island") is a volcanic island off the southern coast of Iceland. At 63.4° N 20.3° W it is also the southernmost point of Iceland. It was formed in a volcanic eruption which began 130 meters below sea level, and reached the surface on 14 November 1963. The eruption may have started a few days earlier and lasted until 5 June 1967, when the island reached its maximum size of 2.7 km². Since then, wind and wave erosion has seen the island steadily diminish in size: As of 2007, its surface area is 1.4 km² in size.
The new island was named after the fire god Surtr from Norse mythology, and was intensively studied by volcanologists during its creation and, since the end of the eruption, has been of great interest to botanists and biologists as life has gradually colonised the originally barren island. The undersea vents that produced Surtsey are part of the Vestmannaeyjar (Westmann Isles) submarine volcanic system, part of the fissure of the sea floor called the Mid-Atlantic Ridge. Vestmannaeyjar also produced the famous eruption of Eldfell on the island of Heimaey in 1973. The eruption that created Surtsey also created a few other small islands along this volcanic chain, such as Jólnir and other unnamed peaks. Most of these eroded away fairly quickly.
The explosive phreatomagmatic eruptions caused by the easy access of water to the erupting vents threw rocks up to a kilometre away from the island, and sent ash clouds as high as 10 km up into the atmosphere. The loose pile of unconsolidated tephra would quickly have been washed away had the supply of fresh magma dwindled, and large clouds of dust were often seen blowing away from the island during this stage of the eruption.
By early 1964, though, the continuing eruptions had built the island to such a size that sea water could no longer easily reach the vents, and the volcanic activity became much less explosive. Instead, lava fountains and flows became the main form of activity. These resulted in a hard cap of extremely erosion-resistant rock being laid down on top of much of the loose volcanic pile, which prevented the island being washed away rapidly. Effusive eruptions continued until 1965, by which time the island had a surface area of 2.5 km²
28 December 1963 saw the onset of submarine activity 2.5 km to the north-east of Surtsey, which formed a ridge 100 m high on the sea floor. This seamount was named Surtla, but never reached sea level. Eruptions at Surtla ended on 6 January 1964, and it has since been eroded from its minimum depth of 23 m to 47 m below sea level.
6. Mount St. Helens
The 1980 eruption of Mount St. Helens, a volcano located in Washington state, in the United States, was a major volcanic eruption. The eruption was the most significant to occur in the contiguous 48 U.S. states (VEI = 5, 0.3 cu mi, 1.2 km3 of material erupted), in terms of power and volume of material released, since the 1915 eruption of California's Lassen Peak.
The eruption was preceded by a two-month series of earthquakes and steam-venting episodes, caused by an injection of magma at shallow depth below the mountain that created a huge bulge and a fracture system on Mount St. Helens' north slope.
An earthquake at 8:32 a.m. on May 18, 1980, caused the entire weakened north face to slide away, suddenly exposing the partly molten, gas- and steam-rich rock in the volcano to lower pressure. The rock responded by exploding into a very hot mix of pulverized lava and older rock that sped toward Spirit Lake so fast that it quickly passed the avalanching north face.
The May 18, 1980, event was the most deadly and economically destructive volcanic eruption in the history of the United States. Fifty-seven people were killed and 200 homes, 47 bridges, 15 miles (24 km) of railways and 185 miles (300 km) of highway were destroyed. U.S. President Jimmy Carter surveyed the damage and stated it looked more desolate than a moonscape. A film crew was dropped by helicopter on St. Helens on May 23 to document the destruction. Their compasses, however, span in circles and they quickly became lost. A second eruption occurred the next day (see below), but the crew survived and were rescued two days after that.
In all, St. Helens released an amount of energy equivalent to 27,000 Hiroshima-sized nuclear weapons and ejected more than 1 cubic mile (4 km³) of material. A quarter of that volume was fresh lava in the form of ash, pumice, and volcanic bombs while the rest was fragmented, older rock. The removal of the north side of the mountain (13% of the cone's volume) reduced St. Helens' height by about 1,313 feet (400 m) and left a crater 1 to 2 miles (2 to 3 km) wide and 2,100 feet (640 m) deep with its north end open in a huge breach.
