A volcanic winter is a reduction in global temperatures caused by volcanic ash and droplets of sulfuric acid and water obscuring the Sun and raising Earth's albedo after a large explosive volcanic eruption. Long-term cooling effects are dependent upon injection of sulfur gasses into the stratosphere where they undergo a series of reactions to create sulfuric acid which can nucleate and form aerosols. Volcanic stratospheric aerosols cool the surface by reflecting solar radiation and warm the stratosphere by absorbing terrestrial radiation; the variations in atmospheric warming and cooling result in changes in tropospheric and stratospheric circulation. The effects of volcanic eruptions on recent winters are modest in scale, but have been significant. 1991: Most the 1991 explosion of Mount Pinatubo, a stratovolcano in the Philippines, cooled global temperatures for about 2–3 years.1883: The explosion of Krakatoa created volcanic winter-like conditions. The four years following the explosion were unusually cold, the winter of 1887–1888 included powerful blizzards.
Record snowfalls were recorded worldwide. 1815: The 1815 eruption of Mount Tambora, a stratovolcano in Indonesia caused what came to be known as the "Year Without a Summer" of 1816. Europe, still recuperating from the Napoleonic Wars, suffered from food shortages. Food riots broke out in the United Kingdom and France, grain warehouses were looted; the violence was worst in landlocked Switzerland, where famine caused the government to declare a national emergency. Huge storms and abnormal rainfall with flooding of Europe's major rivers are attributed to the event, as is the August frost. A major typhus epidemic occurred in Ireland between 1816 and 1819, precipitated by the famine. An estimated 100,000 Irish perished during this period. A BBC documentary, using figures compiled in Switzerland, estimated that the fatality rates in 1816 were twice that of average years, giving an approximate European fatality total of 200,000 deaths; the corn crop in Northeastern North America failed, due to mid-summer frosts in New York State and June snowfalls in New England and Newfoundland and Labrador The crop failures in New England and parts of Europe caused the price of wheat, meat, butter and flour to rise sharply.
1783: The eruption of the Laki volcano in Iceland released enormous amounts of sulfur dioxide, resulting in the death of much of the island's livestock and a catastrophic famine which killed a quarter of the Icelandic population. It has been estimated. Northern hemisphere temperatures dropped by about 1 °C in the year following the Laki eruption; the winter of 1783–1784 was severe, estimated to have caused 8,000 additional deaths in the UK. The meteorological impact of Laki continued, contributing to several years of extreme weather in Europe. In France, the sequence of extreme weather events contributed to an increase in poverty and famine that may have contributed to the French Revolution in 1789. Laki was only one factor in a decade of climatic disruption, as Grímsvötn was erupting from 1783 to 1785, there may have been an unusually strong El Niño effect from 1789 to 1793. A paper written by Benjamin Franklin in 1783 blamed the unusually cool summer of 1783 in North America on volcanic dust coming from this eruption, though Franklin's proposal has been questioned.1600: The Huaynaputina in Peru erupted.
Tree ring studies show. Russia had its worst famine in 1601–1603. From 1600 to 1602, Switzerland and Estonia had exceptionally cold winters; the wine harvest was late in 1601 in France, in Peru and Germany, wine production collapsed. Peach trees bloomed late in China, Lake Suwa in Japan froze early.1452 or 1453: A cataclysmic eruption of the submarine volcano Kuwae caused worldwide disruptions. 1315-1317: The Great Famine of 1315–1317 in Europe may have been precipitated by a volcanic event that of Mount Tarawera, New Zealand, lasting about five years.1257: The 1257 Samalas eruption in Indonesia. The eruption left with Lake Segara Anak inside it; this eruption had a Volcanic Explosivity Index of 7, making it one of the largest eruptions of the current Holocene epoch. An examination of ice cores showed a large spike in sulfate deposition around 1257; this was strong evidence of a large eruption having occurred somewhere in the world. In 2013, scientists proved; this eruption had four distinct phases, alternately creating eruption columns reaching tens of kilometres into the atmosphere and pyroclastic flows burying large parts of Lombok Island.
