Volcano - SpeedyLook encyclopedia

Description

Structures and reliefs

A volcano is formed various structures which one in general finds at each one of eux :

a magmatic Room supplied with magma coming from the coat and playing the part of tank and place of differentiation of the magma. When this one is emptied following an eruption, the volcano can subside and give rise to a Caldeira. The magmatic rooms are between ten and fifty kilometers of depth in the lithosphere;
a principal chimney which is the place of privileged transit of the magma of the magmatic room towards surface.
a summit crater where emerges the principal chimney;
one or more secondary chimneys on the basis of the magmatic room or of the chimney principal and leading in general to the sides of the volcano, sometimes at its base. They can give rise to small secondary cones;
of the side cracks which are longitudinal fractures in the side of the volcano caused by its swelling or its deflation. They can allow the emission of lava in the form of an eruption fissurale.

Form volcanos

According to the type of lava that they emit and the type of eruption, the volcanos can take different formes :

volcano bouclier : the diameter is much higher than the height because of the fluidity of their lava which can traverse kilometers before stopping. The Mauna Kea, the Erta Ale or the Piton of the Furnace are examples;
stratovolcan : the diameter is balanced more compared to the height because of the greatest viscosity of their lava. They are the volcanos to the explosive eruptions like the Vesuvius, the Mont Fuji, the Merapi or the Holy Mont Helens;
volcano faille : it is in general about an opening linear in the earth's crust or oceanic by which escapes from the fluid lava. The volcanos of the dorsal are just like presented in the form of fault the Laki or the Krafla.

Igneous materials

Magma

Tephras and lava

Volcanic gases

The magma S contain dissolved gases. The degasification of the magmas is a phenomenon determining in the release of an eruption, and in the type of eruption. Degasification makes assemble the magma (analogy with a bottle of champagne), which gives the explosive character and violent one of an eruption.

The gases are mainly:

the Steam, HO (50 to 90%);
the CO {{ind|2}} (5 to 25%);
SO {{ind|2}} (3 to 25%).

Then come from other volatile elements: CO, HCl, H {{ind|2}}, H {{ind|2}} S. The degasification of in-depth magma, can be translated on the surface by Fumerolle S, around whose sulfur crystals can be formed. This volcanic degasification of the terrestrial coat is at the origin of the primitive atmosphere of the Ground, which was to be very rich in CO.

Frequency of the eruptions

The birth of a volcano corresponds to its first volcanic eruption which makes it leave the Lithosphère. The birth of a new volcano is a relatively rare phenomenon but which could be observed in 1943 with the Paricutín : a fracture letting escape from smoke and the Lave in a field gave rise to a 460 height meters volcano in nine months. In 1963, the underwater Volcan of Surtsey emerged in the south of Iceland thus forming a new island and a new terrestrial volcano.

A volcano is known as deadened when it any more did not enter in eruption during several hundred years and said extinct when its last eruption goes back to at least 50,000 years and that it is subjected to the erosion.

In a general way, the volcanos undergo several eruptions during their life. But their frequency is very variable according to the volcan : some know only one eruption in a few centuries (Ponta C Pico, with the the Azores), others are in permanent eruption like the Stromboli or the Merapi.

It happens that volcanos are formed only in only one eruption, then falls asleep or dies out during tens or hundreds of thousands of years. It is about volcanos monogenic. The volcanos of the Chaîne of Puys in the Massif Central were formed between 11500 front J. - C. and 5000 av. J. - C. during one only eruption for each volcanic building and showed never again of sign of activity.

The frequency of the eruptions makes it possible to evaluate the Aléa, i.e. the probability that an area can undergo the violence of an eruption. This risk, combined with the type of demonstration volcanic and the presence of populations and its Vulnerability, makes it possible to evaluate the volcanic risk.

Origin of volcanicity

According to the theory of the Plate tectonics , volcanicity is closely related to the movements of the tectonic plates. Indeed, it is in general at the continental border between two plates or oceanic that the conditions are met for the formation of volcanos.

Volcanicity of divergence

In the Rift of the dorsal , the spacing of two tectonic plates thins the lithosphere, and there is an increase of rocks of the coat. Those, already very hot (1200 °C), start to melt partially, because of the Décompression. That gives magma, which goes up by the normal Faille S . Between the two edges of the Rift, one observes traces of important volcanic activities such as “Pillow washed” or “washes in cushion”, which are formed by emissions of fluid Lave in a cool water.

