A pyroclastic flow flows down the southwest flank of Merapi volcano (Java, Indonesia) in May 2006 (Photo by Tom Pfeiffer, Volcano Discovery, Getty Images).
Living room of an abandoned house in the town of San Miguel Los Lotes, largely buried by ash after the eruption of Volcán de Fuego (Guatemala, June 2018 - photo by Daniele Volpe). The photo came second at the Photo World Press 2019, in the "General news, singles" category.In common parlance, the term "dangerousness" generically indicates the possibility that some event or someone constitutes a danger, or causes damage.
However, when we talk about natural phenomena, this word takes on a precise connotation: it indicates the probability that a certain phenomenon occurs, in a given time interval and in a given place, with a certain intensity. Estimating volcanic hazard is a very complex task. The same volcano can generate very different eruptions, both in terms of size - that is, quantity/mass of magma involved, also referred to as "magnitude" - and in terms of eruptive style (from effusive to very explosive, with a whole range of explosive intermediate). Even during a single eruption, very different phenomena can follow and coexist, each of which has a different impact on the territory and its inhabitants. Due to this complexity, volcanic hazard cannot be summarized with a single number or a single map. To have a complete picture, it is necessary to consider the danger of each of the expected phenomena, some of which may occur even before the eruption begins.
The main dangerous phenomena associated with volcanic activity are:
pyroclastic flows (pyroclastic currents of density)
mudflows (lahars) and floods
ballistics fallout
dispersion and fallout of ash and lapilli
lava flows
formation and explosion of lava domes
ground deformation (also pre-eruptive)
volcanic earthquakes (also pre-eruptive)
emission of volcanic gases (also pre-eruptive)
structural and slope collapses
tidal waves of volcanic origin
Volcanic hazard is typically reported and estimated for each of the hazards listed above. For example, the lava flow hazard is the probability that a given region will be affected by the flow of a lava flow in a given time interval.
In general, a distinction is made between the long-term hazard, based mainly on the eruptive history of the volcano, and the short-term hazard, essentially based on the data recorded by monitoring and surveillance networks.
Volcanic hazard is one of the three components that determine volcanic risk.
The volcanic risk, in fact, is given by the product of three factors: the danger, the value exposed and the vulnerability.
risk = danger x exposed value x vulnerability
Hazard is the probability that a certain dangerous phenomenon affects a certain area in a certain period of time.
The exposed value describes what is exposed to the hazard. It is given by the number of people, the number and type of buildings, the surface of agricultural land, the infrastructures, etc., which are present in the territory potentially affected by the dangerous phenomenon.
Vulnerability is the percentage of exposed value that it is estimated will be lost as a result of a given dangerous phenomenon.