3. Why do volcanic eruptions occur?

Volcanoes are created due to the inner dynamics of the Earth. When underground melted rocks (magma) migrate through the fractures in the outer layer of the Earth (lithosphere) and reach the surface of the Earth crust a volcano is created.

A scheme of the Earth’s interior showing lithosphere and astenosphere in diverging and converging plate boundaries. Source: http://dilu.bol.ucla.edu/

The magma has very singular characteristics. Once it has reached the surface, it can have temperatures between 700 – 1200 °C and a density between 2300 – 2700 kg/m3. Viscosity is the attitude of the Magma to flow (high viscosity = low flow; low viscosity = high flow) and depends on its chemical composition. Magmas having high content of silica and gases and low iron and magnesium contents produce highly viscous lavas (rhyolites). Magmas with large content of magnesium (Mg), calcium (Ca) and iron (Fe) produce low viscous lavas (basalts).

As magma viscosity and gas quantity are the main factors controlling the eruptions, it can be stated that low viscous magmas (basaltic) can produce gentle eruptions with lava flows spreading slowly over large distances.

Highly viscous magmas (rhyolitic) can produce high pressures and strong deformations of the volcano, with large explosions and limited lava flows.