More than 4 billion board feet (14.6 km³) of timber was damaged or destroyed, mainly by the lateral blast. At least 25% of the destroyed timber was salvaged after September 1980. Downwind of the volcano, in areas of thick ash accumulation, many agricultural crops, such as wheat, apples, potatoes, and alfalfa, were destroyed. As many as 1,500 elk and 5,000 deer were killed, and an estimated 12 million Chinook and Coho salmon fingerlings died when their hatcheries were destroyed. Another estimated 40,000 young salmon were lost when they swam through turbine blades of hydroelectric generators when reservoir levels were lowered along the Lewis River to accommodate possible mudflows and flood waters.
A volcanic ash column rose high into the atmosphere and deposited ash in 11 U.S. states. At the same time, snow, ice, and several entire glaciers on the mountain melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly fifty miles (eighty kilometers) to the south. Less severe outbursts continued into the next day only to be followed by other large but not as destructive eruptions later in 1980.
By the time the ash settled, 57 people (including innkeeper Harry Truman and geologist David A. Johnston; and thousands of animals were dead. Hundreds of square miles were reduced to wasteland, over a billion U.S. dollars in damage had occurred ($2.74 billion in 2007 dollars), and the face of Mount St. Helens was scarred with a huge crater on its north side. At the time of the eruption, the summit of the volcano was owned by the Burlington Northern Railroad, but afterward the land passed to the United States Forest Service. The area was later preserved, as it was, in the Mount St. Helens National Volcanic Monument. As the avalanche and initial pyroclastic flow were still advancing, a huge ash column grew to a height of 12 miles (19 km) above the expanding crater in less than 10 minutes and spewed tephra into the stratosphere for 10 straight hours. Near the volcano, the swirling ash particles in the atmosphere generated lightning, which in turn started many forest fires. During this time, parts of the mushroom-shaped ash-cloud column collapsed, sending additional pyroclastic flows speeding down St. Helens' flanks. Later, slower flows came directly from the new north-facing crater and consisted of glowing pumice bombs and very hot pumiceous ash. Some of these hot flows covered ice or water which flashed to steam, creating craters up to 65 feet (20 m) in diameter and sending ash as much as 6,500 feet (1980 m) into the air.
Strong high-altitude wind carried much of this material east-northeasterly from the volcano at an average speed of about 60 mph (100 km/h). By 9:45 a.m. it had reached Yakima, Washington, 90 miles (145 km) away, and by 11:45 a.m. it was over Spokane, Washington. A total of 4 to 5 inches (100 to 130 mm) of ash fell on Yakima, and areas as far east as Spokane were plunged into darkness by noon where visibility was reduced to 10 feet (3 m) and half an inch (13 mm) of ash fell. Continuing east, St. Helens' ash fell in the western part of Yellowstone National Park by 10:15 p.m. and was seen on the ground in Denver, Colorado, the next day. In time ash fall from this eruption was reported as far away as Minnesota and Oklahoma, and some of the ash drifted around the globe within about 2 weeks.
During the nine hours of vigorous eruptive activity, about 540 million tons of ash fell over an area of more than 22,000 square miles (60,000 km²). The total volume of the ash before its compaction by rainfall was about 0.3 cubic miles (1.3 km³). The volume of the uncompacted ash is equivalent to about 0.05 mile³ (208,000,000 m³) of solid rock, or about 7% of the amount of material that slid off in the debris avalanche. By around 5:30 p.m. on May 18, the vertical ash column declined in stature, but less severe outbursts continued through the night and for several days.
7. Mount Pinatubo is an active stratovolcano located on the island of Luzon in the Philippines, at the intersection of the borders of the provinces of Zambales, Tarlac, and Pampanga. Ancestral Pinatubo was a stratovolcano made of andesite and dacite. Before 1991, the mountain was inconspicuous and heavily eroded. It was covered in dense forest which supported a population of several thousand indigenous people, the Aeta, who had fled to the mountains from the lowlands when the Spanish conquered the Philippines in 1565.
