5:15 PM - 6:45 PM
[SVC31-P02] Analysis of bubble and crystal distributions in Unzen lava: Do large bubbles always appear with crystals?
Keywords:bubble, crystal, magma
Outgassing from magma governs eruption styles. There are only a few volume changes of liquid and solid phases due to decompression associated with magma ascent, but the gas phase expands greatly and leads to explosive eruption. However, if outgassing through bubble networks or fractures occurs, it leads to effusive eruption. Thus, the mechanism to develop the interconnected bubbles by coalescence and enhance permeability is extensively investigated. Here, if bubbles can exist stably on the surface of the crystals, crystals may help to sustain the interconnected structure of bubbles. We investigate the positional relationship between bubbles and crystals to understand the importance of crystals on bubble coalescence.
We here analyzed the lava samples from the 1991-1995 eruption of the Unzen Fugen-dake Volcano: a breadcrust bomb ejected during the 11 June 1991 Vulcanian explosion and two kinds of rocks of lava spine which extruded the end of the series of eruption activities. We segmented 3D images of X-ray computed tomography (CT) in 8-bit grayscale into bubbles, crystals, and melt using the ImageJ plug-in Weka. Based on the segmented images, we examined the positional relationship between bubbles and crystals, and bubble size distribution. Bubble size is calculated by the radius of the sphere with a volume equivalent to the bubble volume. We also observed lava spine thin sections by synchrotron X-ray laminography.
We found that in more than 90 vol% of bubbles contacting crystals. The size distribution of bubbles contacting crystals follows the power law, and there is a wide range of sizes, while that for bubbles not contacting crystals is exponential, and there are only small bubbles. Observations of the lava spine by synchrotron X-ray laminography show that bubbles surround microlites.
From these results, we infer that large bubbles had nucleated at a depth using crystals as nucleation sites and experienced growth and coalescence to make their size distribution power law. That is, the large bubbles contact crystals because they are formed by heterogeneous nucleation using crystal surfaces. In contrast, bubbles not contacting crystals may be nucleated at shallow depths of the conduit so that the bubble size does not vary significantly. The bubbles surrounding crystals show interconnected structures. This structure may have enhanced the permeability and led to outgassing.
We here analyzed the lava samples from the 1991-1995 eruption of the Unzen Fugen-dake Volcano: a breadcrust bomb ejected during the 11 June 1991 Vulcanian explosion and two kinds of rocks of lava spine which extruded the end of the series of eruption activities. We segmented 3D images of X-ray computed tomography (CT) in 8-bit grayscale into bubbles, crystals, and melt using the ImageJ plug-in Weka. Based on the segmented images, we examined the positional relationship between bubbles and crystals, and bubble size distribution. Bubble size is calculated by the radius of the sphere with a volume equivalent to the bubble volume. We also observed lava spine thin sections by synchrotron X-ray laminography.
We found that in more than 90 vol% of bubbles contacting crystals. The size distribution of bubbles contacting crystals follows the power law, and there is a wide range of sizes, while that for bubbles not contacting crystals is exponential, and there are only small bubbles. Observations of the lava spine by synchrotron X-ray laminography show that bubbles surround microlites.
From these results, we infer that large bubbles had nucleated at a depth using crystals as nucleation sites and experienced growth and coalescence to make their size distribution power law. That is, the large bubbles contact crystals because they are formed by heterogeneous nucleation using crystal surfaces. In contrast, bubbles not contacting crystals may be nucleated at shallow depths of the conduit so that the bubble size does not vary significantly. The bubbles surrounding crystals show interconnected structures. This structure may have enhanced the permeability and led to outgassing.