Not all phenomena in nature can be clearly categorised. There are no clear boundaries between different types of volcanos and eruptions. During a volcanic eruption, the nature of volcanic activity and volcanic products may change within minutes, hours, days or months.

Molten rock in the Earth's interior is called magma. If it flows out of a volcano, lava generates (effusive volcanism). During an explosion it is hurled out as tephra (explosive volcanism). The manner in which magma reaches the surface and/or is released into the atmos¬phere is called the eruption process and is controlled by a number of different mechanisms and influencing factors. These are mainly the tectonic situation, the composition and temperature of the magma, the depth of the reservoir as well as the spatial conditions at the volcano.

Particularly important is the chemical composition of the magma. Low-silica magma has low viscosity (runny) and a low content of volatile substances (volatiles). It is hence associated in volcanic activity with a mostly calm and steady effusive outpour of lava. High-silica magma, in contrast, has high gas content and is therefore highly viscous. The expansion of the gases in the volcanic conduit causes fragmentation of the magma, resulting in explosive volcanism and the formation of pyroclastic material.

                                                Volcano and eruption types

                                                            Eruption products
                       Lava flows                 Tephra Fallout                   Volcanic gases

                                                               Eruption types
                Hawaiian     Fissure eruptions     Strombolian    Volcanian
                  Phreatomagmatic   Hydrothermal explosions   Sub-/Plinian

                                       Secondary phenomena associated with volcanoes
                               Earthquake  Landslides  Lahars  Tsunami  Forest fires

                                                                  Volcano types
       Mid-ocean ridges and deep-sea mountains  Flood basalts  Shield volcanoes
                Lava domes Stratovolcanoes Slag/Ash cones  Crater lakes  Calderas

Text: Christina Bonanati, GEOMAR Helmholtz Centre for Ocean Research Kiel