These volcanic rocks then constitute a good part of the oceanic young person crust in formation.

When the rift is continental, as it is the case in East Africa (Vallée of the great rift), of the effusive volcanos are born then like the Ardoukoba or the Erta Ale.

Volcanicity of subduction

When two tectonic plates overlap, the oceanic plate, slipping under the other oceanic or continental plate, plunges in the coat and sudden of the mineralogical transformations. The water contained in the plunging lithosphere escapes from it then and comes to hydrate the coat, causing its partial fusion by lowering its point melting. This magma goes up and crosses the overlapping plate, creating volcanos. If the overlapping plate is a oceanic plate, a insular Arc volcanic will be formed when the volcanos give rise to islands. It is the case of the Aleutian Islands, the Japan or the the Antilles. If the overlapping plate is a continental plate, the volcanos will be located on the continent, in general in a Cordillère. It is the case of the volcanos of the the Andes or the Rocheuses. This type of volcano, with the andesitic lava and viscous, is in general gray, explosive and dangerous volcanos. The “Ceinture of fire of the Pacific” is formed in quasi majority of this type of volcano.

Volcanicity of hot spot

It happens sometimes that volcanos are born far from very limiting from plate lithospheric. They are in general volcanos of Point hot. The hot spots are plumes of magma in fusion coming from the depths of the coat and boring the plates lithospheric. Being fixed and the plate lithospheric moving on the coat, of the volcanos are created successively and are aligned then, most recent being most active because plumb with the hot spot. When the hot spot emerges under an ocean, it will give rise to a string of islands aligned as it is the case for the archipelago of Hawaii or the Mascareignes. If the hot spot emerges under a continent, it then will give rise to a series of aligned volcanos. It is the case of the Mont Cameroun and its neighbors. Exceptional case, it arrives that a hot spot emerges under a limit of plate lithospheric. In the case of the Iceland, the effect of a hot spot combines with that of the dorsal the médio-Atlantic, thus giving rise to an immense stacking of lava allowing the emergence of the dorsal. The the Azores or the Galápagos are other examples of hot spots emerging under a limit of plate lithospheric, in fact of the dorsals.

Traditional course of an eruption

An volcanic eruption occurs when the magmatic Chambre under the volcano is pressurized with the arrival of magma coming from the coat. It can then more or less eject gas which it contained according to its filling in magma. The setting under pressure is accompanied by a swelling of the volcano and of very surface seisms localized under the volcano, signs that the magmatic room becomes deformed. The magma generally goes up by the principal chimney, and undergoes at the same time a degasification, which causes a Trémor, i.e. a constant and very light vibration of the ground. This is with small seisms whose hearths are concentrated along the chimney.

The eruption begins at the time when the Lave reaches the free air. According to the type of magma, it runs out on the sides of the volcano or accumulates instead of emission, forming a stopper of lava which can give volcanic volcanic clouds and/or plumes when this one explodes. According to the power of the eruption, the morphology of the ground, the proximity of the sea, etc it can occur of other phenomena accompanying the éruption : important seisms, landslides, Tsunami S,…

The possible presence of water in solid form (Icecap, Glacier, Snow) or liquid (lake of crater, Ground water, River, sea or ocean, etc) will cause in contact with igneous materials (magma, lava or will tephras) their explosion or will increase their explosive capacity. By splitting up materials and by brutally increasing volume while being transformed into vapor, water acts as a aggravator of the explosive capacity of an volcanic eruption which will then be described as phreatic or of magmato-phreatic. The cast iron of ice or snow by the heat of the magma can also cause Lahar S when water involves Tephra S or Jökulhlaup S as it was the case for the Grímsvötn in 1996.

The eruption finishes when the lava is not emitted any more. The lava flows, ceasing being fed, are immobilized and start to cool and the ashes, cooled in the atmosphere, fall down on the surface of the ground. But the changes in the nature of the grounds by the covering of the grounds by the lava and the Tephra S sometimes on tens of meters thickness can create destroying and fatal phenomena. Thus the ashes fallen on cultures destroy them and sterilize the ground for a few months at a few years, a lava flow blocking a valley can create a lake which will drown inhabited or cultivated areas, rains falling on ashes can carry them in the rivers and create Lahar S,…

An volcanic eruption can last of a few hours to several years and eject volumes of magma of several hundred cubic kilometers. The intermediate duration of an eruption is of one month and half but the many ones last only one day. The absolute records are that of the Stromboli which has been almost in eruption for approximately 2,400 years.