The flows destroyed human habitations, including the city of Pamatan. Ash from the eruption fell as far away as Java Island; the volcano deposited more than 10 cubic kilometres of material. The eruption was witnessed by people who recorded it on the Babad Lombok. Volcanic activity created additional volcanic centres in the caldera, including the Barujari cone that remains active; the aerosols injected into the atmosphere reduced the solar radiation reaching the Earth's surface, which cooled the atmosphere for several years and led to famines and crop failures in Europe and elsewhere, although the exact scale of the temperature anomalies and their consequences is still debated. It is possible. 535: The extreme weather events of 535–536 are most linked to a volcanic eruption. The latest theorised explanation is the Tierra Blanca Joven eruption of the Ilopango caldera in central El Salvador. A proposed
Plinian eruptions or Vesuvian eruptions are volcanic eruptions marked by their similarity to the eruption of Mount Vesuvius in 79 AD, which destroyed the ancient Roman cities of Herculaneum and Pompeii. The eruption was described in a letter written by Pliny the Younger, after the death of his uncle Pliny the Elder. Plinian/Vesuvian eruptions are marked by columns of volcanic debris and hot gases ejected high into the stratosphere, the second layer of Earth's atmosphere; the key characteristics are ejection of large amount of pumice and powerful continuous gas-driven eruptions. According to the Volcanic Explosivity Index, Plinian eruptions have a VEI of 4, 5 or 6, sub-Plinian 3 or 4, ultra-Plinian 6, 7 or 8. Short eruptions can end in less than a day, but longer events can continue for several days or months; the longer eruptions begin with production of clouds of volcanic ash, sometimes with pyroclastic surges. The amount of magma erupted can be so large that it depletes the magma chamber below, causing the top of the volcano to collapse, resulting in a caldera.
Fine ash and pulverized pumice can deposit over large areas. Plinian eruptions are accompanied by loud noises, such as those generated by the 1883 eruption of Krakatoa; the sudden discharge of electrical charges accumulated in the air around the ascending column of volcanic ashes often causes lightning strikes as depicted by the English geologist George Julius Poulett Scrope in his painting of 1822. The lava is rhyolitic and rich in silicates. Basaltic, low-silicate lavas are unusual for Plinian eruptions. Pliny described his uncle's involvement from the first observation of the eruption: On August 24th, about one in the afternoon, my mother desired him to observe a cloud which appeared of a unusual size and shape, he had just taken a turn in the sun and, after bathing himself in cold water, making a light luncheon, gone back to his books: he arose and went out upon a rising ground from whence he might get a better sight of this uncommon appearance. A cloud, from which mountain was uncertain, at this distance, was ascending, the appearance of which I cannot give you a more exact description of than by likening it to that of a pine tree, for it shot up to a great height in the form of a tall trunk, which spread itself out at the top into a sort of branches.
This phenomenon seemed to a man of such learning and research as my uncle extraordinary and worth further looking into. Pliny the Elder set out to rescue the victims from their perilous position on the shore of the Bay of Naples, launched his galleys, crossing the bay to Stabiae. Pliny the Younger provided an account of his death, suggested that he collapsed and died through inhaling poisonous gases emitted from the volcano, his body was found interred under the ashes of the eruption with no apparent injuries on 26 August, after the plume had dispersed, confirming asphyxiation or poisoning. The June 2009 eruption of Sarychev Peak in Russia; the 1991 eruption of Mount Pinatubo in Zambales, Central Luzon, Philippines. The 1982 eruption of El Chichón in Chiapanecan Volcanic Arc, Mexico; the 1980 eruption of Mount St. Helens in Washington in the United States; the 1886 eruption of Mount Tarawera in New Zealand. The 1883 eruption of Krakatoa in Sunda Strait, Indonesia; the 1815 eruption of Mount Tambora in the island of Sumbawa, Indonesia.
