Volcano
A volcano is a relief land , underwater or extraterrestrial formed by the ejection and stacking of materials from the rise of magma as lava and tephra such as ash. This magma comes from the fusion of partial mantle and exceptionally the crust. The accumulation may reach thousands of meters thick, thus forming mountains or islands. Depending on the nature of the materials, the type of eruption , their frequency and orogeny , volcanoes take various forms but generally has the appearance of a conical mountain, surmounted by a crater or caldera . The submarine volcanoes are much more numerous.
The "volcano" is the set of phenomena associated with volcanoes and the presence of magma. The " Volcano "or" Volcanology is the science of the study, observation and prevention of risks from volcanoes.
The term "volcano" originated from Vulcano , one of the Aeolian Islands , named in honor of Vulcan , the Roman god of fire whose equivalent in the Greek pantheon is Hephaestus.
Description
Structures and landforms
A volcano is composed of different structures found in general at each:
- a magma chamber fed by magma from the mantle and acts as a reservoir and place of magma differentiation. When it is empty after an eruption , the volcano may collapse and give birth to a caldera. The magma chambers are located between ten and fifty kilometers deep in the lithosphere ;
- a volcanic vent , which is the preferred place of transit of the magma of the magma chamber to the surface;
- a crater or caldera summit which opens the volcanic chimney;
- one or more volcanic vents from side of the magma chamber or the main volcanic vent and opening usually on the flanks of the volcano, sometimes at its base and may give rise to small secondary cones;
- lateral cracks are longitudinal fractures in the flank of the volcano caused by the swelling or bursting, and may permit the emission of lava in the form of a fissure eruption.
Volcanoes form
Depending on the dish they emit and the type of eruption , volcanoes can take different forms:
- in shield volcano when its diameter is greater than its height because of the fluidity of the lavas that can travel for miles before stopping, the Mauna Kea , the Erta Ale or the Piton de la Fournaise are examples ;
- in stratovolcano when its diameter is more balanced compared to its height because of the higher viscosity of the lava and it is explosive eruptions of volcanoes such as Vesuvius , the Mount Fuji , the Merapi or Mount Saint Helens ;
- in volcanic fault line formed by an opening in the oceanic crust or by which fluid escapes from the lava, volcanoes and ridges occur as fault as the Laki or Krafla .
Igneous material
Magma
The magma is the material consistency fluid viscous under pressure containing volcanic gases , not crystallized which was formed from the merger or part of the mantle ( anatexis ) at a point of heat as a point hot , decompression as a dorsal and / or water supply as a subduction trench. Generally, this magma rises to the surface and be stored in the lithosphere , forming a magma chamber. In this room, it can undergo crystallization or partial and / or outgassing, which begins to turn into lava. If the pressure is enough to be ejected to the surface, it rises along a volcanic chimney to be issued in the form of lava , that is to say, totally or partially degassed .
Tephra and lava
Depending on whether the magma comes from the melting of mantle or part of the lithosphere , it will not have the same mineral composition, or the same amount of water in volcanic gas , or the same temperature. Moreover, according to the type of terrain it crosses rising to the surface and the duration of his stay in the magma chamber , it will either load or discharge in minerals, water and / or gas and is more or less cool. For all these reasons, tephras and lavas are never exactly the same from one volcano to another, or even sometimes a rash to another on the same volcano.
Materials emitted by volcanoes are generally rocks composed of microlites embedded in a glass magma. In basalt , the most abundant minerals are silica , the pyroxene and feldspar , whereas the andesite is richer in silica and feldspar. The structure of the rock also varies: if the crystals are frequently small and few in basalts, however they are generally larger and more numerous in the andesites, indicating that magma remained longer in the magma chamber . 95% of the materials emitted by volcanoes are basalts and andesites.