Classification of the volcanos

At the time of the beginnings of the Volcanology, the observation of some volcanos was at the origin of the creation of categories based on the aspect of the eruptions and the type of emitted Lave. Each type is named according to the volcano referent. The great defect of this classification is to be rather subjective and badly to take account of the changes of the type of eruption of a volcano.

The cataclysmic term of “” can be added when the power of the eruption involves heavy environmental and/or human damage as it was the case for the Santorin towards 1600 av. J. - C. which destroys the Minoan Civilization, the Vesuvius in 79 which destroyed Pompéi, the Krakatoa in 1883 which generated a Tsunami of forty meters, the Holy Mont Helens in 1980 which shaved hectares of forest,…

In order to introduce a concept of comparison between the various volcanic eruptions, the volcanic Indice of explosiveness (or scale VEI) was developed by two volcanologists of the Université of Hawaii in 1982. The scale, opened and on the basis of zero, is defined according to the volume of ejected materials, the height of the cloud of eruption, and the qualitative data.

There exist two great types of volcanic eruptions depending on the type of magma émis : effusive associated with the “red volcanos” and explosives associated with the “gray volcanos”. The eruption surtseyenne is a phréato-magmatic eruption which can occur on any type of volcano since it is covered with water (sea, lake, etc).

Effusive eruptions

The effusive eruptions emit lava basaltic, low in Silice and thus very fluid and releasing their gases easily. The eruptions are relatively calm, without great explosions and producing great lava flows. These “red volcanos” are those of the points hot like those of Hawaii or the Piton of the Furnace or represented by the Etna. The only danger to the populations are the lava flows which can advance with several tens of kilometers per hour. But in general, the populations have time to evacuate quietly by carrying some businesses.

Eruption hawaïenne

It is characterized by Lave S very fluid, basaltic and low in Silice what sometimes allows a flow along the sides of the volcano on tens of kilometers. The degasification of the lava is very easy and its ejection can be done either in the shape of fountains of lava of several hundred meters height and the regular flow, or in the shape of a lake of more or less temporary lava fascinating places in a crater.

Not very dangerous, these eruptions can nevertheless cause extensive damage when human infrastructures are touched by castings. The human risk is on the other hand quasi null because there is no risk of explosion and the lava leaves time to evacuate.

The volcanos having eruptions of the hawaïen type are the Mauna Kea, the Mauna leasing, the Piton of the Furnace, the Nyiragongo, the Erta Ale,…

The volcanic index of explosiveness of this type of eruption goes from 0 to 1.

Strombolian eruption

Intermediate style between the hawaïen types and Vulcanian, the Strombolian type emits fairly fluid lava in the form of run and of will tephras (volcanic bombs, slags, etc) projected by frequent explosions. A cloud of ashes can rise to a few hundred meters height. The dangerosity depends on the proximity of the human establishments.

The volcanos having eruptions of the Strombolian type are the Stromboli, the Etna (although having sometimes Vulcanian eruptions),…

The volcanic index of explosiveness of this type of eruption goes from 1 to 2.

Explosive eruptions

The explosive eruptions emit as for them lava andesitic, rich in silica and thus very viscous and releasing their gas with difficulty. These eruptions do not form lava flow but are accompanied rather by explosions producing of great quantities of ashes giving rise to volcanic volcanic clouds and plumes. Approximately 80% of the volcanic eruptions are held on this type of volcanos. Sometimes very dangerous because unforeseeable, these types of eruptions do not leave time to evacuate the populations threatened by gases and ashes extreme. The most representative volcanos are the “gray volcanos” of the “Ceinture of fire of the Pacific”: Pinatubo, Krakatoa, Mayon, Merapi,…

Vulcanian eruption

The fluid basaltic lava runs out with difficulty in the Vulcanian type because they are richer in silica and their degasification is less easy. Fountains and projections of lava give rise to castings which go down along the volcano and can reach constructions downwards.

The human risk is higher bus of projections of stone sandpapers, ashes and bombs can occur and rise with several kilometers height. The standard eruption is the last eruption of the Vulcano between 1888 and 1890.