The 1707 eruption of Mount Fuji in Japan. The 1667 and 1739 eruptions of Mount Tarumae in Hokkaido, Japan; the 180 AD Lake Taupo eruption in New Zealand. The 79 AD eruption of Mount Vesuvius in Pompeii, Italy, it was the prototypical Plinian eruption. The 400s BC eruption of the Bridge River Vent in British Columbia, Canada; the 1645 BC eruption of Thera in the south Aegean Sea, Greece. The 4860 BC eruption forming Crater Lake in Oregon, United States; the Long Valley Caldera eruption in Eastern California, United States, which happened over 760,000 years ago. The 1600 eruption of Huaynaputina in Peru; the 946 eruption of Paektu Mountain in China/North Korea The 1257 eruption of Mount Samalas in Lombok, Indonesia According to the Volcanic Explosivity Index, a VEI of 6 to 8 is classified as "ultra-Plinian". Eruptions of this type are defined by ash plumes over 25 km high and a volume of erupted material 10 km3 to 1,000 km3 in size. Eruptions in the ultra-Plinian category include the Lava Creek eruption of the Yellowstone Caldera, Lake Toba, Krakatoa, Akahoya eruption of Kikai Caldera and the 1991 Mount Pinatubo eruption in the Philippines.
Peléan eruption, related to the explosive eruptions of the Mount Pelée Types of volcanic eruptions List of largest volcanic eruptions USGS Photo Glossary Entry for Plinian Eruptions Volcanic mesocyclones
A limnic eruption termed a lake overturn, is a rare type of natural disaster in which dissolved carbon dioxide erupts from deep lake waters, forming a gas cloud capable of suffocating wildlife and humans. A limnic eruption may cause tsunamis as the rising CO2 displaces water. Scientists believe earthquakes, volcanic activity, other explosive events can serve as triggers for limnic eruptions. Lakes in which such activity occurs are referred to exploding lakes; some features of limnically active lakes include: CO2-saturated incoming water A cool lake bottom indicating an absence of direct volcanic interaction with lake waters An upper and lower thermal layer with differing CO2 saturations Proximity to areas with volcanic activityInvestigations of the Lake Monoun and Lake Nyos casualties led scientists to classify limnic eruptions as a distinct type of disaster event though they can be indirectly linked to volcanic eruptions. Due to the invisible nature of the underlying cause behind limnic eruptions, it is difficult to determine to what extent eruptions have occurred in the past.
In recent history, this phenomenon has been observed twice. The first recorded limnic eruption occurred in Cameroon at Lake Monoun in 1984, causing asphyxiation and death of 37 people living nearby. A second, deadlier eruption happened at neighbouring Lake Nyos in 1986, this time releasing over 80 million m3 of CO2, killing around 1,700 people and 3,500 livestock, again by asphyxiation. A third lake, Lake Kivu, rests on the border between the Democratic Republic of the Congo and Rwanda, contains massive amounts of dissolved CO2. Sediment samples from the lake taken by Professor Robert Hecky showed an event caused living creatures in the lake to go extinct around every 1000 years, caused nearby vegetation to be swept back into the lake. Limnic eruptions can be detected and quantified on a CO2 concentration scale by taking air samples of the affected region; the Messel pit fossil deposits of Messel, show evidence of a limnic eruption there in the early Eocene. Among the victims are preserved insects, turtles, birds, insectivores, early primates, paleotheres.
For a lake to undergo a limnic eruption, the water must be nearly saturated with gas. CO2 was the primary component in the two observed cases. In Lake Kivu, scientists are concerned about the concentrations of methane gas as well. CO2 may originate from volcanic gas emitted from under the lake or from decomposition of organic material. Before a lake is saturated, it behaves like an unopened carbonated beverage: the CO2 is dissolved in the water. In both the lake and the soft drink, CO2 dissolves much more at higher pressure; this is. In the case of lakes, the bottom is at a much higher pressure; therefore huge amounts of CO2 can be dissolved in deep lakes. CO2 dissolves more in cooler water, such as that found at a lake bottom. A small rise in water temperature can lead to the release of a large amount of CO2. Once a lake is saturated with CO2, it is unstable, but a trigger is needed to set off an eruption. In the case of the 1986 Lake Nyos eruption, landslides were the suspected triggers, but a volcanic eruption, an earthquake, or wind and rain storms are potential triggers.