The best-known material emitted by volcanoes is lava in the form of castings. Type basalt from the mantle melting in the case of a volcanic hotspot , the dorsal or rift or andesite from the melting of the lithosphere in the case of volcanism in subduction , rarely type carbonatic , they are formed of fluid lava flowing along the flanks of the volcano. The temperature of lava is between 700 and 1200 C and flows can reach tens of kilometers in length, a speed of fifty miles per hour and progress in Lava. They can be smooth and satiny, then called " pahoehoe lava "or" washing strung, "or a rough and cut, then called" aa lava. The flows of lava, which sometimes several feet thick, can take decades to cool completely . In some exceptional cases, the lava can fill the crater or major secondary crater and form a lava lake. Survival of lava lakes results from a balance between supply of lava from the magma chamber and overflow outside the crater associated with a permanent mixing of gases by volcanic mountain in order to limit the hardening of the lava. These lava lakes that are born during Hawaiian eruptions , the large flow of lava to the formation and maintenance of these phenomena. The Kilauea in Hawaii and Piton de la Fournaise on Runion are two volcanoes have lava lakes at some of their eruptions. The Erta Ale in Ethiopia and Mount Erebus in Antarctica are among the few volcanoes in the world with a lava lake in a quasi-permanent. In some eruptions of Erta Ale's lava lake is emptied or otherwise overflow its level goes up and train flows on the slopes of the volcano .
The most common materials are composed of volcanic tephra and are the volcanic ash , and lapilli , the slag , the pumice stones , and volcanic bombs , boulders and basalt, obsidian , etc.. These magma and pieces of rock torn from the volcano that are sprayed and thrown up to tens of kilometers high in the atmosphere. The smallest is the ashes, they sometimes go around the Earth , carried by the winds prevailing. Bombs, volcanic ejecta larger, can be the size of a house and generally fall near the volcano. When volcanic bombs are ejected while still molten, they can take a spindle shape during their passage through the atmosphere, cow dung on their impact on soil or crust of bread in the presence of water . Lapilli, which resemble small pebbles, can accumulate in thick layers and thus form the pozzolan. Pumice, lava real foam are so small and contain so much air they can float on water. Finally, when fine droplets are ejected lava and carried by the winds, they can stretch into long filaments called Pele's hair ".
Volcanic gases
The magmas contain volcanic gases dissolved. Degassing of magma is a phenomenon critical in triggering an eruption and the eruptive type. Degassing magma pushed up along the volcanic pipe which may give the explosive and violent eruption in the presence of a viscous magma.
Volcanic gases are primarily composed of :
- water vapor content of 50 to 90%;
- carbon dioxide content of 5 to 25%;
- sulfur dioxide content of 3 to 25%.
Then come other volatile elements such as carbon monoxide , the hydrogen chloride , the hydrogen , the hydrogen sulfide , etc.. Degassing of magma at depth can lead to the surface by the presence of fumaroles around which crystals , mostly of sulfur , can form.
Frequency of eruptions
The "birth" of a volcano is the first volcanic eruption which removes it from the lithosphere. The birth of a new volcano is a relatively rare but could be observed in 1943 with the Paricutn : fracture allowing the escape of volcanic gases and lava in a field gave birth to a volcano, 460 meters top to nine months. In 1963 , the submarine volcano of Surtsey emerged in southern Iceland , thus forming a new island volcano and a new earth.
A volcano is "active" when it last erupted a few decades at most, no longer asleep when erupted for several hundred years and died when its last eruption dates back to at least 50 000 years and is subject to erosion.
In general, volcanoes undergo several eruptions during their life. But their frequency varies according to the volcano eruption some know only a few centuries as the Ponta do Pico in the Azores , while others are erupting permanent as Stromboli in Italy or Merapi in Indonesia.
Sometimes volcanoes are formed in a single eruption can fall asleep or go out for tens or hundreds of thousands of years. These monogenic volcanoes. Volcanoes of the Chaine des Puys in the Massif Central were formed between 11,500 BC. BC and 5000 BC. AD during a single eruption for each volcano and have never shown any sign of activity.
The frequency of eruptions can assess the hazard , ie the probability that an area may suffer one of the manifestations of an eruption. This hazard, combined with the type of volcanic event and the presence of people and its vulnerability , assesses the risk volcano.
Origin of volcanism
According to the theory of plate tectonics , volcanism is intimately linked to the movement of tectonic plates. Indeed, it is usually at the boundary between two plates that conditions are ripe for the formation of volcanoes.
Volcanism of divergence (or accretion)
In the rift of the dorsal , the distance between two tectonic plates thins the lithosphere , causing a rise in rocks of mantle. These, already very hot at about 1200 C, begin to melt partially due to decompression. This gives the magma seeping through normal faults. Between the two edges of rift , traces of volcanic activity such as " pillow lava "or" pillow lava "formed by the emission of washing fluid in cold water. These volcanic rocks are as much of the crust oceanic training.
