Washing
Lava is a rock melts, more or less fluid, emitted by a volcano during an eruption. The dish originated from a magma , molten rock reserves located in the thickness of solid rock layers of the planet.
The lava at the time they are issued, which reach temperatures depending on their chemical composition varies from 700 to 1200 C. They solidify by cooling rapidly in contact with soil, air or water and then form volcanic rocks , such as basalts or rhyolites.
Volcanic rocks formed by cooling lava surface, belong to all igneous rocks , together with the plutonic rocks that are formed by cooling and crystallization of magma at depth (such as granites or gabbros ).
The accumulation of lava and eruptive ejected material ( bombs , ash and lapilli ) is building the characteristic cone of volcanoes whose shape depends on the viscosity of the lava and its gas content.
Summary |
Appearance lava fields
When a lava flow, or viscous fluid, is created during an eruption, the appearance of the lava and its composition are not always the same.
Indeed, the viscosity of lava depends on its content of silica (SiO 2), where the lava is highly charged with silica, it is viscous and vice versa. This allows us to explain the principle of volcanoes such effusive and explosive.
Among volcanoes type explosive , lava emitted during a casting is viscous and rich in silica. During an eruption, a pyroclastic flow can be created. Gas bubbles form and rise of the magma chamber. This surge of gas to the surface is called the gas pressure. During the ascent up the chimney, the pressure is very high (weight of the column of magma) and the viscosity of the lava important. When the gas pressure becomes greater than the pressure of the column of magma, the gases released suddenly, causing the explosive ejection of particles and molten rock fragments (bombs, lapilli and ash). Rock obtained during such eruptions is a rock ryolithique.
Type volcanoes in effusive , whose lava is low in silica, the lava emitted during a casting is fluid. During the ascent of magma in the chimney, the gases have no difficulty in escaping to the crater in the fluid lava. So there is no increase in pressure during the ascent of magma, which can spring fountain of lava. This type of volcano can also submit their crater a permanent lake of lava (eg, Kilauea, Mauna). Rock obtained during an effusive lava flow is a basaltic rock.
For effusive lavas formed in the open air, there are mainly two types that are in volcanology that their names had already given the Hawaiians :
- aa: for rough lava flows and chaotic that form on cooling scopes difficult to navigate (the Hawaiian word 'a'a with multiple meanings: "Rocky," "burning", "brave", etc..)
- pahoehoe: for fluid lava cools, forming a smooth surface "that can go barefoot (the Hawaiian word which means pahoehoe" smooth and sweet ") Types of lava
There are two main types of lava in Volcanology : Lava low in silica, after an effusive volcano and lava rich in silica, after an explosive volcano.
When a volcano effusive in between eruptions , it emits lava fluid characteristics, poor silicates. The temperature of lava coming out of the crater is about 1200 C. This lava basalt flows about 20 km / h leaving the crater and cools slowly on contact with air, soil or water, further reducing their speed. The flows are channeled and relatively slow. During an eruption near a populated area, the damage can be ascertained, but the slowness of these flows allow people to flee during an eruption.
Pyroclastic volcano Mayon When an explosive volcano erupts, it emits pyroclastic flows , composed of ash from lapilli , with piles of burning lava and lava goo. Very rich in silicate , very few smooth, hard, this lava accumulates most often at the top of the volcano , creating a lava dome. This dish can go explosively at temperatures of 1000 C outdoors, see more in a magma chamber. When an eruption large enough fires, entire flank of the volcano's lava dome or break off and hurtle down the slopes to tens of kilometers per hour. These flows can be very dangerous for humans, especially the speed of its cast, the size, but mostly through the stem of the eruption, which can bury and destroy an entire city within hours. Moreover, these flows can go very far from the epicenter of the eruption, which increases the chances of destroying a populated area near the volcano, even miles away from it.
We distinguish:- lava plain (or pahoehoe as the Hawaiian word): very fluid lava with a smooth surface, rough and often ridged;
- lavas strung: very fluid lava flow whose surface shows overlapping and intersecting ribs (the thin outer skin of the casting is deformed by the advancing lava in the center of the casting);
- lavas Prism: Some lavas, mainly basalts, speak into hexagonal columns. These prisms appear during the cooling of lava (cracks of withdrawal). When they are large, we talk about organs or colonnades. The prisms are vertical, sometimes in radial jet;
- the a'a (as the Hawaiian word): lava flows with a chaotic surface, bristling with tilted blocks;
- pillow lava (or (in) pillow lava) lava implemented during a submarine eruption;
- the domes , or needles: built by very viscous lavas.
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References
- Hawaii Center for Volcanology: General Information about Hawaiian volcanoes fhield (fr)
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