The volcanic index of explosiveness of this type of eruption goes from 2 to 5.

Pelean eruption

In this type of eruption, the pasty lava almost does not run out and tends to form a Dôme of lava. This one, under the pressure of the magma, can disaggregate or explode by producing volcanic volcanic clouds and plumes. Very fatal because of the unstable character of the eruption and the speed of the volcanic clouds, the standard eruption is that of the Peeled Montagne which made: 28,000 died in 1902 in Martinique.

The volcanos having Pelean eruptions are the Peeled Montagne, the Soufrière of Montserrat, the Soufrière of the Guadeloupe,…

The volcanic index of explosiveness of this type of eruption goes from 1 to 8.

Eruption plinienne

In this type of eruption, the lava is extremely pasty because very rich in Silice. The gases which cannot release itself, the pressure increase in the Chambre magmatic and produced explosions which pulverize the Lave and sometimes the volcano by projecting ashes with tens of kilometers height, thus reaching the Stratosphère. The plume falls down in general under its own weight and devastates the sides of the volcano to kilometers with the round. The presence of ground water on the way of the lava increases the explosive risk and dangerosity of these volcanos of which the first description was that of the Vesuvius in 79 by Pline the Young person and who destroyed Pompéi.

The volcanos having eruptions pliniennes are the majority of those forming the “Ceinture of fire of the Pacific” : Merapi, Krakatoa, Pinatubo, Holy Mount Helens, Mount Augustine,…

The volcanic index of explosiveness of this type of eruption goes from 3 to 8.

Eruption surtseyenne

The eruptions surtseyennes are phréato-magmatic eruptions which blame of great quantities of water. They are in general eruptions underwater or under-lake close to surface (in general less than one hundred meters of depth) or subglacial when the heat of the magma manages to dissolve great quantities of Glace.

The underwater or under-lake volcanos managing to reach the surface emergent of water to form an island during an eruption surtseyenne. The island of Surtsey, which gave its name to this type of eruption was born from the kind in 1963.

At the time of an eruption surtseyenne, the surface of the volcano is with a few meters or a few tens of meters under water surface. The pressure of water is not then sufficient any more to avoid the explosion of the Lave to its contact. Explosions “cypressoïdes” (in form of Cypress) occur then, mixing lava and cooled Tephra S, liquid water and Steam. Once the island emerged, the eruption is prolonged in a traditional way according to the type of magma.

If the eruption is subglacial, it is necessary that the melt water is found trapped above it volcano to cause an eruption surtseyenne. The Nevado del Ruiz did not cause an eruption surtseyenne at the time of its eruption in 1985 because water coming from the cast iron of the ices at the top of the volcano descended the slopes of the volcano by forming Lahar S which destroyed the town of Armero. On the other hand the eruption of the Grímsvötn in 1996 under the Vatnajökull was transformed into eruption surtseyenne because the melt waters of the Icecap formed a lake above the volcano. At the time of the arrival of the magma on the surface, projections cypressoïdes crossed the ice and the lake was emptied in the form of a Jökulhlaup.

The volcanic index of explosiveness of this type of eruption goes from 2 to 5 but it depends largely on the type of magma (basaltic or andesitic).

Volcanic geomorphology

In addition to the volcano in itself, various geological formations are directly or indirectly related to the volcanic activity.

Certain reliefs or landscapes result from the direct product of the eruptions. They are the volcanic cones by forming mountains to them-even or islands, domes and lava flows solidified, tunnels of lava, “Pillow washed S” and the Guyot S of the underwater volcanos, the Trapp S and the Horst S forming of the plates, of accumulations of Tephra S in Tuf S, of the crater S left by the exit of the Lave,…

Other reliefs result from a erosion or an evolution of the products of the eruptions. These case of the Dyke S, Neck S, Sill S, intrusive rocks, mesas and Planèze S released by erosion, of the Caldeira S and circuses resulting from the collapse of part of the volcano, lakes of crater or made upstream of a stopping made up of the products of the eruption, Atoll S coral S surrounding the vestiges of a broken down underwater volcano,…