The "axial volcanic chains" in Afar are of type "ocean" (the plane tectonic and magmatic) and provide the relay between the axial valley of the Red Sea and the Gulf of Aden, so that the plate boundary between Africa and Arabia does not "at sea" by the Strait of Bab-el-Mandeb, but on land through the Afar. The nature of the tectonics and volcanism in Afar is thus distinguished from that of the African Rift, which is a "rift" has not led to the generation of new oceanic crust.
Subduction volcanism
When two tectonic plates overlap, the oceanic lithosphere , sliding under the other oceanic or continental lithosphere plunges into the mantle and undergoes mineralogical transformations. The water in the lithosphere plunging escapes and then comes hydrate the mantle, causing it to melt partially by lowering its melting point. This magma rises through the lithosphere and overlapping, creating volcanoes. If the oceanic lithosphere is thrust, a volcanic arc island will be formed, giving rise to the volcanoes of the islands. This is the case Aleutians , from Japan or the West Indies. If the continental lithosphere is thrust, the volcanoes are located on the mainland, usually in a mountain range. This is where the volcanoes of the Andes or the Rockies. These volcanoes are volcanoes usually gray, explosives and dangerous. This is due to their viscous lava because rich in silica , which is hard to escape; more dating magmas are rich in dissolved gases (water and carbon dioxide), whose sudden release can form pyroclastic flows. The " Pacific Ring of Fire "is formed in near majority of this type of volcano.
Hot spot volcanism
Sometimes volcanoes are born far from any boundary of lithospheric plate. These volcanoes usually hot. The hot spots are plumes of magma melt from deep within the mantle and piercing the lithospheric plates. The hot spots are fixed, while the lithospheric plate moves over the mantle, volcanoes are formed successively and then line up, the latest being the most active since plumb the hot spot. When the hot spot opens on an ocean, it will give rise to a fringe of islands aligned as is the case for the islands of Hawaii or Mascarenes. If the hot spot opens as a continent, it will then give rise to a series of volcanoes aligned. This is the case of Mount Cameroon and its neighbors. Exceptional case, sometimes a hot spot opens in a lithospheric plate boundary. In the case of Iceland , the effect of a hot spot combines with that of the Mid-Atlantic , giving rise to a huge pile of washing to the emergence of the ridge. The Azores or the Galapagos are other examples of emerging hot spots in a lithospheric plate boundary, ie ridges .
Course of a typical eruption
A volcanic eruption occurs when magma chamber beneath the volcano is pressurized with the arrival of magma from the mantle. It can then eject more or less volcanic gases contained therein by refilling its magma. Pressurization is accompanied by swelling of the volcano and earthquakes located very shallow beneath the volcano, the signs that magma chamber is deformed. The magma rises generally the main stack and undergoes simultaneously degassing which causes tremor , that is to say, a constant vibration and very light soil. This is due to small earthquakes whose foci are concentrated along the chimney.
The eruption began when the lava reached the open air . Depending on the type of magma, it flows on the flanks of the volcano or accumulates instead of issue, forming a plug of lava that can give pyroclastic and / or volcanic plumes when it explodes. Depending on the strength of the eruption, the morphology of the terrain, the proximity of the sea, so it can happen to other phenomena accompanying the eruption: major earthquakes, landslides , tsunamis , etc..
The possible presence of water in solid form as an ice cap , a glacier , the snow or liquid such as a crater lake , an aquifer , a river , a sea or ocean will cause the contact of igneous material such as magma, lava or tephra explode or increase their explosive power. Fragmenting materials and increasing the volume suddenly turning into steam , water acts as a multiplier of the explosive power of a volcanic eruption that will be described as table or table-magmato. The melting of ice or snow by the heat of magma may also cause lahars when water causes tephra or jkulhlaups as was the case for Grmsvtn in 1996.
The eruption ends when the lava is no longer issued. The lava flows ceased to be hungry, stop and begin to cool and ash , cooled in the atmosphere , fall to the ground surface. But changes in the nature of the ground by the recovery of soils by lava and tephra sometimes tens of meters thick can create destructive and deadly phenomenon. And the ash fell on crops destroy and sterilize the soil for several months to several years, a lava flow blocking a valley can create a lake that drowned areas inhabited or cultivated, rain falling on the ashes can be carried in the rivers and create lahars , etc..
A volcanic eruption can last from several hours to several years and eject volumes of magma hundreds of cubic miles. The average duration of an eruption is one month and a half, but many only last one day. The absolute record is that of Stromboli , which is almost erupted since about 2400 years .
