There are about 1500 active volcanoes Volcano: volcanism definition given by an American geologist: "This is all the physico-chemical phenomena that accompany the ascent of magmas." on Earth including sixty erupt every year.
Volcanoes can cause massive climate disasters, causing a sudden cooling of the Earth's mean temperature. A volcano is composed of three parts: a reservoir of magma at depth, one or more volcanic vents which communicate with the interior of the Earth and the volcanic mountain.
Volcanic eruptions give rise to popular beliefs mingled superstitious terrors and fantastic legends.
Some rashes can be catastrophic to the point of disappearing civilizations.
A volcano is a relief of the crust underwater or air formed by ejecting and stacking of materials from the rise of magma as lava and tephra The tephra represent all materials (solids and liquids, driven by gases) emitted by volcanoes, except lava, they come from different types of explosive eruptions.. A volcano is powered by one or more reservoirs of magma. This magma derived from partial melting of the mantle. It is through the chimneys as the molten rock rises to the surface.
The accumulation of these rocks often reaches thousands of meters thick forming mountains and even islands. Depending on the nature of the materials, the type of eruption frequency and orogenesis The orogenesis (Greek oros, mountain and genesis, formation) is the set of processes that lead to the formation of mountain ranges., volcanoes have different shapes.
In general they are conical mountains ending in a crater or caldera. The eruption of material out of the volcanic vent usually located on the top, but sometimes as openings occur on the flanks of the volcano as Mount Etna. Volcanoes are classified into two types:
the "red volcanoes" to effusive eruptions relatively calm and emit fluid lava in the form of flows, they are "hot spot" volcanoes and submarine volcanoes mid-ocean ridges;
the "gray volcanoes" to explosive eruptions and emit pasty lava and ash in the form of pyroclastic flows and volcanic plumes.