Volcanic eruptions occur when the magma rising from inside Earth, spills out onto the surface. This can be from the volcano’s vent – in the case of Vesuvius – or vents opening at various points as with Phlegraen Fields or Etna. Volcanic eruptions last for varying periods of time: it can be for a few hours to dozens of years. For example Kilauea volcano in the Hawaii Islands has been erupting since 1986.
Forewarning phenomena. Volcanic eruptions are generally preceded and accompanied by several phenomena, including:
• fractures opening up (earthquakes) caused by induction of mechanical stress in the rocks;
• swelling or change in the shape of the volcanic edifice caused by the intrusion of magma;
• changes in the gravimetric and magnetic field surrounding the volcanic edifice;
• increased and altered composition of gaseous emissions from craters and ground;
• changes in the physico-chemical characteristics of the water table.
These phenomena, accompanying the magma as it rises, can be recorded by appropriate fixed instrumental systems, recording data 24 hours a day, or else by regular repeated measurement campaigns.
Classification. Volcanoes do not have a magnitude scale similar to the one used for earthquakes, but there are various measures as well as information helping to classify eruptions. An initial classification distinguishes volcanic eruptions into effusive and explosive. The first are characterized by low explosivity and emissions of fluid magma flowing down the volcano flanks. In the second, the magma is fragmented into shreds of various sizes, called pyroclasts, violently expelled from the volcano.
Another classification of volcanic eruptions is had by combining quantitative data (such as volume of products emitted, fragmentation of magma and height of eruptive plume) and from qualitative observations. These are expressed through the Volcanic Explosivity Index ((VEI) – an empirical index classifying the energy of the explosive eruptions with degrees going from 0 to 8. On the basis of this classification, eruptions are distinguished in: Hawaiian, Strombolian, Strombolian/Vulcanian, Vulcanian, Sub-plinian, Plinian, Krakatoian, Ultra-plinian.
Products. Effusive eruptions mainly generate lava flows. They flow on top of the ground at temperatures of between 700°C and 1200°C and at a speed that depends on the magma’s viscosity.
Whilst with explosive eruptions we have coarse materials (bombs and blocks) and fine materials (ashes and lapilli) falling. Volcanic bombs are fragments of lava which, upon being expelled from the volcano, cool and solidify before they reach the ground, taking on aerodynamic shapes during their flight. Whilst blocks are fragments of rocks of various sizes, torn from the walls of the volcanic conduit during the explosion. Lapilli and ashes are likewise fragments of magma expelled during an explosive eruption but are much finer materials. Ashes in particular are minute and can be born by the wind for many kilometres.
Explosive eruptions can generate elevated eruptive plumes. Pyroclastic flows are often originated by the collapse of said plumes, that is clouds denser than air comprising fragments of rock and gas and characterized by high temperature and speed.
The pyroclastic material deriving from explosive eruptions can lead to the formation of mudflows – or lahars – if mixed with water, which flow at high energy and speed down the slopes of the volcano and then mainly along the river valleys.
There may also be steam and other volcanic gas emissions near to the craters and flanks of active volcanoes as well as in hydrothermal areas where volcanic centres are no longer active. These come out of small but deep fissures in the ground where they reach temperatures going from about 100 up to 900°C. The gases condense in contact with the air due to a significantly reduced temperature, forming the typical “plumes” and concretions.
Effects on the territory. A volcano’s activity may be characterized by emissions of small quantities of magma, with limited effects on the environment, or to the contrary by catastrophic eruptive events capable of causing significant changes to the environment surrounding the volcano and in the climate, also at a global level.
There are also phenomena which, even though not directly connected with volcanic activity and not very frequent, can be dangerous and bring about significant changes to the territory.
Rocky or loose materials moving or falling due to the effects of gravity can generate landslides. All the volcanic edifices with steep flanks often made up of loose rocks therefore easily mobilized are prone to these phenomena of instability, which can create profound transformations and trigger off seismic activities or eruptions subsequent to the intense fracturing.
Submarine volcanic activity, submarine earthquakes and landslides pouring into the sea can give rise to tsunami. The energy propagated by this series of waves is constant and varies according to height and speed. Therefore when the wave approaches land it will increase in height but diminish in speed. In the worst cases the waves travel at high speeds, up to 700 km/h, and can rise to a height of up to 30 m upon reaching the coastline.
Lastly, fires can also be generated by incandescent material falling onto land with vegetation or as the lava flow progresses.