Colors of Volcanic Ashes From 3 Different Summit Eruptions of Sakurajima Volcano, Japan.

Colors of volcanic ashes from summit eruptions of Sakurajima volcano were measured quantitatively and their relationship to eruption styles was investigated. Three eruption styles (I: strombolian eruptions which precede vulcanian explosions, II: vulcanian explosions and III: continuous ash eruptions after the vulcanian explosions) were distinguished from seismic waveform data measured by Sakurajima Volcano Research Center of Kyoto university. Volcanic ash samples from these eruptions have different color trends in CIE L*a*b* color space, although they have almost the same bulk chemical compositions. The colors of volcanic ashes from strombolian eruptions (I) have higher b* (yellow) values than the others, and the colors of ashes from vulcanian explosions (II) have larger dispersion in a* (red) values than those of continuous eruptions (III). By using heavy liquid and magnetic mineral separations, origins of the color differences were considered to be mainly due to the groundmass (vesiculated pumice-like grains + lithic fragments). The point counting under SEM of the groundmass revealed the abundance of pumice-like grains in the ash I. On the other hand, the lithic fragments are generally rich in the ashes II and III. Therefore, the high b* values of ash I can be originated from the pumice-like grains. The colors of groundmasses of ashes II and III after mineral separation have varying a* values with low b* values almost the same as the natural ashes before the separation. The selective dissolution of iron from the natural ashes resulted in the significant decrease of a* values approaching to the similar a* and b* values. The ferrous contents in their groundmasses determined by wet chemical analyses decreased with increasing a* values. Therefore, the a* variation can be attributed to the iron oxidation at the surface of the groundmass. In fact, the color variation trend of ashes II and III is similar to that of rhyolitic glass powders oxidized by heating in air. By comparing with the reported geophysical model, the ash I might be generated from vesiculated magma that erupted rapidly, while the ashes II and III experienced oxidation process in the volcanic vent covered possibly by a cap rock.