Phenomena paravolcanic

Consequence of volcanicity on the history of the Earth

Volcanicity was born at the same time as the Ground, at the time of the phase of Accrétion of its formation there is 4,6 billion years. Starting from a certain mass, the materials in the center of the Earth undergo important pressures, thus creating heat. This heat, accentuated by the degradation of the radioactive elements, causes the fusion of the Earth which dissipates twenty times more heat than today. After a few million years, a solid film is formed on the surface of the Earth. It is torn in many places by floods of lava and by great granitoid masses which will give the futures Continent S. Thereafter, the lithospheric plates lately created will tear preferentially at precise places where will be formed the volcanos. During hundred million years, the volcanos will reject into thin the atmosphere of the time of great quantities of gaz : Diazotizes, Carbon dioxide, Steam, oxide of sulfur, Hydrochloric acid, Acid hydrofluoric, etc There is 4,2 billion years, in spite of the 375 °C and the pressure 260 times higher than that of today, the steam condenses and gives rise to the Océan S.

The role of the formation of the first organic molecules and the appearance of the life on Earth can be charged to the volcanos. Indeed, the underwater hot sources or the Sulfur spring S and others Geyser S offer favorable conditions to the appearance of the vie : water which washed carbonaceous molecules, minerals, heat and energy. Once the life widespread and diversified on the surface of the Earth, the volcanos could have caused contrary to large extinctions : the age of the great extinctions of alive coincides with the age of the Trapp S. These trapps could have been caused by the fall of Météorite S or the exceptional eruption of hot spots. The combined effects of gases and particles dispersed in the atmosphere would have caused the disappearance of many species by a volcanic Hiver (screening of the solar rays) followed by a rise of the Greenhouse effect by the changes in the gas composition of the atmosphere.

One of the theories most accepted for the appearance of the man would be the opening of the African rift : uniformly wet on the level of the equator, the African climate would have been drained in the east of the rift which stops the clouds coming from the West. The Hominides, adapting to their new formed medium of a Savanna, would have developed the Bipédie to escape their predatory.

Still nowadays, the volcanos take part in the dissipation of the internal heat of the Earth and in the biogeochemical Cycle world by releasing gases, the steam and minerals absorbed in the coat on the level of the pits of subduction.

Incidence of volcanicity on the human activities

Beliefs and myths related to the volcanos

Since the appearance of the Agriculture and the sedentarisation of the companies, the men always côtoyé the volcanos. Renting for the fertile grounds that they offer, they also fear them for their eruptions and deaths which they cause. Quickly, by ignorance of a natural phenomenon, the volcanos are feared, déifiés, regarded as the entry of the kingdom of deaths, the Enfer S and the populated underground worlds of malfaisants spirits and are the object of Légende S and Mythe S following the various cultures.

In the tribes of Asia, Oceania and America alive near the Belt of fire of the Pacific, the volcanic eruptions are regarded as being the demonstrations of supernatural or divine forces. Into the Mythology maori, the volcanos Taranaki and Ruapehu fell both in love with the volcano Tongariro and a violent argument burst between the two. This is why no maori lives between the two volcanos coleric, of fear of finding itself taken in the middle of the argument.

Among other myths and legends, one can announce that of the Devils Tower which would have been drawn up to save seven Amerindian young girls of Ours who would have scratched the rock faces or the history of the goddess Pélé who, driven out Tahiti by her sister Namakaokahai, found refuge in the Kilauea and since, of rage, pours floods of Lave of a simple blow of heel.

At the INCA S, the whims of the Misti were worth to him to have its crater blocked by a stopper of ice, punishment inflicted by the Sun. The Chagas of Tanzania tells that the Kilimandjaro, exceeded by its neighbor the volcano Mawensi, struck it with great blow of rammer, which was worth its cut out top to him. At the Amerindians of the Oregon, the Mont Mazama was the malefic residence of the god of fire and the Mont Shasta that of the beneficial god of snow. One day the two divinities entered in conflict and the god of fire was overcome and decapitated, thus creating the Crater Lake as a sign of defeat.

