Directed blasts and pyroclastic surges usually move a large mass of air at very high speed, and produce death and sudden destruction of buildings, infrastructure and the environment.
Tephra (rocky materials projected in the atmosphere) can be responsible for deaths and injuries once they fall to the ground, covering large areas and damaging structures and the environment.
Volcanic ash exploded from a volcanic vent is less than 2mm in size, like small sharp glass-particles that damage anything they encounter. Volcanic ash is very harmful to the environment around the volcano: due to heavy ash-rains, people and animals may die by lack of oxygen and houses and buildings may collapse. Ash clouds can also be very dangerous for airplanes as they can damage engines and control instruments.
Volcanic gases can be noxious for human and animal health and for the natural environment.
Lava flows move slowly so they can hardly be a danger for human life, provided that adequate safe behaviour is respected.
Pyroclastic flows, mudflows (lahars), debris avalanches and landslides produce large flows of water, rocks and debris along the hills of the volcano and can be very dangerous for human life. People caught in their path have a high risk of death from severe crush injuries, drowning or asphyxiation or can be burned in a pyroclastic flow. The only protective measures from such hazards in the case of an eruption are to evacuate the potentially involved people from the exposed area.
Volcanic earthquakes can produce human losses where the quality and maintenance of buildings are scarce. In such a condition, small volcanic earthquakes can already produce damage or destruction to buildings, killing and injuring people.
Fires produced by eruptions can be responsible for among other, destruction of forests, wood stocks, buildings, factories, producing large damage to urban and rural areas.
Eruptions can also produce volcanic tsunamis: large sea waves reaching coastal areas and going far inland causing death, large-scale damage and destruction of coastal regions.
Volcanic eruptions can have strong negative effects for the environment. They can produce large destruction of the environment as lava flows and pyroclastic materials cover large portions of natural environments, destroying them for decades.
Furthermore, toxic gases emitted during strong activity can have an additional impact and in particular carbon dioxide emitted from volcanoes can have a significant impact on increasing greenhouse effect.
Emitted gases consist mainly of water vapour (H2O), carbon dioxide (CO2) and sulphur dioxide (SO2) and other gases typically found in volcanic ashes are hydrogen sulphide (H2S), hydrogen chloride (HCl, which becomes hydrochloric acid in contact with humidity), hydrogen fluoride (HF) and carbon monoxide (CO).
In particular, sulphur dioxide is converted in the stratosphere into sulphuric aerosols which reflect solar radiation absorbing heat but also take part in chemical reactions generating ozone destructive material, globally affecting the climate, and producing acid rains.
A typical example of a volcanic eruption that caused substantial environmental damage is the 1991 Mount Pinatubo eruption (90kms northwest of Manila in the Philippines), the second largest volcanic eruption of the twentieth century.
Large eruptions can cover prehistorical and historical remains, preserving them up to our days. Very famous examples of historical remains and archaeological sites damaged by volcanic eruption are known in the world: the Vesuvius area, inhabited since the Ancient Greek period, suffered eruptions with consequences on monuments and archeological sites (i.e. Pompeii, Herculaneum and prehistorical settlements near the Vesuvius).