伊豆大島1986年LA溶岩流の表面形態

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The 1986 eruption in Izu-Oshima produced three different types of lava flows (LA, LB, LC) from different craters. As for LA lava flow which overflowed from the summit lava lake to caldera floor, precise survey on surface morphology and modeling on its formation process are still lacking. We discuss the factor controlled the surface morphology and crystallinity distribution of LA lava flow. LA lava flows consists of four flows, LA-I to LA-IV named in order of overflow, and they are all classified in aa flow. Contrary to general aa flows whose surface clinkers are scoriacious, LA surface mainly consisted of dense platy or blocky clinkers with red oxidized surface. Scoriacious clinkers distributed only flow edges, i.e., near flow front and on outer flank of levee. They became rounder and their average long axis sizes tended to decrease with distance. With the facts that the distribution boundary between dense and scoriacious clinkers was unclear and no clinker had their transitional texture, we conclude that LA surface clinkers did not originate from usual scoriacious clinkers but from autobreccia formed on the summit lava lake. Next, we examined crystallinity in LA lava groundmass. Both plagioclace and clinopyroxene tended to increase with flow distance. If the temperature before overflow and the following cooling rate was the same, the crystallinity should be independent of flow distance, because the time available for nucleation and crystal growth was the same anywhere. We further classified the plagioclase in groundmass into microphenocysts and microlites by their shapes and sizes; microphenocryst is thought to nucleate through rise from magma chamber, and microlite after effusion from vent. As for microphenocrysts the crystallinity seemed to be almost constant or rather decrease with distance, and the number density and the mean length did not change systematically. On the other hand, microlite tended to increase their number density and decrease their mean length. These may reflect the difference of thermal history the lava had suffered before overflow. The present results imply that the front (i.e. the earlier overflowed) lava suffered higher degree of undercooling when microlite nucleated. If the microlite nucleated in the lava lake, the difference of their number density and size may correspond to the difference in the time spent near the surface; the later effused lava was not exposed at the shallower part of the lava lake longer, where the degree of undercooling was high.論文
東北大学の論文
2004-03-19

伊豆大島1986年LA溶岩流の表面形態

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