Sustained explosive activity

Overview This chapter reviews some of the physical processes responsible for the injection of large volumes of volcanic tephra and gas high into the atmosphere, following sustained explosive eruption from a volcanic vent. The resulting volcanic plumes or columns can disperse ash and aerosols over vast distances, and cause global perturbations to climate. In contrast, incandescent lava fountains are common manifestations of the low-intensity end of the spectrum of explosivity. This chapter presents a series of modeling approaches, from dimensional analysis to fully time-dependent, three-dimensional numerical treatments, in order to develop some quantitative understanding of these phenomena. Introduction When sustained explosive eruptions discharge tephra from a volcanic vent, the erupting material may form a convecting plume that rises high into the atmosphere; these flows are known as volcanic eruption columns (Fig. 8.1(a)). At moderate to high discharge rates subplinian and plinian columns rise to 10−40 km, penetrating the stratosphere, and erupt ejecta volumes of ~0.1 to ~10 km 3 , whereas very high ultraplinian discharges produce columns > 40−50 km high and erupt volumes of ≫ 10 km 3 . At low discharge rates, hawaiian activity (named after the archetypal eruption style of the Hawaiian basaltic volcanoes) erupts volumes of 10 4 −10 8 m3 as incandescent lava fountains tens to hundreds of meters high (Fig. 8.1(b)), with weak plumes of fine pyroclasts rising above the fountains to heights of order 1−5 km.