Another possible cause of a limnic eruption is gradual gas saturation at specific depths which can trigger spontaneous gas development. For any of these cases, the trigger pushes some of the gas-saturated water higher in the lake, where pressure is insufficient to keep CO2 in solution; as bubbles start forming the water is lifted higher in the lake, where yet more CO2 comes out of solution. This process forms a column of gas, at which point the water at the bottom of this column is pulled up by suction, it, loses CO2 in a runaway process; this eruption discharges CO2 into the air and can displace enough water to form a tsunami. Limnic eruptions are exceptionally rare for several reasons. First, a CO2 source must exist. Second, the vast majority of lakes are holomictic. Only meromictic lakes do not remain stratified, allowing CO2 to remain dissolved, it is estimated. A lake must be deep enough to have sufficient pressure to dissolve large amounts of CO2. Once an eruption occurs, a large CO2 cloud forms above the lake and expands to the surrounding region.
Because CO2 is denser than air, it has a tendency to sink to the ground displacing breathable air, resulting in asphyxia. CO2 can make human bodily fluids acidic and cause CO2 poisoning; as victims gasp for air, they accelerate asphyxia by inhaling CO2 gas. At Lake Nyos, the gas cloud descended into a nearby village where it settled, killing nearly everyone. A change in skin color on some bodies led scientists to hypothesize the gas cloud may have contained dissolved acid such as hydrogen chloride, though this hypothesis is disputed. Many victims were found with blisters on their skin, thought to have been caused by pressure ulcers, which were caused by low blood oxygen levels in those asphyxiated by carbon dioxide. Nearby vegetation was unaffected, except any growing adjacent to the lake. There, vegetation was da
1980 eruption of Mount St. Helens
On May 18, 1980, a major volcanic eruption occurred at Mount St. Helens, a volcano located in Skamania County, in the U. S. state of Washington. The eruption was the most significant volcanic eruption to occur in the contiguous 48 U. S. states since the much smaller 1915 eruption of Lassen Peak in California. It has been declared the most disastrous volcanic eruption in U. S. history. 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 volcano that created a large bulge and a fracture system on the mountain's north slope. An earthquake at 8:32:17 a.m. PDT on Sunday, May 18, 1980, caused the entire weakened north face to slide away, creating the largest landslide recorded; this allowed the molten, high-pressure gas- and steam-rich rock in the volcano to explode northwards toward Spirit Lake in a hot mix of lava and pulverized older rock, overtaking the avalanching face. An eruption column rose 80,000 feet into the atmosphere and deposited ash in 11 U.
S. states. At the same time, snow and several entire glaciers on the volcano melted, forming a series of large lahars that reached as far as the Columbia River, nearly 50 miles to the southwest. Less severe outbursts continued into the next day, only to be followed by other large, but not as destructive, eruptions that year. Thermal energy released during the eruption was equal to 26 megatons. 57 people were killed directly, including innkeeper Harry R. Truman, photographers Reid Blackburn and Robert Landsburg, geologist David A. Johnston. Hundreds of square miles were reduced to wasteland, causing over $1 billion in damage, thousands of animals were killed, Mount St. Helens was left with a 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 preserved, as it was, in the Mount St. Helens National Volcanic Monument. Mount St. Helens remained dormant from its last period of activity in the 1840s and 1850s until March 1980.
Several small earthquakes, beginning on March 15, indicated that magma may have begun moving below the volcano. On March 20, at 3:45 p.m. Pacific Standard Time, a shallow magnitude 4.2 earthquake centered below the volcano's north flank, signaled the volcano's violent return from 123 years of hibernation. A building earthquake swarm saturated area seismographs and started to climax at about noon on March 25, reaching peak levels in the next two days, including an earthquake registering 5.1 on the Richter scale. A total of 174 shocks of magnitude 2.6 or greater were recorded during those two days. Shocks of magnitude 3.2 or greater occurred at a increasing rate during April and May with five earthquakes of magnitude 4 or above per day in early April, eight per day the week before May 18. There was no direct sign of eruption, but small earthquake-induced avalanches of snow and ice were reported from aerial observations. At 12:36 p.m. on March 27, phreatic eruptions ejected and smashed rock from within the old summit crater, excavating a new crater 250 feet wide, sending an ash column about 7,000 feet into the air.