The volcanos were even the place of human sacrifices : children thrown in the crater of Bromo in Indonesia, Christians sacrificed for the Mount Unzen to the Japan, virgins precipitated in the lake of lava of the Masaya to the Nicaragua, child thrown in a lake of crater to calm the under-lake volcano of Ilopango to the El Salvador,…

At the Greek and the Roman , the volcanos is the place of life of Vulcan/Héphaïstos. The eruptions are explained as being a divine demonstration (anger of the Gods, predict, forging mills of Vulcan/Héphaïstos in activity, etc). No scientific explanation or not utilizing the Gods was retained. For the Romans, the forging mills of Vulcan were under Vulcano while for the Greeks, those of Héphaïstos were located under the Etna. The Greek Cyclops S could be an allegory of the volcanos with their summit crater while the name of Héraclès drift of will hiera or Etna , the Greek word being used to indicate the volcanos.

Among the Greek myths putting in scene volcanos, most famous is that told by Plato in the Timée and the Critias . These accounts report the disappearance of the Atlantis, absorbed by the floods in gigantic a Earthquake followed by a Tsunami. Not putting directly concerned a volcano, this myth seems to have found its origin in the eruption of the Santorin towards 1600 av. J. - C. which almost entirely destroyed the island and which could have caused or have taken part in the fall of the Minoan Civilization. However no observation of the eruption of Santorin was consigned, it is only at the beginning of the 20th century that one realized of the importance of the eruption.

The Roman poet Virgile, taking as a starting point the Greek myths, reported that at the time of the Gigantomachie, Encelade, in escape, was buried under the Etna by Athéna as a punishment for its disobedience to the Gods. Grondements of Etna thus constituting the tears of Encelade, the flames its breathing and the Trémor its attempts to release itself. Mimas, another giant, as for him was absorbed under the Vesuvius by Héphaïstos and the blood of the other overcome giants spouted out of the Champs Phlégréens in the vicinity.

In the popular Christianity, in spite of some attempts at prescientific explanations, the volcanos were often regarded as the work of Satan and the eruptions like signs of the anger of God. Certain numbers of Miracle S allotted to some Saint S are associated in the catholic tradition with éruptions : Thus in 253, the town of Catane was saved when the floods of Lave of the Etna were divided into two front the procession transporting the Relique S of Sainte Agathe. But in 1669, the procession with same the relics could not avoid the destruction of the great majority of the city.

In 1660, the eruption of Vesuvius made rain in the neighborhoods of the black crystals of Pyroxène. The population took them for Crucifix and allotted this sign to Saint January which became patron saint and guard of Naples. Since with each eruption, a procession ravels in Naples to beseech the protection of the Saint. Moreover, three times per annum take place the phenomenon of the liquefaction of the blood of Saint January who, according to the tradition, if it occurs, protects the town of any eruption of Vesuvius.

Still today with the religious processions are associated with the volcanos and their activity. With each eruption of the Vesuvius, catholic processions request Saint January, with Hawaii the inhabitants still venerate Pélé and the Mont Fuji is the crowned mountain of the Shintoïsme just as the Bromo for the hindouists Indonesia NS.

Eruptive forecast

Dangeureuses volcanic demonstrations

; Run Lava They in general make more property damages than of victims because even if they can be very fast (several tens of kilometers per hour), their behavior is generally foreseeable, leaving time to the populations to evacuate. In 2002, the lake of lava of the crater of the Nyiragongo is emptied with the favor of faults which opened in the volcano: two cast reaches the town of Goma to the democratic Congo, makes 147 dead and destroys 18% of the city. These matter rivers in fusion leave little chance to the vegetation and constructions being on their passage, consuming them and burying them in a gangue of rock.

; volcanic Clouds Also called pyroclastic castings, these are the gray clouds which descend the slopes of the volcanos to several hundred kilometers per hour, reach the 600 °C and traverse kilometers before stopping. Born from the collapse of a dome or a volcanic needle, these clouds made up of gas and Tephra S slip on the ground, cross peaks and consume all on their passage. Stackings of materials transported by the volcanic clouds can accumulate on tens of meters thickness and the beginning of are extended from Ignimbrite S. One of these pyroclastic castings born from the Peeled Montagne of Martinique shaved the town of Saint-Pierre in 1902 and killed its: 28,000 inhabitants. More recently the alarm clock of the Soufrière of Montserrat caused the destruction of Plymouth, the capital of the island, and classified uninhabitable the large majority of the island because of the repeated passages of volcanic clouds.

