Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 213-226

ArticleVariations of Color and Leachate Contents of Volcanic Ashes
from Sakurajima Volcano, JapanIsoji MIYAGI＊, Hiroshi SHINOHARA＊ and Junʼichi ITOH＊(Received September 20, 2011 ; Accepted June 20, 2012)
To understand magma degassing processes near the top of volcanic conduit, we investigated a series of volcanic
ashes from the Sakurajima volcano, Japan. We describe temporal changes in the color and the amount of leachates (Cl,F, S) of ash erupted from 1981 to 2011. Based on the amount of leachates present, ash samples are classified into twomajor groups: one is with a molar S/Cl ratio of 〜10, and another with S/Cl of 〜1 and that is relatively depleted in S.
Ash samples that were erupted during 1981-1991 from the Minamidake summit crater belong to the latter group.
Ashes erupted from the Showa crater in early 2008 belong the former group, of which the S content was found to
decrease systematically through time, although in 2011, ashes of the latter group erupted for the first time from thiscrater. Based on coloration, the ash samples in this study are classified into two groups: one with a yellowish color,
another with a less yellowish color. The coloration of the former group can be explained by the existence of yellowish
native sulfur as well as other hydrothermally altered minerals. We observed positive correlations between the intervalof successive eruptions and both the yellowness and the amount of ash leachates. Our interpretation is that the observed
temporal changes in volcanic ash result from a transition in the amount of fumarolic sulfur accumulation in partly
solidified magma near the top of volcanic conduit, which we interpret to reflect the mean residence time of the magma.
The magma probably periodically renewed in response to ash eruption and/or magma convection near the top of thevolcanic conduit.
Key words : Sakurajima; volcanic ash, color, volcanic gas, magma degassing, S, F, Cl

1．Introduction
Developing a sound understanding of magma degassing
processes can ultimately yield clues to the mechanismsthat cause volcanic eruptions (Eichelberger et al., 1986;
Eichelberger and Westrich, 1981; Jaupart, 1998; Kazahayaet al., 1994; Melnik et al., 2005; Papale et al., 1998; Wilsonet al., 1980; Woods and Koyaguchi, 1994). The Sakurajima volcano is one of the best places for conducting
petrological investigations of magmatic degassing processes, especially because fresh ash samples are frequently
discharged by eruptions at relatively short time intervals.
The volcanic activitiy of Sakurajima has been extensively
monitored in recent years, including for example, thetiming of eruptions, the height of volcanic clouds, thermal
infrared imaging of the vent, SO2 gas fluxes, volcanic
earthquakes, and crustal deformation (e.g., Iguchi et al.,2008; Yokoo, 2009, Kazahaya et al., this volume, Japan
Metrological Agency (JMA)). Here we describe temporalchanges in the color and amount of leachates (Cl, F, S)
absorbed on the ash. The analysis of ash leachates (Withamet al., 2005) has been used in the past to estimate the composition of volcanic gases, and also in monitoring thestages of eruption of a volcano (e.g., Nogami et al., 2002).
＊

Geological Survey of Japan, Tsukuba Central 7, 1-1-1
Higashi, Tsukuba, Ibaraki 305-8567, Japan.

In this study, the ashes were collected shortly after eacheruption took place, without having exposed to rain. The
analysis of the color of volcanic ash can provide information about the temperature and oxidation state of the post
eruptive magma (Miyagi et al., 1998; Miyagi and Tomiya,
2002; Moriizumi et al., 2008; Yamanoi et al., 2008).
2．Recent activity of Sakurajima
Over the past three decades, eruptions of the Sakurajima
volcano occurred at the Showa crater and the Minamidake summit crater. The current center of volcanic activity is the Showa crater (N31° 3443, E130° 3945) located about 750 m above sea level on the eastern hill of thesummit of Minami-dake. Volcanic activity within the Showa
crater commenced on 26 October 1939, and subsequentlyceased by 1948. Since 1948, the Minami-dake summit
crater has been the only active volcanic center of theSakurajima volcano, until eventually recommencement ofvolcanic activity in the Showa crater took place in June
2006. On 3 February 2008, explosive eruptions began
within the Showa crater. Since mid-April 2008, red-hot
ejecta have been observed at night by network cameras
installed around the volcano. Seemingly in response to theCorresponding author: Isoji Miyagi
e-mail: miyagi.iso14000@aist.go.jp