Introduction
Explosive activity at volcanoes is characterized by ejection into the atmosphere products derived from the fragmentation of magma and preexisting rocks. Dispersal and fragmentation of the products have been used to classify the eruption style, with the most dispersed and fragmented products being related to the most intense events (Walker, 1973; Fig. 1). While many fields of the resulting classification diagram have been largely explored in past years, eruptions with deposits characterized by intermediate dispersal have not been studied in large detail (shaded area in Fig. 1).
Figure 1. Classification diagram of explosive volcanic eruptions
Classification diagram of explosive volcanic eruptions based on the amount of magma fragmentation (F) and dispersal (D) of tephra. The shaded area corresponds to deposits of mid-intensity eruptions, characterized by intermediate dispersal and efficient magma fragmentation.
These deposits can vary from thick accumulations of lapilli-sized material to very fine ash beds. While in the past two decades fine-grained deposits have been generally interpreted as the result of the explosive interaction between external fluids and magma, the direct observations of the phenomena clearly demonstrated that ash production is instead very often related to primary fragmentation driven by magmatic degassing.
As a matter of fact, in nature we observe the occurrence of eruptions characterized by largely contrasting fragmentation of the products, associated to a similar dispersal. This suggests the possibility that mid-intensity eruptions cannot be explained by simply referring to a single modality of magma ascent and fragmentation, like for example in the case of the models explaining dynamics of low intensity, basaltic activity or Plinian-like, silicic activity. Rather, different eruption processes have to be devised, each corresponding to a different type of eruption style (and corresponding products and deposits) observed.
Recent observations of volcanic crises all around the world have confirmed that mid-intensity explosive activity is associated with a large range of eruptive styles and magma compositions, suggesting that the dynamics of magma degassing, fragmentation and tephra dispersal can be largely variable.
Even though the hazard implications of mid-intensity eruptions are clearly lower than those associated with pumice and ash deposition related to the large Plinian or sub Plinian events (Santacroce, 1987), they assume however an important role in the definition of the scenario for short-term, mid-magnitude, expected events at a large number of volcanoes worldwide.
In addition, ash-dominated events can produce a strong environmental impact and affect wide areas up to thousands of kilometres from their source due to the large dispersal of ash plumes.
The fallout of only a few millimeters of volcanic ash may cause serious temporary pollution in pasture-lands and in water supply systems, increasing water acidity through the leaching of the volatile components (essentially chlorine and sulfur) (Blong, 1984). Moreover, the potential for the inhalation of volcanic ash as a hazard to human respiratory health has become increasingly apparent (Baxter et al 1999; Bernstein et al. 1986; Rojas-Ramos et al. 2001; Horwell et al. 2003). Also the socio-economical impact of such small-size, ash producing eruptions can be impressive. The 1995–1996 Vulcanian eruptions of Mt. Ruapehu, New Zealand (Johnston et al. 2000), or the 1994 eruption of Rabaul Caldera (Papua-New Guinea) may provide an up-to-date picture of the problem. During these eruptions, only a few millimeters of ash caused very high economic costs, with damage to hydroelectric power facilities, the interruption of air and road traffic, and water and grass contamination (Johnston et al. 2000). The implications of ash emission on air traffic have been discussed in recent papers (Bursik et al. 2009; Papp et al. 2005) and have been recently brought to world attention during the eruption (still in progress at the moment of writing this paper) of the volcano Eyjafjallajökull (Southern Iceland) which shut down air travel in Europe for several days.
The past and more recent activity of Italian volcanoes is dotted with this type of activity, which, also due to the very intense population of the area since pre-historical times, has had a strong impact on human life. The paper deals with the general problems related to this class of explosive activity and presents a large case history from several volcanoes.