By this date a 16,000-foot-long eastward-trending fracture system had developed across the summit area. This was followed by more earthquake swarms and a series of steam explosions that sent ash 10,000 to 11,000 feet above their vent. Most of this ash fell between three and twelve miles from its vent, but some was carried 150 miles south to Bend, Oregon, or 285 miles east to Spokane, Washington. A second, new crater and a blue flame were observed on March 29; the flame was visibly emitted from both craters and was created by burning gases. Static electricity generated from ash clouds rolling down the volcano sent out lightning bolts that were up to two miles long. Ninety-three separate outbursts were reported on March 30, strong harmonic tremors were first detected on April 1, alarming geologists and prompting Governor Dixy Lee Ray to declare a state of emergency on April 3. Governor Ray issued an executive order on April 30 creating a "red zone" around the volcano; this precluded many cabin owners from visiting their property.
By April 7, the combined crater was 500 feet deep. A USGS team determined in the last week of April that a 1.5-mile-diameter section of St. Helens' north face was displaced outward by at least 270 feet. For the rest of April and early May this bulge grew by five to six feet per day, by mid-May it extended more than 400 feet north; as the bulge moved northward, the summit area behind it progressively sank, forming a complex, down-dropped block called a graben. Geologists announced on April 30 that sliding of the bulge area was the greatest immediate danger and that such a landslide might spark an eruption; these changes in the volcano's shape were related to the overall deformation that increased the volume of the volcano by 0.03 cubic miles by mid-May. This volume increase corresponded to the volume of magma that pushed into the volcano and deformed its surface; because the intruded magma remained below ground and was not directly visible, it was called a cryptodome, in contrast to a true lava dome exposed at t
A phreatic eruption called a phreatic explosion, ultravulcanian eruption or steam-blast eruption, occurs when magma heats ground or surface water. The extreme temperature of the magma causes near-instantaneous evaporation to steam, resulting in an explosion of steam, ash and volcanic bombs. At Mount St. Helens, hundreds of steam explosions preceded a 1980 plinian eruption of the volcano. A less intense geothermal event may result in a mud volcano. Phreatic eruptions include steam and rock fragments; the temperature of the fragments can range from cold to incandescent. If molten magma is included, it is classified as a phreatomagmatic eruption; these eruptions create broad, low-relief craters called maars. Phreatic explosions can be accompanied by carbon hydrogen sulfide gas emissions; the former can asphyxiate at sufficient concentration. A 1979 phreatic eruption on the island of Java killed 140 people, most of whom were overcome by poisonous gases. Phreatic eruptions are classed as volcanic eruptions because a phreatic eruption could bring juvenile material to the surface.
It is believed that the 1883 eruption of Krakatoa, which obliterated most of the volcanic island and created the loudest sound in recorded history, was a phreatic event. Kilauea, in Hawaii, has a long record of phreatic explosions. Additional examples are the 1963–65 eruption of Surtsey, the 1965 eruption of Taal Volcano, the 1982 Mount Tarumae eruption, the 2014 eruption of Mount Ontake and on May 7, 2013, at 8 a.m. Mayon Volcano produced a surprise phreatic eruption lasting 73 seconds. Types of volcanic eruptions – Basic mechanisms of eruption and variations Phreatic Hydrothermal explosion – Explosion of superheated ground water converting to steam Steam cannon
A pyroclastic flow is a fast-moving current of hot gas and volcanic matter that moves away from a volcano about 100 km/h on average but is capable of reaching speeds up to 700 km/h. The gases can reach temperatures of about 1,000 °C. Pyroclastic flows are a devastating result of certain explosive eruptions, their speed depends upon the density of the current, the volcanic output rate, the gradient of the slope. The word pyroclast is derived from the Greek πῦρ, meaning "fire", κλαστός, meaning "broken in pieces". A name for pyroclastic flows which glow red in the dark is nuée ardente. Pyroclastic flows that contain a much higher proportion of gas to rock are known as "fully dilute pyroclastic density currents" or pyroclastic surges; the lower density sometimes allows them to flow over higher topographic features or water such as ridges, hills and seas. They may contain steam and rock at less than 250 °C. Cold pyroclastic surges can occur when the eruption is from a vent under the sea. Fronts of some pyroclastic density currents are dilute.