; Volcanic ash Expelled by volcanic plumes, they can fall down and cover with the whole areas under a thickness of several meters, causing the destruction of the cultures and the appearance of Famine S (eruption of the Laki of 1783 in Iceland), the collapse of the roofs of the dwellings on their occupants, the formation of Lahar S in the event of rains,…

; Seisms They can be caused when following the draining of the magmatic Chambre, the volcano crumbles on itself and forms a Caldeira. The multiple slips of the walls of the volcano then generate seisms which cause the collapse of the buildings sometimes weakened by falls of ash.

; Tsunami S They can be generated multiple manners at the time of an volcanic eruption: explosion of a underwater Volcano or with water flower, falls of walls or of volcanic clouds in the sea, collapse of the volcano on itself putting in direct contact water with the magma of the magmatic room, movements of grounds related to the draining of the magmatic room, etc In 1883, the explosion of the Krakatoa generated a tsunami which made: 36,000 victims.

; Landslides With the manner of the volcanic clouds, they can cause fatal avalanches. In rare cases, it is a great part or the majority of the volcano which disaggregates under the pressure of the lava. In 1980, the Holy Mont Helens has surprised the volcanologists whole world when half of the volcano was dislocated. Certain scientists, believing themselves in the shelter on surrounding hills, were made trap and perished in gigantic the volcanic Cloud which followed.

; Volcanic gases It is the most underhand danger of the volcanos. They are sometimes emitted without any other sign of volcanic activity at the time of a limnic eruption. In 1986, to the Cameroun, a tablecloth of Carbon dioxide left the Lac Nyos. Being heavier than the air, this gas descended the slopes of the volcano and killed 1 800 villagers and several thousands of heads of Cattle in their sleep by Asphyxiation.

; Lahar S They are formed muddy castings of water, Tephra S (as a majority of ashes), cold or extreme, very dense and heavy and carting quantity of remains (rock blocks, tree trunks, remainders of buildings, etc). The lahars are formed when important rains (cyclone, prolonged rains, etc) fall down on volcanic ash. They can occur of the years after an volcanic eruption as long as ashes can be trained. In 1985: 23,000 inhabitants of the city Colombia of Armero were absorbed under a lahar born on the slopes from the Nevado del Ruiz.

; Jökulhlaup It is a type of raw particularly powerful and brutal. They are formed when an volcanic eruption occurs under a Glacier or a Icecap and that the heat of the magma or Lave manages to dissolve great quantities of ices. If the melt water cannot be evacuated, it forms a lake which can be emptied when the barrier which retains it (rock face, glacier, etc) breaks. A flood mixing lava, Tephra S, Mud, Ice and blocks rock escapes then from the glacier, carrying all on their passage. The most frequent jökulhlaup are held in Iceland, under the Vatnajökull.

; Acidification of the lakes It is about another possible consequence of the presence of a volcano. The acidification causes to eliminate any form of life from water and their accesses and can even constitute a danger to the bordering populations. This phenomenon occurs when gas emanations emerge at the bottom of a lake, this one then will trap them by dissolution what acidifies water.

; volcanic Winters The ashes, sulphuric gas and droplets of Acid and Acid hydrofluoric expelled in the atmosphere by volcanic plumes can cause volcanic acid rains and “winters” which lower the temperatures and can cause famines, rigorous winters or cold summers on a worldwide scale (eruptions of the Tambora in 1815 and Krakatoa in 1883).

Assets related to the volcanos

By certain aspects, the man can benefit from the presence of the volcans :

Exploitation of the geothermal energy (production of electricity, heating of the buildings, the Greenhouse S for the cultures, etc).
Supply of construction materials, or of industrial use:

the Basalte is used as stones of construction, of ballast, of engraved crushed.
Pounce and Pouzzolane is useful, inter alia, of insulator in the concretes.
ores of Sulfur, Copper, Iron, Platinum, Diamond S, etc

Fertilization of the grounds: the slopes of the Etna constitute an area with very strong agricultural density because of the fertility of the volcanic grounds. Immense citrus fruits orchards are established there.

A volcano also contributes to the Tourisme by proposing a Panorama, destinations of Randonnée, Thermalisme or even a place of Pèlerinage to the visitors.

Even in the artistic field, their influence is made sentir : certain strongly transmitting eruptions of volcanic ash (as that of the Tambora in 1815) generated spectacular sunsets during several years. Certain painters as Turner knew to collect this light through original works which announce the Impressionnisme.

