Explosive volcanic eruptions - a new classification scheme

Abstract

A classification scheme is proposed based on measurements made on the resulting pyroclastic fall deposits, the significant parameters being the area of dispersal and degree of fragmentation of the material. An empirical measure of the first is the area enclosed by the 0.01 Tmax isopach (where Tmax is the maximum thickness of the deposit), called D, which ranges from less than 10 km2 for deposits of strongly cone-building type to more than 1000 km2 for deposits of strongly sheet-forming type. An empirical measure of the second is the percentage of material finer than 1 mm in the deposit, or more simply at the point where the 0.1 Tmax isopach crosses the dispersal axis. The latter value, called F, varies from less than 20 for deposits in which fragmentation was mainly achieved by the tearing apart of magma, to more than 80 where it was largely due to thermal shock resulting from the quenching of lava by water. Three kinds of pyroclastic fall deposit are characterised on the basis of their D and F values: hawaiian/strombolian, with low D and low F; surtseyan, with low D and high F; and plinian, with high D and low or moderate F. A distinction based on D is proposed between the strombolian and hawaiian types, and one based on F between normal and violent strombolian. A new, sub-plinian, type is proposed intermediate in character between strombolian and plinian. Others, still to be characterised, are thought to occupy the field outlined by these types. © 1973 Ferdinand Enke Verlag Stuttgart.