The word volcano find its etymology in the name of Roman of fire Vulcan, Hephaistos in Greek
A volcano is made of three parts:
- The tank of magma, located in-depth;
- One or more chimneys making communicate the tank and surface;
- The volcanic mountain located at surface, this one being able to be various forms.
volcanicity is the whole of the phenomena which accompany the increase by magma in a volcano. volcanology is the study of its phenomena and one volcanologist is the researcher who studies them, so, inter alia, to envisage the episodes and the effects of them often devastators.
* See the detailed article Volcanology, for the types of eruption and the phenomena volcanic (lava flow, volcanic cloud, lahar...)
The volcanic eruptions are classified in families, according to the various ways in which are emitted the lava (castings, explosions weak or strong, plume of ashes or not, etc.). Formerly, whereas volcanology was only at its beginnings, one used a typology based on the study of some volcanos. The thorough observation of a greater number of volcanos shows a more complex classification (see the paragraph "New criteria of classification").
It is based on the observation of a small number of volcanos and is determined by the products expelled by the crater:
- Vulcanian eruptions
The emitted lava is rather viscous, which supports the explosive demonstrations. Those are short and form plumes of ashes of generally moderate width. The prototype is the eruption of 1888-1890 of Vulcano, located in , in Wind islands.
- Strombolian eruptions
Their characteristics are intermediate between eruptions "hawaiienne" and "Vulcanian". They appear by more or less important explosions projecting large quantity of bombs. This explosive activity can be accompanied by lava flow. The prototype is the activity of Stromboli, located onisland same name with broad of Sicily
- Pelean eruptions
This category draws its name from the eruption of 1902-1904 from Peeled mountain, famous volcano which made almost 30 000 dead in . This activity is marked by the formation of a dome pulverized by a very powerful side explosion, followed pyroclastic castings (what is called also one " volcanic cloud »).
- the eruptions pliniennes
These eruptions draw their name from that from Vesuvius in August 79 and of which Pline the Young person made description. Generally preceded by explosions of the underground sheets of water (phreatic eruptions), the eruptions pliniennes are characterized by violent explosions with open crater projecting a plume of ashes and of pounces at high altitude (from 10 to 20 km in general). After a few times, this plume breaks down under its own weight, generating castings descending at very high speed the sides of the volcano, in particular in the bed of . The destroying capacity such eruptions is often higher than that of the Pelean eruptions because of the volume of emitted rock materials and the violence of the explosions.
- the eruptions hawaiiennes
They are effusive eruptions, continuous and characterized by fluid lava flows.
- One also speaks about cataclysmic eruption in the most destroying cases, as for the eruption which destroyed Pompéi or Saint-Pierre in Martinique.
New criteria of classification
One can classify the volcanos according to the principle of and to cross a multitude of criteria in order to obtain a description more faithful to reality. One can thus retain several factors to categorize the same volcano:
- According to its genesis : two current situations:
- one rift suboceanic gives a generally effusive volcanicity: on the oceanic floors (The Atlantic for example), the underwater volcanos produce a great quantity of and is at the origin of the accretion phenomenon.
- the volcanicity of tectonic arcs is rather explosive: Peeled mountain with Lesser Antilles, archipelagoes .
- certain volcanos escape from these localizations on the limits from plates : they are the hot points (of which most famous is that of) or volcanicity intracontinental (plate of Dekkan in , Tibesti in ).
- According to its type of activity:
- Extinct/active volcano: the active volcanos are distinguished, which appear, or appeared at one historical time, by the extinct eruptions, and volcanos (like chain of Puys in , or the volcano ofAgde), in sleep since geological times. It is considered that the volcano is extinct if its last eruption goes up with more than 50 000 years. It is subjected to erosion.
- For the active volcanos, one analyzes his activity in time present: it can be in sleep or in activity. This activity can be permanent or very irregular.
- According to the nature of the magma, one will be able to observe a basic or acid volcanicity.
- According to the situation of the volcano, one will find a volcanicity oceanic or continental
- According to the nature of its activity, the volcano can be effusive or explosive.
- According to the shape and the slope of the volcano, which depend themselves on the history of the volcano, emitted materials and the natural environment:
- the cumulo-volcanos have a strong slope and resemble domes.
- the volcano-shields have weak slope: the very fluid magma flows in all the directions. The eruptions are effusive. Examples: Hawaii and .
- the stratovolcanoes have a slope moderated with a more complex architecture, primarily built by slags.
- According to the conduit of the volcanic building:
- the central volcanos have one conduit.
- the linear or fissuraux volcanos have a fracture as a conduit, situation which one finds in the areas of rift.
- According to the frequency of the eruptions:
Thus, according to these criteria, one will say that:
- The volcano Etna (Sicily, ) is one stratovolcano in activity permamente (quasi continuous gas emission), of oceanic origin and rather effusive activity. It has several craters (polygenic volcano)
- The volcano of Holy Mount Helens (The United States, in the state of ) was a long time in sleep, but its explosive activity began again in 1980.
Geomorphology of the volcanic areas
For the various aspects of wash (pillow lava, tunnel of lava, wash in flagstones...), to see this article. The volcanic activity produces magmatic rocks (also qualified the igneous ones or the eruptive ones) formed by the solidification of magmas, of which:
The volcanic building
Its form depends on its history, of the nature of the lava and the eruptions, erosion, the natural environment: thus underwater volcanicity almost never gives cone...