A pyroclastic flow is a type of gravity current. There are several mechanisms that can produce a pyroclastic flow: Fountain collapse of an eruption column from a Plinian eruption. In such an eruption, the material forcefully ejected from the vent heats the surrounding air and the turbulent mixture rises, through convection, for many kilometers. If the erupted jet is unable to heat the surrounding air sufficiently, convection currents will not be strong enough to carry the plume upwards and it falls, flowing down the flanks of the volcano. Fountain collapse of an eruption column associated with a Vulcanian eruption; the gas and projectiles create a cloud, denser than the surrounding air and becomes a pyroclastic flow. Frothing at the mouth of the vent during degassing of the erupted lava; this can lead to the production of a rock called ignimbrite. This occurred during the eruption of Novarupta in 1912. Gravitational collapse of a lava dome or spine, with subsequent avalanches and flows down a steep slope.
The directional blast when part of a volcano explodes. As distance from the volcano increases, this transforms into a gravity-driven current; the volumes range from a few hundred cubic meters to more than 1,000 cubic kilometres. The larger ones can travel for hundreds of kilometres, although none on that scale have occurred for several hundred thousand years. Most pyroclastic flows are around travel for several kilometres. Flows consist of two parts: the basal flow hugs the ground and contains larger, coarse boulders and rock fragments, while an hot ash plume lofts above it because of the turbulence between the flow and the overlying air and heating cold atmospheric air causing expansion and convection; the kinetic energy of the moving cloud will flatten buildings in its path. The hot gases and high speed make them lethal, as they will incinerate living organisms instantaneously: The cities of Pompeii and Herculaneum, for example, were engulfed by pyroclastic surges on August 24, 79 AD with many lives lost.
The 1902 eruption of Mount Pelée destroyed the Martinique town of St. Pierre. Despite signs of impending eruption, the government deemed St. Pierre safe due to hills and valleys between it and the volcano, but the pyroclastic flow charred the entirety of the city, killing all but two of its 30,000 residents. A pyroclastic surge killed volcanologists Harry Glicken and Katia and Maurice Krafft and 40 other people on Mount Unzen, in Japan, on June 3, 1991; the surge started as a pyroclastic flow and the more energised surge climbed a spur on which the Kraffts and the others were standing. On 25 June, 1997 a pyroclastic flow travelled down Mosquito Ghaut on the Caribbean island of Montserrat. A large energized pyroclastic surge developed; this flow could not be restrained by the Ghaut and spilled out of it, killing 19 people who were in the Streatham village area. Several others in the area suffered severe burns. Testimonial evidence from the 1883 eruption of Krakatoa, supported by experimental evidence, shows that pyroclastic flows can cross significant bodies of water.
However, that might be a pyroclastic surge, not flow, because the density of a gravity current means it cannot move across the surface of water. One flow reached the Sumatran coast as much as 48 km away. A 2006 BBC documentary film, Ten Things You Didn't Know About Volcanoes, demonstrated tests by a research team at Kiel University, Germany, of pyroclastic flows moving over water; when the reconstructed pyroclastic flow hit the water, two things happened
Mount St. Helens
Mount St. Helens is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States, it is 96 miles south of Seattle, Washington. Mount St. Helens takes its English name from the British diplomat Lord St Helens, a friend of explorer George Vancouver who made a survey of the area in the late 18th century; the volcano is located in the Cascade Range and is part of the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire that includes over 160 active volcanoes. This volcano is well known for pyroclastic flows. Mount St. Helens is most notorious for its major 1980 eruption, the deadliest and most economically destructive volcanic event in U. S. history. Fifty-seven people were killed. A massive debris avalanche triggered by an earthquake measuring 5.1 on the Richter scale caused an eruption that reduced the elevation of the mountain's summit from 9,677 ft to 8,363 ft, leaving a 1 mile wide horseshoe-shaped crater. The debris avalanche was up to 0.7 cubic miles in volume.
