Continuous Monitoring of Ash fall from Showa Crater by Time Series Sampling at Sakurajima Volcano, SW Japan

Strombolian and Vulcanian eruptions are characterized by unsteady surface manifestation such as the alternation of fountaining and successive ash emission with/without explosions. The time evolution of these surface eruptive phenomena is strongly dependent on the history of magmas ascending in volcanic conduits as well as that of conduit condition itself. In this regard, real time monitoring of active volcanoes is fundamental for understanding and predicting unsteady volcanic eruptions. Various geophysical data, such as seismic data and ground deformation, have been obtained and enabled us to figure out depth and motion of magma. However, they are all indirect information of magma itself. We now developed a new sampling system of ash fall for continuous monitoring of magma at active volcanoes. We have been successful in collecting ash samples emitted from Showa crater everyday for more than two years (2008-2010) at Sakurajima volcano, SW Japan. We investigated the evolution of the amount of ash fall, chemistry, particle size, and microstructure of ash particles. There have been several findings as follows; (1) Sakurajima volcano has been very active with waxing in intensity so that the amount of ash fall increased in accelerating manner. The accumulation rate of ash fall in 2009 was ca. 5 times that of 2008, and that of 2010 so far is ca. 10 times that of 2009. (2) Each ash sample mainly consists of crystalline andesite with variable crystallinity, color, and vesicularity. The matrix glass of these main constituents have similar range of variations in chemical compositions, but dominant composition changes with time, indicative of continuous mixing at shallow conduit system of two similar but slightly different magmatic sources at depths. (3) The daily monitoring of ashfall at a fixed location shows different grain size characteristics. There are two variation trends between the amount of ash fall and the fraction of fine particles (<1mm in diameter) which may be correlated with different eruption styles. The sequence of the activity from 2006 to date of Showa crater is very similar to that of the 1939-46 eruption which resulted in voluminous effusion of lava, and we will continue monitoring by this method for eruptive products that would greatly help understanding how eruption styles change with time.