Volcanology

The volcanology or vulcanology is the science which studies the volcanic phenomena, their products and their settings in place : volcanos, Geyser S, Fumerolle S, volcanic eruptions, magma S, Lava S, Tephra S,…

A volcanologist or vulcanologist is the scientist specialist in this discipline related on the Géophysique, the Sismologie and the Géologie of which it is a speciality.

The objectives of this science are to include/understand the origin and the operation of the volcanos and the comparable phenomena in order to establish a diagnosis (for one given period) on the risks and the dangers incurred by the populations and the human activities. The studies and research initially proceed on the ground in order to carry out collections of information in the form of observations, measurements and samplings and in the second time in laboratory in order to analyze and to interpret the data and the samples.

Indeed, the management of the effects even of an eruption once it occurs is impossible. Only some operations of diversion of run Lave succeeded on the Etna and with Heimaey.

Only the Prévention makes it possible to limit or avoid the effects of an volcanic eruption. This prevention passes by an observation of the volcano and harbingers of a éruption : gas emissions, swelling and deflation of the volcano, minor seisms, thermic anomalies, etc the evacuation of manner temporary and in the urgency of the zones in danger is the means of prevention more employed. Nevertheless, there exist means of long-term prevention like the total evacuation of the zones most exposed to the volcanic risks, the development of prevention plans, evacuation, help and sensitizing of the populations,…

Underwater volcanos

Extraterrestrial volcanos

The Ground is not the only planet of the solar system to know a volcanic activity. Venus knows an intense volcanicity with: 500,000 volcanic buildings, Mars comprises more the high summit of the Solar system : the Mount Olympe, a volcano considered as extinct and high of 27 kilometers, the the Moon is covered with immense fields of basalte : the “lunar Maria”. Volcanos also exist on satellite of Jupiter and Neptune : in particular Io and Triton. The probe Voyager I made it possible to photograph in March 1979 an eruption on Io, while Voyager 2 made discover in August 1989, on Triton, of the traces of Cryovolcanisme and the geysers. One knows also cryovolcans on Encelade.

The cameras of space probes provided invaluable images revealing the existence of an important volcanicity in the solar system, in particular on Venus, Mars, Io, Triton. The astrophysicists strip the data of this fantastic harvest which extends the field of study of volcanology. The knowledge of the phenomenon such as it occurs on the Earth passes from now on by the odyssey of space.

The chemical composition varying considerably between planets and the satellites, the type of ejecta is very different from those emitted on Earth (Soufre, ice of Azote, etc).

Volcanos in the media

The eruption of a volcano near a populated zone is very often lived like an major event in the life of a country because, in addition to the spectacular and unexpected character of an eruption, this one requires a monitoring and sometimes the evacuation and the assumption of responsibility of the people in danger.

The volcanos are sometimes the main actors of certain films catastrophes like “the Peak of Dante” and “Volcano” (1997) or the docu-fiction Supervolcan (2004) of BBC and of Discovery Channel which puts in scene the alarm clock of the supervolcan of Yellowstone in an eruption of volcanic Indice of explosiveness of 8.

More usually, the volcanos are the subject of many documentary televised scientific, informative or of popularization.

Records

higher volcan :

altitude cumulée : Mauna Kea with 10 230 meters (altitude 4205 meters)
altitude absolue : Nevados Ojos del Salado, with the Chile, with 6887 meters.

greater eruption (in volume of ejected materials) : Toba there is: 73,000 years (2800 km 3)
smaller eruption (in volume of ejected materials) : geothermic drilling with Námafjall in Iceland in 1977 (1.2 m3 of basalt,).
Volcano more actif : the Kilauea and the Piton of the Furnace dispute the record with a eruption all the one year at one year and half
Plus young person volcan : Ardoukoba (first eruption in November 1978), the Paricutín knew its first large eruption in 1943
Plus Caldeira or larger terrestrial volcanic crater: Toba formed there is: 73,000 years (hundred kilometers length on thirty kilometers broad)
greater number of victimes : Tambora on the island of Sumbawa in Indonesia, 1816 (: 88,000 related directly to the eruption and: 200,000 per famine)
Volcanic eruption more bruyante : Krakatoa in Indonesia, on August 27th, 1883, where the explosion was heard to the island of Rodrigues to 500 km in the east of the island Maurice, is with: 4,811 kilometers of distance from the eruption
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