The Mount St. Helens National Volcanic Monument was created to preserve the volcano and allow for the eruption's aftermath to be scientifically studied; as with most other volcanoes in the Cascade Range, Mount St. Helens is a large eruptive cone consisting of lava rock interlayered with ash and other deposits; the mountain includes layers of basalt and andesite through which several domes of dacite lava have erupted. The largest of the dacite domes formed the previous summit, off its northern flank sat the smaller Goat Rocks dome. Both were destroyed in the 1980 eruption. Mount St. Helens is 34 miles west in the western part of the Cascade Range; these "sister and brother" volcanic mountains are 50 miles from Mount Rainier, the highest of Cascade volcanoes. Mount Hood, the nearest major volcanic peak in Oregon, is 60 miles southeast of Mount St. Helens. Mount St. Helens is geologically young compared with the other major Cascade volcanoes, it formed only within the past 40,000 years, the pre-1980 summit cone began rising about 2,200 years ago.
The volcano is considered the most active in the Cascades within the Holocene epoch. Prior to the 1980 eruption, Mount St. Helens was the fifth-highest peak in Washington, it stood out prominently from surrounding hills because of the symmetry and extensive snow and ice cover of the pre-1980 summit cone, earning it the nickname "Fuji-san of America". The peak rose more than 5,000 feet above its base, where the lower flanks merge with adjacent ridges; the mountain is 6 miles across at its base, at an elevation of 4,400 feet on the northeastern side and 4,000 feet elsewhere. At the pre-eruption tree line, the width of the cone was 4 miles. Streams that originate on the volcano enter three main river systems: the Toutle River on the north and northwest, the Kalama River on the west, the Lewis River on the south and east; the streams are fed by abundant snow. The average annual rainfall is 140 inches, the snow pack on the mountain's upper slopes can reach 16 feet; the Lewis River is impounded by three dams for hydroelectric power generation.
The southern and eastern sides of the volcano drain into an upstream impoundment, the Swift Reservoir, directly south of the volcano's peak. Although Mount St. Helens is in Skamania County, access routes to the mountain run through Cowlitz County to the west and Lewis County to the north. State Route 504, locally known as the Spirit Lake Memorial Highway, connects with Interstate 5 at Exit 49, 34 miles to the west of the mountain; that north–south highway skirts the low-lying cities of Castle Rock and Kelso along the Cowlitz River, passes through the Vancouver, Washington–Portland, Oregon metropolitan area less than 50 miles to the southwest. The community nearest the volcano is Cougar, Washington, in the Lewis River valley 11 miles south-southwest of the peak. Gifford Pinchot National Forest surrounds Mount St. Helens. During the winter of 1980–1981, a new glacier appeared. Now named Crater Glacier, it was known as the Tulutson Glacier. Shadowed by the crater walls and fed by heavy snowfall and repeated snow avalanches, it grew rapidly.
By 2004, it covered about 0.36 square miles, was divided by the dome into a western and eastern lobe. By late summer, the glacier looks dark from rockfall from the crater walls and ash from eruptions; as of 2006, the ice had an average thickness of 300 feet and a maximum of 650 feet, nearly as deep as the much older and larger Carbon Glacier of Mount Rainier. The ice is all post-1980, making the glacier young geologically. However, the volume of the new glacier is about the same as all the pre-1980 glaciers combined. With the recent volcanic activity starting in 2004, the glacier lobes were pushed aside and upward by the growth of new volcanic domes; the surface of the glacier, once without crevasses, turned into a chaotic jumble of icefalls criss-crossed with crevasses and seracs caused by movement of the crater floor. The new domes have separated the Crater Glacier into an eastern and western lobe. Despite the volcanic activity, the termini of the glacier have still advanced, with a slight advance on the western lobe and a more considerable advance on the more sha