volcanoes: origin and location

  The volcano is a terrestrial or maritime geological group. Generally it is a mountain that emits melt or solid matter. It is composed of three essential parts:

* The magma reservoir (or magma chamber) placed at the depth of the earth
* One or more chimneys through which the magmas pass from the reservoir to the surface of the tectonic plates
* The volcanic unit

  There are thousands of volcanoes on Earth. Some are "extinct" (case of French volcanoes in the Massif Central). Others are active, either permanently or intermittently (they are then considered as drowsy).

  Volcanoes differ from each other. In the early days of volcanology, the observation of a few volcanoes led to the creation of categories based on the appearance of eruptions and the type of lava emitted. Each type is named according to the referent volcano.

  1500 volcanoes are active on our planet. Their location corresponds closely to the zones where the terrestrial plates meet in the basement.

  700 million people live under the permanent threat of a volcano. Most live in high-density areas, mainly in poor countries, but not only. Iceland, for example, is an entirely volcanic island on which live 300,000 people, among the richest in the world.