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Gigantic SO₂ emission from Miyakejima volcano, Japan, caused by caldera collapse

¹ Geological Survey of Japan, Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japan
² Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan
³ Volcano Fluid Research Centre, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-0033, Japan

An extremely large amount of volcanic gas has been released since mid-August 2000 from the volcanic island of Miyakejima, Japan, after formation of a summit caldera of 1.6 km diameter. The volcanic gas emission was continuous with very little extrusive magma activity. Variation of the SO₂ emission rate was monitored by repeated measurements with an airborne correlation spectrometer. In December 2000, the SO₂ emission rate averaged for the month peaked at 54 kt/d, which is twice the global SO₂ emission rate from nonerupting volcanoes evaluated before this activity. The SO₂ emission rate gradually decreased, almost linearly when plotted on a log scale, to 7 kt/d by the end of 2002, and then remained constant until at least December 2003. The total SO₂ emission amounts to 18 Mt, comparable to the emission of a large explosive eruption such as Pinatubo in 1991. A theoretical evaluation, based on the model of magma convection in a conduit, suggests that extremely large volcanic gas emissions can be caused by formation of a magma pathway with a slightly larger diameter than exists in common systems, because the magma-transport rate is proportional to the fourth power of the conduit radius. Because volcanic gas emissions were initiated by formation of a summit collapse caldera of 1.6 km diameter, the creation of a large magma-conduit system through fractures formed during caldera collapse is likely the underlying cause of the extremely large volcanic gas emissions from the volcano.

Key Words: SO₂ flux • volcano • degassing process • magma convection • conduit

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