A volcano, derived from the Italian term vulcano, which means Vulcan (the Roman god of fire), is an opening in the Earth’s crust through which magma from the Earth's mantle is expelled.
Magma is a mixture of molten rocks and dissolved gases (water vapor, carbon dioxide (|CO_2|) and sulfur dioxide (|SO_2,| |SO_3|).
A volcanic eruption occurs when a volcano bursts. Then there is expulsion of magma or ash outside the volcano.
A volcanic eruption occurs in a few stages.
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Magma makes its way into the lithosphere. It accumulates in the magma chamber, which is a huge reservoir of magma located between |10| and |30| km deep in the lithosphere. Gases mix with the magma, creating an increase in pressure.
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When the pressure inside the volcano becomes too strong, the magma is expelled outside the volcano. An explosion combined with an earthquake announces the start of the eruption.
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Most of the magma passes through the main vent, while some of the magma can also make its way through secondary vents.
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When the magma reaches air, it exits the crater as lava.
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The volcanic eruption can expel hundreds of tons of lava, ash, and gas into the air (ash cloud).
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The volcanic eruption may also cause the formation of lava flows, a slow flow of lava down the slopes of the volcano. Lava fragments may also be ejected from the volcano during the eruption (volcanic bomb).
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The volcano ceases its activities and the lava cools to form, with the hardened ash, the cone of the volcano.
When magma rises, it simultaneously undergoes a degassing, which causes a constant and very light vibration of the ground called a tremor. When lava reaches the open air, it is the start of an eruption and it ends when no more lava is emitted. Eruptions are classified according to the type of lava and the type of volcanic projections emitted. There may be other phenomena accompanying the eruption: major earthquakes, landslides, tsunamis, etc. A volcanic eruption can last from a few hours to several years, and it can eject volumes of magma, of several hundred cubic kilometres.
The gases that volcanoes emit mainly consist of water, chlorides, carbonates and sulphates. They can also contain carbon dioxide and carbon monoxide, methane, ammonia and hydrochloric acid. Low gas emissions are called fumaroles during periods of calm.
There are two types of lava.
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FLUID LAVA |
VISCOUS LAVA |
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Volcanic projections are classified by their size. The following table lists the main volcanic projections.
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Volcanic projections |
Description of the volcanic projection |
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Pyroclastic rocks |
Fire debris that solidifies in air |
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Ashes |
Fine particles of rocks and minerals of less than |2| mm in diameter |
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Lapilli |
Lava fragments of a few millimetres in diameter |
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Slag |
Lava fragments of |5| to |30| cm in diameter |
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Bombs |
Large oval blocks in a viscous state |
The convection movements of magma inside the Earth's mantle are linked to the movement of tectonic plates and to the creation of volcanoes. Volcanoes can be formed in different places.
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At the edge of two converging plates, the Earth’s crust becomes more fragile, because it is subjected to different types of stress (compression, folding, etc.). It is then easier for the magma to create a path towards the surface of the Earth. This is called a subduction zone; the most dangerous volcanoes appear there.
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At the edge of two divergent plates, there are also tensions and fractures in the Earth's crust that allow magma to seep into the lithosphere and form volcanoes. This is the case with ocean ridges which form when two oceanic plates move away from each other; this creates mid-oceanic ridges.
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About fifty volcanoes appear in the centre of a tectonic plate. These volcanoes are located above what are called hot spots (hot magma that rises to the surface of the Earth). In these places, the heat is very intense and it manages to pierce the Earth’s crust.
Volcanoes form on continents and in oceans, but it is where tectonic plates meet that |95\%| of them are formed.
There are more than |1\ \500| active volcanoes on the Earth's surface, in addition to those formed underwater. The vast majority of these volcanoes are located along the Ring of Fire, a string of volcanoes that lie along different tectonic plates.
There are two main types of volcanic activity.
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Effusive eruptions are relatively calm eruptions. They do not produce an explosion but the lava flows are very abundant. The lava contains a lot of basalt (volcanic rock resulting from a quickly cooled magma in contact with water or air) but it is poor in silica. Since the lava does not contain much silica, it releases its volcanic gases very easily and it is very fluid. Volcanoes that have effusive eruptions are called red volcanoes (Ex: Hawaii volcanoes, Piton de la Fournaise (Peak of the Furnace), Etna…).
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Explosive eruptions are much more spectacular. They are very dangerous because they are unpredictable. These eruptions do not form a lava flow. However, they are accompanied by strong explosions, which produce a large quantity of ash that generates fiery clouds and volcanic plumes. The lava during these eruptions contains a lot of andesite (igneous rock generally grey in color) and silica. This lava is, therefore, very viscous and does not release volcanic gases easily. Volcanoes that have explosive eruptions are called grey volcanoes (Ex: Pinatubo, Krakatoa, Mayon, Merapi…).