Classification of volcanoes
In the early days of Volcanology , observation of some volcanoes was behind the creation of categories based on the appearance of eruptions and the type of lava emitted. Each type is named after the volcano referent. The great defect of this classification is to be fairly subjective and poorly reflect changes in the type of eruption of a volcano.
The term " cataclysmic "can be added when the power of the eruption causes heavy environmental damage and / or human as was the case for the Santorini to 1600 BC. BC , which contributed to the fall of the Minoan civilization , the Vesuvius in 79 that destroyed Pompeii , the Krakatoa in 1883 which generated a tsunami of forty meters in height, Mount St. Helens in 1980 that destroyed acres of forest etc..
To introduce a notion of comparison between the different volcanic eruptions, the a href = "% Indice_d 27explosivit% C3% A9_volcanique"> index of volcanic explosion, also called VEI, was developed by two volcanologists from the University of Hawaii in 1982 .
There are two main types of volcanic eruptions, depending on the type of magma emitted: effusive associated with " red volcanoes "and associated with the explosive" volcano gray " . The eruptions are effusive eruptions, Hawaiian and Strombolian explosive while are Vulcanian , pelenne and Plinian. These eruptions can occur in the presence of water and then take the characteristics of eruptions table , Surtseyan , subglacial , diving and limnic.
Volcanic geomorphology
Apart from the volcano itself, different formations geological directly or indirectly related to volcanic activity.
Some landforms and landscapes resulting from the direct product of eruptions. These volcanic cones themselves forming mountains or islands , the domes and lava solidified and Lava , the " pillow lavas "and seamounts of underwater volcanoes , the traps forming trays , accumulations of tephra in tuff , the craters left by the exit of the washer , etc..
Other landforms are the result of erosion or changes in products of eruptions. For these dykes , necks , sills , intrusive rocks , mesas and planzes released by erosion of the caldera and circuses of the collapse of part of the volcano, the crater lakes or formed upstream of a dam consisting of products of the eruption of atolls reefs surrounding the remains of a submarine volcano collapsed, and so on.
Phenomena paravolcaniques
Some activities geothermal can precede, accompany or follow a volcanic eruption. These activities are usually present when a residual heat from a magma chamber heats the water table sometimes up to boiling. Surface occurs when geysers , fumaroles , mud pools , mofettes , solfataras or mineral deposits . These phenomena can be grouped in "volcanic fields. These volcanic fields are formed when ground water is heated by magma reservoirs located at shallow depth. This is the case supervolcanoes as Yellowstone in the U.S. and Campi Flegrei in Italy or in geothermal fields as Haukadalur in Iceland.
At mid-ocean ridges , seawater seeps into cracks in the seafloor , warms, becomes laden with minerals and appears at the bottom of the oceans in the form of black smokers or white smokers.
In a crater with a degassing activity and fumaroles, a lake acid can be formed by collecting rainwater. The lake water is highly acidic with a pH from April to January, sometimes very hot with temperatures of 20 to 85 C and only cyanobacteria are able to live in these waters then tinted blue-green. This type of lake is common at large chain of volcanoes as the Pacific Ring of Fire and the Great Rift Valley.
Consequently the history of volcanism on Earth
The volcanism was born at the same time as the Earth , during the phase of accretion from its formation 4.6 billion years ago. From a certain mass, the materials in the center of the Earth undergo significant pressure , creating heat. This heat, exacerbated by the degradation of radioactive elements , causes the melting of the Earth that dissipates heat more than twenty times today. After a few million years, a solid film forms on the surface of the Earth. She is torn in many places by streams of lava and large masses granitoids that will give future continents. Subsequently, the tectonic plates will tear newly created preferentially at specific locations where volcanoes form. For a hundred million years, volcanoes reject the lean atmosphere of the era of large quantities of gases: nitrous , carbon dioxide , water vapor , sulfur dioxide , hydrochloric acid , hydrofluoric acid , etc.. There are 4.2 billion years, despite the 375 C and pressures 260 times greater than today, the water vapor condenses and gives rise to the oceans.
The role of the formation of the first organic molecules and the appearance of life on Earth can be attributed to volcanoes. Indeed, submarine hot springs or solfataras and other geysers provide conditions conducive to the emergence of life: water that leached carbon molecules, minerals, heat and energy. Once widespread and diverse life on the surface of the Earth, volcanoes could cause the opposite of mass extinctions : the age of mass extinctions of life coincides with the age of traps. These traps could be caused by the fall of meteorites or the eruption of outstanding hot spots. The combined effects of volcanic gases and particles dispersed in the atmosphere have caused the extinction of many species by a volcanic winter followed by a rise in the greenhouse by changes in the gaseous composition of the atmosphere.