- The sides of the volcano give him a more or less conical aspect. One finds there:
- Volcanic rocks: , obsidian, pyroclastites (still called will tephra or ejecta). One classifies the pyroclastites in three categories, according to their size:
Forms and types of relief created by the volcanic activity
- Basaltic plates: Trapps of Deccan in , Plate of Columbia in the United States.
- Plates ofignimbrites are the vast wide ones covered by the successive deposits with volcanic clouds overflowing with very acid magma.
- tuff is a rock made up of materials mixed with the pyroclastites which are cemented under the action of water. According to ejecta accumulated, one distinguishes the vitric tuff, the pumice tuff, the chaotic tuff (blocks and ashes).
- dyke and sill
Forms of eroded volcanic relief
is more or less strong according to the natural environment; it depends on several factors which can combine:
- gravity (slopes of the volcano)
- action of the vegetation
- action of water (river, glacier, sea)
The volcanic activity produces itself of the erosive forces: the lava flows modify the landscape unceasingly, the explosions can increase the crater and to transform it into will caldera.
Risks related to volcanicity
Risks for the Men and their activities
- lava flow: they destroy all on their passage.
- projection of will tephra (ejecta)
- volcanic clouds : avalanches dry of blocks very dangerous: that of Peeled mountain in made nearly 30 000 died and traversed the slopes of the volcano with 600 Km/heure.
- gas: the imprudent tourists expose themselves to the risk of asphyxiation
- muddy casting or lahar : at the time of the eruption of the Holy Mount Helens, it , the mud flows submerged and destroyed tens of thousands of hectares of forests, carrying enormous tree trunks and rocks which destroyed all the road infrastructures. The old valley Columbia was filled by 150 m of deposits.
- : the eruption of Krakatoa in ( ) generated one tidal wave whose vagueness was perceptible until in .
Prevention of the risks
- See the detailed article Vulcanology
- The observation makes it possible to envisage an eruption more easily than a seism. It makes it possible to engage the evacuation of the threatened zone.
- Attempt at diversion of the lava: Etna.
Assets related to volcanicity
- The deposits of ashes make the grounds fertile: the farmers included/understood it well since Antiquity.
- The volcanic products are used as building materials: the cathedral of Clermont-Ferrand is built thanks to the volcanic stone.
- The paravolcanic activities are also exploited by the Men: geothermal energy is used in Iceland and in the United States; geysers park of Yellowstone thousands of tourists attract. The sources of hot water allow the development of hydrotherapy (Japan, Iceland etc.).
Some famous volcanos
The majority of the volcanos concentrate on the belt of fire of the Pacific. Iceland hopes more than 200 volcanos (see the detailed article Volcanos of Iceland).
In the solar system
Apart from , one knows one to date celestial body where active volcanos were observed: , one of . However, the volcanicity of Io is different from that of the Earth: it is due to the intensity of the forces of tide what exerts on him Jupiter (the largest planet of the solar system).
On Triton, the moon of , the probe To travel 2 however observed material ejections (nitrogenizes liquid, methane, dust) at altitudes which could reach 8 km and which are perhaps assimilable to volcanos.
The planet accomodates an extinct but gigantic volcano: Olympus Mons with its 27 km in height is thus the most mountain of . However, observation of recent castings (in geological terms 30 million years) on the sides of Olympus Mons thus that the rate a too high methane little in the Martian atmosphere suggests a possible activity. On one also finds: Arsia Mons, Pavonis Mons, Ascraeus Mons, Elysium Mons, Albor Tholus, Hecates Tholus etc.
The planet is also candidate with the presence of active extraterrestrial volcanos, but the observations remain difficult. One only knows -- since the observations radar of the probe Magellan -- that the volcanic cones are very numerous there (more than 7000) and the activity tectonics y is still sensitive (wrinkles and faults are the witnesses).
Volcanos and media
The media are often interested in the volcanos. The catastrophes which they cause or the spectacular aspect of their eruption are often the object of reports of information or popularization.
- Supervolcan (English supervolcano). This term, unsuitable in vulcanology, was employed for the first time in 2004 in documentary of BBC and of Discovery Channel to describe catastrophic eruptions on a continental or world scale, being able entrainer of disappearances of species or a climatic change. These eruptions are classified 8 onVolcanic index of Explosiveness or VEI, most recent are that of Lake Toba in Indonesia there are 70 000 years and that Yellowstone, approximately 640 000 years ago.
- Jacques-Marie Bardintzeff, Volcanos, Minerva, 2004, ISBN 2830707559
- Bernhard Edmaier, Volcanos, Fernand Nathan, 2004, ISBN 2092610996
- Jacques Kornprobst, Christine Laverne, Does the Volcanos, how it go?, Editions of the BRGM, 2002, ISBN 2847030174
- Maurice Krafft, Fires of the ground: Stories of volcanos, Discovered Gallimard, 2003, ISBN 2070429008
- Patrick de Wever, Volcanicity: Cause of dead and source of life, Vuibert, 2003, ISBN 2711752933