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 to give rise to one will 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;
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.
According to the type of lava that they emit and the type of eruption, the volcanos can take various forms:
volcano shield: the diameter is much higher than the height because of the fluidity of their lava which can traverse kilometers before stopping. Mauna Kea, Erta Ale or the Piton of the Furnace are examples;
stratovolcano: 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 Vesuvius, the Fuji Mount, Merapi or the Holy Mount Helens;
volcano is necessary: 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 dorsals are just like presented in the form of fault Laki or Krafla.
The magma is the fluid material of consistency to viscous, under pressure, container of gas, not crystallized which was formed starting from fusion partial or total coat (anatexie) on the level of a point of heat (not hot), decompression (dorsal) and/or a contribution of water (pit of subduction). Generally, it goes up towards surface and is stored in the lithosphere by forming a magmatic room. In this room, it can undergo a total or partial crystallization and/or a degasification which start to transform it into lava. If the pressure becomes sufficient so that it is ejected on the surface, it goes up along a volcanic chimney and is emitted in the form of lava, i.e. completely or partially degassed.
Tephras and lava
According to whether the magma comes from the fusion of the coat or part of the lithosphere, it will have neither the same mineral composition, neither the same water content or of gas, nor the same temperature. Moreover, according to the type of ground which it crosses to go up on the surface and the duration of its stay in the magmatic room, it either will be given the responsability or to discharge out of minerals, water and/or gas and more or less will cool. For all these reasons, will tephras them and the lava is never exactly the same ones of a volcano with another, nor even sometimes of an eruption to another on the same volcano.
The materials emitted by the volcanos are generally rocks made up of microlites drowned in magmatic glass. In basalt, the most abundant minerals are silica, pyroxenes and the feldspars whereas the andesite is richer in silica and feldspars. The structure of the rock also varies: if the crystals are frequently small and very few in basalts, they are on the other hand generally larger and more in the andesites, signs that the magma remained longer in the magmatic room. 95% of materials emitted by the volcanos are basalts or andesites.
The most known material emitted by the volcanos is the lava in the form of castings. Of basaltic type (coming from the fusion of the coat in the case of a volcanicity of hot spot, of dorsal or rift) or andesitic (coming from the fusion of the lithosphere in the case of a volcanicity of subduction), more rarely of carbonatic type, they are made of fluid lava which runs out along the sides of the volcano. The temperature of the lava lies between 700 and 1.200 °C and castings can reach tens of kilometers length, a speed of fifty kilometers per hour and progress in tunnels of lava. They can have a smooth and glossed aspect (“pahoehoe” or “washes twisted”) or a rough and cutting aspect (“aa”). Castings of this lava, making several meters thickness sometimes, can put tens of years to be cooled completely. In certain exceptional cases, lava in fusion can fill the principal crater or a secondary crater and form a lake of lava. The survival of the lakes of lava results from a balance between contribution of lava coming from the magmatic room and overflow outside the crater associated with a permanent mixing by volcanic gas increase in order to limit the hardening of the lava. These lakes of lava are born only at the time of eruptions hawaïennes, the great fluidity of the lava allowing the formation and the maintenance of these phenomena. Kilauea in Hawaii and the Piton of the Furnace to the Meeting are two volcanos which have lakes of lava at the time of some of their eruptions. Erta Ale in Ethiopia is the only volcano in the world to have a lake of lava in a quasi-permanent way. At the time of certain eruptions, the lake of lava is emptied or on the contrary its level goes up until overflowing and forming castings on the slopes of the volcano.
Generally, the volcanic materials are composed of will tephras (or ejected): ashes, lapilli, slags, stones pounces, volcanic bombs, rock or basaltic blocks, obsidian, etc They are magma and torn off pieces of the volcano which are pulverized and projected sometimes until tens of kilometers height in the atmosphere. Smallest being ashes, it sometimes happens to them to make it tower of the Earth, carried by the dominant winds. The volcanic bombs, ejected largest, can have the size of a house and fall down in general near the volcano. When the volcanic bombs are ejected whereas they are still in fusion, they can take the shape in spindle at the time of their way in the atmosphere, in dung of cow at the time of their impact on the ground or in bread crust in the presence of water. The lapilli, which resemble small stones, can accumulate in thick layers and thus form pozzolana. The stones pounces, genuine foam of lava, are so light and contain so much air which they can float on water. Finally when fine drops of lava are ejected and carried by the winds, they can stretch in long filaments called “hair of Shovel”.
The magmas 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, H2O (50 to 90%);
CO2 (5 to 25%);
SO2 (3 to 25%).
Then come from other volatile elements: CO, HCl, H2, H2S. The degasification of in-depth magma, can be translated on the surface by fumerolles, around whose sulfur crystals can be formed. This volcanic degasification of the terrestrial coat is at the origin of the primitive atmosphere of the Earth, which was to be very rich in CO2.