Volcanism

Phenomena involving the expulsion of solid, liquid or gaseous substances from inside the Earth's crust to the surface are subsumed under the term volcanism.

How does a volcanic eruption arise?

Molten magma collects at varying depths within the Earth's crust. Discontinuities in the Earth's crust within weak tectonic zones may result in decompression and cooling of the magma. During this process, the gases contained in the magma are released and are carried with it towards the surface. Finally, the substances are ejected from the volcano via tubular ducts or fissures. Magma that has reached the surface is referred to as lava. The lava can have a temperature of over 1200°C. Further products expelled by volcanoes are pyroclastics and ash. Depending on its composition, lava may either be ejected explosively or flow slowly downhill. Volcanoes occur in two main forms:

• shield volcano (e.g. Hawaii Islands)
• stratovolcano (e.g. Stromboli, Italy)

What damage can occur?

Since the flow paths of the lava can be precisely determined, sufficient time is available for humans to escape. It therefore represents only a slight danger despite its extremely high temperature. However, lava can cause serious damage to material assets. The pyroclastics ejected represent a substantial risk both to humans and residential areas. However, owing to its high density and large area of distribution, the very hot volcanic ash represents the most significant threat. When the ash is saturated with water, mud avalanches may occur, and, when exposed to heavy rainfall, these may continue long after the actual volcanic eruption. Other dangerous volcanic phenomena are pyroclastic flows owing to their unpredictability, and also vapours and gases escaping from the surface of the lava, such as carbon dioxide and sulphur dioxide, which can be fatal to humans.