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A volcano is an opening in the earth's crust from which lava (consisting of magma), gases (sulfur dioxide, carbon dioxide, nitrogen, for example) and ashes are released during eruptions.

The study of volcanoes is volcanology or vulcanology. Volcanic activity is one of the manifestations of the internal activity of the Earth due to the energy stored within it (see "volcanism").

Volcanoes on Earth and in space

On Earth, there are about 600 active volcanoes on the continents, 1,300 if we add those of the seabed. The largest volcano in the solar system is Mount Olympus on Mars. Some volcanoes are considered extinct, others sleep and can see their activity resume.

We distinguish traditionally different types of volcanoes according to their form:

Hawaiian volcanoes (of Hawaii), very flat, from which escape, only by effusion, very fluid lava;

• Strombolian volcanoes (Stromboli, Italy), with pointed cones, formed by debris accumulation and lava flows;

• Vulcanian volcanoes (Vulcano, Italy) formed by explosive debris accumulation;

• Pelican volcanoes (from Mount Pelee in Martinique), with very viscous lava that tends to form needles by climbing through the cracks of previous extrusions. Their eruption under the effect of the thrust of the gases gives rise to the phenomenon of burning cloud, or ejection of melted dust;

• volcanoes formed by cracks.

The morphology of the volcanic apparatus (size and slope of the cone and the crater) results from the accumulation of materials emitted during the eruptions, and depends in particular on the properties of the lava, and the internal conditions of pressure, which can evolve during the activity of the volcano. Small variations in lava composition can result in very different properties, especially for viscosity. Volcanics can be explosive when the pressure exerted by the gases ejects fragments of magma more or less solidified with ash and slag.

Unreliable eruption forecasts

Volcanic activity is extremely difficult to predict; their study has only a few data available, because of the disproportion between the average duration between two eruptions and the human lifespan. The research focuses on the evaluation of deep reservoirs (about 10 km below the surface) that flow through various veins, fissures and conduits. The consequences of an explosion are related to the importance of magma reservoirs. Gas emissions and other warning signs of rising magma can be exploited by active volcano monitoring networks.

The study of outcrops of extinct and eroded volcanic systems also provides information on the mechanisms involved in the activity period.

The different types of volcanoes

Volcano eruptions

There are three types of volcanism:

 Effusive volcanism: It regularly rejects flows of fluid lava. The lava is at about 1200 ° C.

Effusive volcanism

 Explosive volcanism: it rejects a lava thick, viscous and does not flow, forming a "plug" on the crater. The eruption sprays the top of the volcano and unleashes destructive hot clouds. The lava is at about 300 ° C to 900 ° C.

Explosive volcanism

 There is also underwater volcanism that operates differently from continental terrestrial volcanoes. The lava coming to the surface cools immediately forming a sort of cushions called pillow lava. Marine volcanoes sometimes form islands, for example: the island of Reunion. The chimneys are very deep: up to 2 kilometers. Scientists say that there are 20000 volcanoes almost under the sea.

Volcano activity

Volcanoes can also be distinguished according to their activity.

 Active volcanoes: volcanoes are erupting.

 Sleeping volcanoes: they can wake up and erupt if gas accumulates too much.

 The extinct volcanoes: it's the volcanoes that are old and will never be erupted again.

The magma

The magma is formed by melting a rock between 150 and 50 km deep. Only one part of the rock melts giving droplets of liquid. Lighter than the rocks, the droplets slowly migrate upwards and gather to form, under the volcano, in depth, a magmatic chamber.

Once on the surface and released from its gases, the magma takes the name of lava

Famous volcanoes

On earth

 Vesuvius

 Etna

 Stromboli

 The Kilauea

 Mount St. Helens

 The Krakatoa

 Mount Fuji

 The Nevado ojos del salado