One of the most accepted theories for the appearance of man would open the African Rift : evenly moist at the equator , the African climate would dry in the east of the rift that stops the clouds from the West. The hominids , adapting to their new environment formed a savannah , have developed bipedalism to escape predators.
Even today, the volcanoes are involved in the evacuation of the internal heat of the Earth and the biogeochemical cycling world by releasing the gas, water vapor and mineral swallowed up in the mantle at subduction trenches.
Impact of volcanism on human activities
Beliefs and myths associated with volcanoes
Since the advent of agriculture and sedentary societies, men have always rubbed volcanoes. Praising them for fertile land they offer, they also fear for their rash and deaths they cause. Quickly, by ignorance of natural phenomena, volcanoes are feared, deified , seen as the entry from the dead, the underworld and underground worlds populated by evil spirits and are the subject of legends and myths according different cultures.
In the tribes of Asia , from Oceania and America living near the Pacific ring of fire , volcanic eruptions are considered manifestations of divine or supernatural forces. In Mori mythology , volcanoes Taranaki and Ruapehu both fell in love with the volcano Tongariro and a violent argument broke out between the two. That's why no Mori saw between the two volcanoes angry for fear of being caught in the middle of the dispute.
Among other myths and legends, we can report that of the Devils Tower who would be prepared to rescue seven girls Amerindian of bears that have scratched the rock or the history of the goddess Pele who, driven from Tahiti by his sister Namakaokahai , found refuge in the Kilauea and since, rage, poured streams of lava with a flick of his heel.
Among the Incas , the vagaries of Misti earned him have his crater blocked by a plug of ice , punishment imposed by the Sun. The Chagas of Tanzania said that the Kilimanjaro , exasperated by his neighbor volcano Mawensi , the banging of pestle, which earned him the top cut. Among Native Americans of Oregon , the Mount Mazama was the home of the evil god of fire and the Mount Shasta beneficial than the god of snow. One day the two gods came into conflict and the god of fire was defeated and beheaded, thus creating the Crater Lake sign of defeat.
Volcanoes were the same place of human sacrifice : children thrown into the crater of Bromo in Indonesia , Christians sacrificed for the Mount Unzen in Japan , virgins thrown into the lava lake of Masaya in Nicaragua , children thrown into a crater lake to calm the submarine volcano lake of Ilopango in El Salvador , etc..
Among Greeks and Romans , volcanoes are the living place of Vulcan / Hephaestus. The eruptions are explained as a divine manifestation (angry gods, omens, forges of Vulcan / Hephaestus active, etc.). No scientific explanation or not involving the Gods was chosen. For the Romans, the forges of Vulcan were under Vulcano while for the Greeks, those of Hephaestus were located in the Etna. The Cyclops Greek could be an allegory of volcanoes with their crater summit while the name of Herakles or drift hiera Etna, the Greek word used to designate the volcanoes.
Among the Greek myths featuring volcanoes, the most famous is that recounted by Plato in the Timaeus and Critias. These stories tell of the disappearance of Atlantis , swallowed by the waves in a massive earthquake followed by tsunami. Which do not directly involve a volcano, this myth appears to have originated in the eruption of Santorini around 1600 BC. AD , which destroyed almost entirely the island and could have caused or contributed to the fall of the Minoan civilization. However, no observation of the eruption of Santorini was not recorded and it was not until the early twentieth century that we realized the importance of the eruption .
The poet Roman Virgil , inspired by Greek myths, reported that during the gigantomachy , Enceladus , a fugitive, was buried under the Mount Etna by Athena as punishment for his disobedience to the gods. The rumblings of Mount Etna and the tears forming of Enceladus, flame breath and tremor attempts to break free. Mimas , another giant, meanwhile was buried under the Vesuvius by Hephaestus and the blood of other giants defeated erupted from Campi Flegrei nearby.
In Christianity popular, despite some attempts to pre-scientific explanations, volcanoes were often regarded as the work of Satan and rash as signs of the wrath of God. A number of miracles attributed to certain saints are associated in the Catholic tradition to rashes, Thus in 253 , the city of Catania was spared when the waves washed the Etna split into two before the procession carrying the relics of St. Agathe. But in 1669 , the same procession with relics could not avoid the destruction of the vast majority of the city.
