有珠山2000年噴火の推移

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The eruption started on the northwestern flank of Usu volcano on 31 March 2000, following strong seismicity and inflation of the summit area. The initial vigorous phreatomagmatic eruption moved gradually into phreatic eruption with vertical cock's tail jets in early April. The phreatic eruption was replaced by intermittent small explosion in mid-April, and, furthermore, by continuous steaming that often included ash emissions. Although many craters had been formed by late-April, the eruptive activity became concentrated on a few craters, which enlarged with time. As the eruption proceeded, craters became dry and the amount of issued steam decreased with time. Uplifting of the crater area had been evident since 3 April, and the rate of uplifting decreased exponentially with time. The amplitude of volcanic tremors changed, reflecting the variation of eruption types; small during vertical jet eruption and large during small explosions. The mode of eruption this time was similar in many ways to that of the 1910 eruption, when cryptodomes were formed in the northern slope over several months of sustained phreatic eruptions. The temporal change of eruption type probably reflected a decrease in water within the vents and rising explosion levels in the vent. This may have resulted from the successive production of slurry (clay and ash) there and, in addition, from the effective cooling of the magma head by groundwater (beneath the crater area). The volume of magma related to this eruption (about 0.1 km3) was similar to those of the previous two eruptions (1943-44 and 1977-78), though the mode and the duration of eruption were very different. The different environment of the magma's path, such as the abundance of groundwater, may be the reason.

有珠山2000年噴火の推移

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