Scientific Results of Conduit Drilling in the Unzen Scientific Drilling Project (USDP)

Abstract

Directional drilling at Unzen Volcano in Japan during mid of 2004 penetrated the magma conduit and successfully recovered samples of the lava dike that is believed to have fed the 1991-1995 eruption. The dike was sampled about 1.3 km below the volcano's summit vent and is intruded into a broader conduit zone that is 0.5 km wide. This zone consists of multiple older lava dikes and pyroclastic veins and has cooled to less than 200˚C. The lava dike sample was unexpectedly altered, suggesting that circulation of hydrothermal fluids rapidly cools the conduit region of even very active volcanoes. It is likely that seismic signals monitored prior to emergence of the lava dome reflected fracturing of the country rocks, caused by veining as volatiles escaped predominantly upward, not outward, from the rising magma. Geophysical and geological investigation of cuttings and core samples from the conduit and of bore-hole logging data continues. Introduction and Drilling OperationThe volcanic eruption at Unzen, Japan, during 1991-1995 took a heavy toll on life and property through devastating pyroclastic flow events. To understand the structure and growth history of the volcano and to clarify the eruption mechanisms of SiO2-rich viscous magmas, the Unzen Scientif ic Drilling Project ( USDP), a six-year project consisting of two phases, was started in April 1999 (Uto et al., 2000). In the first phase, two holes were drilled into the volcano's flank (USDP-1 and -2 wells). In the second phase, drilling penetrated the magma conduit that fed a lava dome at the summit during the 1991-1995 eruption. The conduit drilling reported here was carried out as a joint research project with the International Continental Scientific Drilling Program (ICDP). The detailed design and targets of the conduit drilling were determined in the first phase. The design and decision process involved negotiations with federal environmental and forestry agencies, explaining the project to local residents, a one-year-long environmental assessment of possible drilling sites, the drilling of a pilot hole (USDP-3 well), and the convening of an international workshop on drilling techniques (Nakada et al., 2001).The magma conduit, especially its upper part, is believed to be the site of effective degassing that is the major factor controlling eruption styles. The pressure-dependent nature of solubility of volatiles, principally water, accelerates vesiculation as magma approaches the surface and produces geophysical signals (earthquakes and inf lation) in the shallow conduit region. Drilling into this region allowed us the first in situ observations and sampling of the still-hot conduit and wallrocks of a recent, well-observed eruption (Nakada and Eichelberger, 2004).Geothermal modeling prior to drilling had suggested a temperature of over 600˚C at the center of the conduit, if it cooled by conduction only. The drilling target was set in the hypocenter region of isolated tremors that occurred prior to magma extrusion in 1991. Drilling started vertically at...

Two-dimensional resistivity structure of Unzen Volcano revealed by AMT and MT surveys

“…This feature is consistent with the previous works by Kagiyama et al (1999) and Srigutomo et al (2008). Figure 7 shows the resistivities along the borehole USDP-4, where conduit drilling was performed (Nakada et al, 2005). Red and black lines represent the resistivities obtained by this study and those obtained by well logging (Ikeda et al, 2008).…”

“…• C was estimated at the bottom of the borehole USDP-4 from temperature logging and fluid inclusion studies (Nakada et al, 2005). Furthermore, geodetic surveys found the pressure source during the 1990-1995 eruption at a depth of about 1.5-3 km beneath the summit (Geodetic Survey Group, Joint Observation by National Universities, 1991;Ishihara, 1993;Hendrasto et al, 1997;Kohno et al, 2008).…”

Tectonic stress and renewed uplift at Campi Flegrei caldera, southern Italy: New insights from caldera drilling

“…The results provide the first direct stress measurements in Campi Flegrei and are consistent with an accumulation of stress since at least the uplift of 1969-72, so that the potential for eruption appears to have increased with successive episodes of unrest. They also extend the database of deep borehole studies in volcanic districts, including Iceland (Haimson and Rummel, 1982), Long Valley caldera in California (Moos and Zoback, 1993), Hawaii (Morin and Wilkens, 2005) and Unzen in Japan (Nakada et al, 2005), and reinforce the importance of borehole data for investigating the structure and evolution of magmatic systems, as well as the interplay between tectonic, magmatic and geothermal processes (Eichelberger and Uto, 2007;Harms et al, 2007).…”

Towards more realistic values of elastic moduli for volcano modelling

“…The borehole at Mt. Unzen was drilled to a depth of 1995 m between 2003 and 2004 (Nakada et al, 2005;Sakuma et al, 2008). The rock types intersected are lava flows, lava dykes, pyroclastic rocks, and volcanic breccias (Sakuma et al, 2008;Figure 22).…”

“…Finally, neutron porosity logs collected at Mt. Unzen during the Unzen Volcano Scientific Drilling Project revealed average in situ porosities of lava flows and dykes of 0.15-0.20(Nakada et al, 2005;Sakuma et al, 2008). Therefore, if values of GSI are known, but porosity or density values…”