In 1660 , the eruption of Vesuvius rained down around the crystals of pyroxene black. The people took them for crucifixes and attributed this sign to San Gennaro , who became patron saint and protector of Naples. Since each eruption, a procession paraded through Naples to implore the protection of the saint. In addition, three times a year takes place the phenomenon of liquefaction of the blood of San Gennaro, which according to tradition, if it occurs, protect the city from any eruption of Vesuvius.
Even today religious processions are associated with volcanoes and their activity. Each eruption of Vesuvius , processions Catholics pray San Gennaro, to Hawaii residents still revere Pele and Mount Fuji is the sacred mountain of Shinto as the Bromo for Hindu Indonesian.
Forecasting eruptive
The goals of volcanology is to understand the origin and functioning of volcanoes and related phenomena in order to establish a diagnosis on the risks and dangers to people and human activities. The volcano predictions require the implementation of knowledge of several scientific disciplines. Current knowledge does not allow today to predict the type of eruptions , without, however, more than a few hours in advance when they will occur, how long they will last and especially their importance (volume of lava Intensity clearances, etc.).
Increasingly, the trend is continuous monitoring active volcanoes deemed dangerous by using remote-controlled devices powered by solar cells. In this regard, the equipment of Piton de la Fournaise , on Reunion , however, deemed non-hazardous, is exemplary. The measurements are transmitted by telemetry to the observatory and all the expansions, tremors and changes in temperature are recorded.
The civilian security of affected countries while trying to find the correct balance between the risks and precautions unnecessary. In many cases, authorities have been unresponsive . There were however some success as 1991 to the eruption of Pinatubo where experts believe the Government Philippines to organize the evacuation of 300,000 people. Despite 500 deaths, 15,000 lives have been saved.
The dangerous volcanic events
Lava
The lava flows are generally more damage than victims, because even if they can be very fast with several tens of kilometers per hour, their behavior is generally predictable, allowing time for people to evacuate. In 2002 , the lava lake in the crater of Nyiragongo is empty thanks to loopholes that have opened in the volcano: two heats reach the town of Goma in the Democratic Republic of Congo , are 147 dead and destroyed 18% of the city. These rivers of molten material leaves little chance to vegetation and structures on their path, burning them and burying them in a matrix of rock.
Pyroclastic
Also known as pyroclastic flows, and pyroclastic flows are gray clouds on the slopes of the volcano at several hundred km / h, reaching the 600 C and walk for miles before stopping. Arising from the collapse of a dome or a hand washing , these clouds consist of volcanic gases and tephra slide on the ground, crossing ridges and burn everything in their path. The stacks of materials transported by pyroclastic flows can accumulate over tens of meters thick and are at the origin of bodies of ignimbrites. One of these pyroclastic flows born of Mount Pelee in Martinique razed the city of Saint-Pierre in 1902 and killed its 28 000 inhabitants. More recently, a revival of the Soufriere of Montserrat has caused the destruction of Plymouth , the capital of the island uninhabitable and classified the vast majority of the island because of the repeated passage of pyroclastic flows.
Volcanic Ash
Expelled by volcanic plumes , the volcanic ash can fall back and cover entire regions in a thickness of several meters, causing destruction of crops and the occurrence of famines as was the case after the eruption of Laki in 1783 in Iceland , the collapse of roofs of houses on their occupants, the formation of lahars in the event of rain, etc..
Earthquakes
The earthquakes may be caused as a result of emptying the magma chamber when the volcano collapsed in on itself, forming a caldera. Multiple shifts walls of the volcano then generate earthquakes that cause the collapse of buildings sometimes weakened by falling volcanic ash.
Tsunamis
The tsunami can be generated in many ways during a volcanic eruption as the explosion of an underwater volcano or bloom, the fall of walls or pyroclastic flows into the sea, the collapse of the volcano on itself into direct contact with the water magma of the magma chamber , movement of land related to the emptying of the magma chamber, etc.. In 1883 , the explosion of Krakatoa generated a tsunami that caused 36 000 victims.
Landslides
In the manner of pyroclastic flows, and landslides can cause deadly avalanches. In rare cases, much or most of the volcano that is disintegrating under the pressure of the lava. In 1980 , the Mount St. Helens has surprised volcanologists around the world where half of the volcano has been dislocated. Some scientists, believing himself free on the surrounding hills, were trapped and perished in the massive pyroclastic flow that followed.
