Vesiculation path of ascending magma in the 1983 and the 2000 eruptions of Miyakejima volcano, Japan |
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| Authors: | T Shimano S Nakada |
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| Institution: | (1) Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan;(2) Present address: Center for Northeast Asian Studies, Tohoku University, 41 Kawauchi, Aoba-ku, Sendai City Miyagi, 980-8576, Japan |
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| Abstract: | The vesiculation of magma during the 1983 eruption of Miyakejima Volcano, Japan, is discussed based on systematic investigations
of water content, vesicularity, and bubble size distribution for the products. The eruption is characterized by simultaneous
lava effusion and explosive sub-plinian (‘dry’) eruptions with phreatomagmatic (‘wet’) explosions. The magmas are homogeneous
in composition (basaltic andesite) and in initial water content (H2O = 3.9±0.9 wt%), and residual groundmass water contents for all eruption styles are low (H2O <0.4 wt%) suggestive of extensive dehydration of magma.
For the scoria erupted during simultaneous ‘dry’ and ‘wet’ explosive eruptions, inverse correlation was observed between vesicularity
and residual water content. This relation can be explained by equilibrium exsolution and expansion of ca. 0.3 wt% H2O at shallow level with different times of quenching, and suggests that each scoria with different vesicularity, which was
quenched at a different time, provides a snapshot of the vesiculation process near the point of fragmentation. The bubble
size distribution (BSD) varies systematically with vesicularity, and total bubble number density reaches a maximum value at
vesicularity Φ ∼ 0.5. At Φ ∼ 0.5, a large number of bubbles are connected with each other, and the average thickness of bubble
walls reaches the minimum value below which they would rupture. These facts suggest that vesiculation advanced by nucleation
and growth of bubbles when Φ < 0.5, and then by expansion of large bubbles with coalescence of small ones for Φ > 0.5, when
bubble connection becomes effective.
Low vesicularity and low residual water content of lava and spatter (Φ < 0.1, H2O < 0.1 wt%), and systematic decrease in bubble number density from scoria through spatter to lava with decrease in vesicularity
suggest that effusive eruption is a consequence of complete degassing by bubble coalescence and separation from magma at shallow
levels when magma ascent rate is slow.
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| Keywords: | Eruption style Basaltic magma Degassing Water content Vesicularity BSD Coalescence |
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