5:15 PM - 6:30 PM
[SVC31-P07] Texture of ejecta of summit eruptions in Izu-Oshima
Keywords:Izu-Oshima, Bubble texture, Microlite
Izu-Oshima, which is an active volcano erupting basaltic magma, have caused many eruptions from both summit and flank craters. The eruptions which occurred in 1986 and 1950 are known as recent middle-scale eruptions. It is also known that large-scale eruptions have occurred every 150-200 years before recent middle-scale eruptions (Nakamura 1964, Isshiki 1984, Koyama and Hayakawa 1996, Endo et al. 1988). There are various kinds of geological and petrological research in Izu-Oshima, and the sequence and magma system of each eruption has gradually become revealed (Kawanabe 1991, Nakano and Yamamoto 1991, Hamada 2014, Ishizuka et al. 2015). On the other hand, there are few studies which analyze the textures of ejecta, and the relationship between eruption intensities and textures of ejecta is not clear. We focused on the summit eruptions in this study, and analyzed the bubble and microlite textures of scoriae of Strombolian eruption (1986 eruption) and sub-Plinian eruptions (Y1, Y2, Y4 and Y6 eruptions). In Y1 eruption, scoria deposits were divided into lower part (Unit A) and upper climax part (Unit C).
We measured scoria density of each eruption by glass beads, and selected one low-density, two middle-density and one high-density samples to create thin sections. Backscattered electron images of thin sections were obtained by FE-EPMA and analyzed by ImageJ.
Bubble textures changed according to the intensities of eruptions. In 1986 eruption, large bubbles were mainly observed. On the other hand, in the sub-Plinian (Y1, Y2, Y4 and Y6) eruptions, there were many small bubbles and the bubble number densities were higher than 1986 eruption. Bubble number density was especially high in Y1 Unit C. Scoria density was highest in Y1 Unit C. Microlites were observed in almost all eruptions, except Y1 Unit C.
From bubble textural analyses, there is a tendency that scoriae include many small bubbles in high-intensity eruptions. It is assumed that magma ascent rates were high and magma was supersaturated in volatiles and bubble nucleation progressed in high-intensity eruptions. In the low-intensity eruption such as 1986 eruption, there were many large bubbles and bubble number density was low as a result of growth, coalescence and degassing of bubbles. There were nearly no microlites in Y1 Unit C. Although it is estimated that magma was highly supersaturated in this Unit, there is possibility that liquidus temperature didn’t become lower sufficiently and crystallization of microlites didn’t occur because magma ascent rate was high and volatiles couldn’t precipitate enough as bubbles.
We measured scoria density of each eruption by glass beads, and selected one low-density, two middle-density and one high-density samples to create thin sections. Backscattered electron images of thin sections were obtained by FE-EPMA and analyzed by ImageJ.
Bubble textures changed according to the intensities of eruptions. In 1986 eruption, large bubbles were mainly observed. On the other hand, in the sub-Plinian (Y1, Y2, Y4 and Y6) eruptions, there were many small bubbles and the bubble number densities were higher than 1986 eruption. Bubble number density was especially high in Y1 Unit C. Scoria density was highest in Y1 Unit C. Microlites were observed in almost all eruptions, except Y1 Unit C.
From bubble textural analyses, there is a tendency that scoriae include many small bubbles in high-intensity eruptions. It is assumed that magma ascent rates were high and magma was supersaturated in volatiles and bubble nucleation progressed in high-intensity eruptions. In the low-intensity eruption such as 1986 eruption, there were many large bubbles and bubble number density was low as a result of growth, coalescence and degassing of bubbles. There were nearly no microlites in Y1 Unit C. Although it is estimated that magma was highly supersaturated in this Unit, there is possibility that liquidus temperature didn’t become lower sufficiently and crystallization of microlites didn’t occur because magma ascent rate was high and volatiles couldn’t precipitate enough as bubbles.