Volcanic gases
The volcanic gases are the most insidious danger of volcanoes. They are sometimes issued without any other signs of volcanic activity at a limnic eruption. In 1986 , in Cameroon , a layer of carbon dioxide was released from Lake Nyos. Is heavier than air, this gas has tumbled down the slopes of the volcano and killed 1,800 villagers and several thousand head of cattle in their sleep by asphyxiation.
Lahars
The lahar mudflows are formed of water , of tephra mainly of volcanic ash cold or hot, very dense and heavy and carting amount of debris such as boulders, tree trunks, remnants of buildings, etc.. The lahars are formed when rain occurs during major hurricanes or lengthy rains fall on volcanic ash. They can occur years after a volcanic eruption as the ash can be trained. In 1985 , 23,000 city residents in Colombia of Armero was buried under a lahar born on the slopes of Nevado del Ruiz.
Jkulhlaup
The jkulhlaup is a type of flood particularly powerful and brutal. They are formed when a volcanic eruption occurs beneath a glacier or ice sheet and the heat of magma or lava manages to melt large quantities of ice. If the melt water can not drain, it forms a lake that can be emptied when the barrier that holds formed by a rock or a glacier breaks. A wave mixing lava, tephra , mud , ice and boulders of the glacier then escapes, carrying away everything in their path. The most frequent jkulhlaup take place in Iceland , under the Vatnajkull.
Acidification of lakes
The acidification of lakes is another possible consequence of the presence of a volcano. Acidification has the effect of eliminating all forms of water life and their surroundings and may even constitute a danger to local residents. This phenomenon occurs when emissions of volcanic gases lead to the bottom of a lake, it will then be trapped by dissolving the acidified waters.
Volcanic winter
The ash , volcanic gases and droplets of sulfuric acid and hydrofluoric acid expelled into the atmosphere by volcanic plumes can cause acid rain and " volcanic winter "that lower temperatures and can cause starvation , harsh winters or cold summers around the world as was the case for the eruptions of Tambora in 1815 and Krakatoa in 1883.
Assets related to volcanoes
In some ways, man can take advantage of the presence of volcanoes with:
- exploitation of geothermal energy for producing electricity , heating buildings or greenhouses for crops;
- the supply of building materials, or industrial use such as:
- the fertilization of soils such as slopes of Etna , which is a region with very high density due to the agricultural fertility of volcanic soils and where vast citrus groves are located.
A volcano also contributes to tourism by offering a panorama , destinations of hiking , the spa or even a place of pilgrimage for visitors.
Even in the arts, their influence is felt: some eruptions strongly emitting volcanic ash as that of Tambora in 1815 generated sunsets spectacular for several years. Some painters like Turner have captured the light through the original works that herald the Impressionism.
Volcanology
The Volcanology Volcanology is the science or studying volcanic phenomena, their products and set up volcanoes, geysers , fumaroles , volcanic eruptions , magma , lava , tephra , etc.. A volcanologist volcanologist or is the scientific expert in this field related to geophysics , the seismology and geology, it is a specialty.
The objectives of this science are to understand the origin and functioning of volcanoes and related phenomena in order to establish a diagnosis, for a specified period, the risks and dangers to people and human activities. The studies and researches are conducted initially on the ground to make collections of information in the form of observations, measurements and sampling and a second time in the laboratory to analyze and interpret data and samples. Indeed, even managing the effects of an eruption once it occurs is impossible. Only a few steps misuse of lava have succeeded on Mount Etna in Italy and Heimaey in Iceland.
Only prevention can reduce or avoid the effects of a volcanic eruption. This prevention is through observation of the volcano and signs of an eruption: emission of volcanic gases , inflation and deflation of the volcano, earthquakes minor thermal anomalies, etc.. The temporary evacuation and emergency areas at risk is the means of preventing the most widely used. Nevertheless, there are ways to prevent long term as the total evacuation of the areas most exposed to volcanic hazards, the development of prevention plans, evacuation, rescue and awareness of people, etc..
Submarine volcanoes
The submarine volcanoes are most numerous on Earth. It is estimated that 75% of volcanoes and igneous material emitted by volcanoes are at mid-ocean ridges . Schedules
Related Articles
External Links
Bibliography
Notes and references
(1 votes, average: 4.00 out of 5, rated)
Loading ...