Suwanosejima, which lies within the Ryukyu Islands, is one of Japan’s most active volcanoes, erupting almost continuously between the 1950s and mid 1990s. It has two active craters, the central Otake crater and the Bunka crater, to the southwest. While the frequency of these eruptions has declined, the volcano remains active, with strombolian and vulcanian type eruptions occurring every 2 to 4 weeks.
Strombolian eruptions, typified by the activity of the Italian volcano Stromboli, consist of distinct bursts of lava from a magma-filled conduit. The explosions are caused by the bursting of large bubbles of gas that are released from a basaltic to basaltic andesite lava. Vulcanian eruptions, on the other hand, occur when the magma is more viscous (andesitic, dacitic, or rhyolitic). Vulcanian eruptions (characteristic of those from the volcanic island of Vulcano) produce large ash clouds and pyroclastic flows (dense currents of hot ash, gas and rock that flow down the flanks of a volcano). The andesitic composition of Suwanosejima’s magma places its activity between the two, with frequent explosions of lava and ash.
The largest historical eruption at Suwanosejima took place over 1813 to 1814, when lava flows from the southwest crater reached the western coast of the island. This eruption culminated in the collapse of the Otake crater, generating a large debris avalanche and a horseshoe-shaped caldera (the Sakuchi caldera) at the volcano’s summit. The photo below, though, shows a more recent eruption that occurred in the Otake crater during November 2010. Explosive eruptions were followed by continuous lava effusion and degassing and large earthquakes associated with the eruption were followed by harmonic tremor lasting minutes to hours.
Iguchi M, Yakiwara H, Tameguri T, Hirabayashi J (2008). Mechanism of explosive eruption revealed by geophysical observations at the Sakurajima, Suwanosejima and Semeru volcanoes. Journal of Volcanology and Geothermal Research. Vol. 178, pp. 1-9.
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