In 2000, there were two major volcanic eruptions in the Japanese island arc. Usu, Hokkaido, erupted at the end of March, with the Miyakejima volcano, near Tokyo, erupting at the end of June (Fig, 1). For the past 300 years, both volcanoes have exhibited eruptive activity on a regular 20-30 year cycle. However, the 2000 eruption of Miyakejima was significantly different than had been experienced over those three centuries, and as such, it provoked a deal of scientific interest which culminated in the publication of a special issue of the Bulletin of Volcanology (2005, v.67, n.3), edited by Setsuya Nakada and Tim Druitt, and containing six in-depth studies of the eruption. As pointed out in the opening editorial, the eruption of Miyakejima was significant for several reasons. First, lateral migration of magma under the sea occurred up to 30 km from the volcanic centre, which resulted in the formation of a summit caldera (Fig. 2). Second, large volumes (>100kg/s) of sulphur dioxide were emitted continuously for more than four years following the eruption. Finally, the availability of modern techniques and monitoring equipment meant that the magma migration and caldera formation could be continuously monitored in detail. Chronologically, submarine eruption offshore first took place on the morning of 27 June 2000, following an earthquake swarm (Fig. 3). Strong earthquakes continued after this, until the middle of August, when they were accompanied by extensive ground deformation. It was deduced that the tectonic framework of the region, in which the Phillippine Sea plate is subducting beneath the Japanese island arc (Fig. la), created a fracturing of the crust which led to magma injection and dyke formation. A phreatic explosion took place on 8 July, accompanied by widespread subsidence of the summit, subsidence that eventually led to the formation of a caldera some 1.7km across, its volume reaching 0.6km~3, two orders of magnitude greater than the total equivalent rock volume of the erupted tephra (0.009 km~3), but less than half the volume of the magma required to intrude the dykes to the north-west of the volcano (approx 1.5 km~3) (Fig. 4). As such, the likely cause of the caldera was the lateral movement of magma to form the dykes. The largest explosion took place on the afternoon of 18 August 2000, with the eruption plume reaching 16 km above sea level. The eruption began with the discharge of basaltic-andesitic magma, subsequently replaced by basalt. It is thought that the initial discharge was a hangover from the 1983 eruption of the volcano.
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