2005 Volume 50 Issue Special Pages S273-S288
Drilling is a powerful scientific tool to directly investigate the interior of volcanoes, and to collect hidden volcanic materials. Two scientific drilling projects were organized in Japan to investigate the internal structure, to reconstruct the eruptive history, and to reveal the magmatic processes. One is at Fuji and the other is at Unzen. Project of scientific drillings at Fuji volcano was a part of three-year term comprehensive study (April 2001-March 2004) on Fuji, whose future eruption may have serious social impacts. Five drillings were made to the depth between 75 and 650m. Three of them were aimed to install geophysical instruments such as seismometers and tiltmeters. Cores obtained from all drillings were examined to reconstruct the eruptive history and magmatic evolution of Fuji and older volcanoes beneath it. A previously unknown andesitic volcanic body was identified beneath the Komitake volcano. Dominance of debris flow deposits may be due to the cooler climate during the growth stage of Fuji experiencing more frequent interactions between erupted hot materials with snows capping the edifice. Unzen Scientific Drilling Project (USDP) was a six-year term international project (April 1999-March 2005) to understand the growth history, subsurface structure and magma ascending mechanism of Unzen Volcano. Four drillings were made in total, but first two boreholes were vertically drilled at the northeastern flank (USDP-1: 752m) and the eastern flank (USDP-2: 1462m) to reveal the three-dimensional structure and the growth history of Unzen. Later two drillings were to drill into the magmatic conduit of 1990-95 eruption to prove the magma ascent model. USDP-3(350m) was a pilot drilling to construct the drilling strategy for the conduit drilling. The conduit drilling (USDP-4) started from the northern slope of Unzen at 840m asl and directional to below the summit. The conduit zone consisting of multiple EW-oriented vertical dikes and explosion volcanic breccias was found near the sea level, ca. 1.3km beneath the summit area. Volcanic materials in the conduit zone were altered hydrothermally. Pyroclastic veins of the similar orientations are abundant in the conduit zone. Relatively fresh dike between 1975 to 1999m was considered to be the solidified magma of 1990-95 eruption. The temperature was as low as 160℃, much cooler than expected due to the effective circulation of hydrothermal fluids. Magma prefers to ascend forming a new dike independent from older conduits in every eruption event as far as in this depth. Pyroclastic veins may be fossils of isolated tremor events generated by the penetration of vesciculated magma into wall rocks at the tip of intruding magmas.