The volcano's eruption in June 1991 produced the second largest terrestrial eruption of the 20th century. The 1991 eruption had a Volcanic Explosivity Index (VEI) of 6, and came some 450-500 years after the volcano's last known eruptive activity (estimated as VEI 5, the level of the 1980 eruption of Mount St. Helens), and some 500-1000 years after previous VEI 6 eruptive activity. Successful predictions of the onset of the climactic eruption led to the evacuation of tens of thousands of people from the surrounding areas, saving many lives, but surrounding areas were severely damaged by pyroclastic flows, ash deposits, and later by lahars caused by rainwater remobilizing earlier volcanic deposits: thousands of houses and other buildings were destroyed.
The effects of the eruption were felt worldwide. It ejected roughly 10 billion metric tons of magma, and 20 million tons of SO2, bringing vast quantities of minerals and metals to the surface environment. It injected large amounts of aerosols into the stratosphere—more than any eruption since that of Krakatoa in 1883. Over the following months, the aerosols formed a global layer of sulfuric acid haze. Global temperatures dropped by about 0.5 °C (0.9 °F), and ozone depletion temporarily increased substantially.
The eruption cloud shortly before the climactic eruption
June 15 saw the onset of the climactic eruption. Large tremors starting at 13:42 saturated all the seismographs at Clark Air Base, and by 14:30 all had been rendered inoperative, mostly by pyroclastic density currents. Intense atmospheric pressure variation was also recorded. On the same day, Typhoon Yunya struck the island, passing about 75 km (50 miles) north of the volcano. The typhoon rains made direct visual observations of the eruption impossible, but measurements showed that ash was ejected to heights of 34 km by the most violent phase of the eruption, which lasted about three hours. Pyroclastic flows poured from the summit, reaching as far as 16 km away from it. Typhoon rains mixed with the ash deposits caused massive lahars.
The ash cloud from the volcano covered an area of some 125,000 km² (50,000 mi²), bringing total darkness to much of central Luzon. Almost all of the island received some ashfall, which formed a heavy, rain-saturated snow-like blanket. Tephra fell over most of the South China Sea and ashfall was recorded as far away as Vietnam, Cambodia and Malaysia.
As the clouds reached Malaysian Borneo, “ a haze span over the land, like none had ever witnessed. This haze penetrated every house, every room, and left a millimeter-blanquet of very fine grey ash everywhere (Obong Jau).”
Vast quantities of minerals and metals were brought to the surface. Overall, introduced to the surface environment, was an estimated 800,000 tons of zinc, 600,000 tons of copper, 550,000 tons of chromium, 300,000 tons of nickel, 100,000 tons of lead, 10,000 tons of arsenic, 1000 tons of cadmium, & 800 tons of mercury.
Dust and gases from major volcanic eruptions can affect world’s climate for a number of years, resulting in a temporary cooling, a change of weather patterns that affect harvests in (some) positive and (mostly) negative ways. Estimated duration of the temporary global cooling is as follows for the cited events:
Laki 2-5 years
Tambora 2-5 years
Krakatau 2-3 years
Katmai 1-3 years
Surtsey 2 cold winters - Northern hemissphere only
St. Helens 1-3 years
Pinatubo 2-5 years
In super-eruptions, the toll on human society and nature is frightening. The question, of how many Tambora-style eruption would balance the global warming, remains unanswered. No doubt there will be enormous volcanic eruptions sooner or later on this planet, but it would be unwise to reserve too much hope that such events – catastrophic, random, unpredictable- will solve our man-made problems.
© 2008 by Franz L Kessler
Picture: A trail of smoke and ash is seen drifting over the Southern Atlantic with the Jet Stream. Compared to the eruptions, the 2008 Chaiten eruption is seen as a minor event.
Picture by NASA