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Fig. 7 | Earth, Planets and Space

Fig. 7

From: Petrological characteristics and volatile content of magma of the 1979, 1989, and 2014 eruptions of Nakadake, Aso volcano, Japan

Fig. 7

A cross section of Aso caldera and results of a density calculation of magma. a Comparison of the magma depth estimated in this study with geophysical observations. This cross section is along the profile in Fig. 1 (profile A2 in Hata et al. 2016). The magma chambers and conduits proposed by geophysical observations are also shown. LD is a low-density cylindrical block with a density contrast of 150 kg m−3, which is lower than that of the surrounding rocks, and is located at 6–17 km depth (bsl) with a radius of 5 km (Komazawa 1995). LV is a low-velocity region that is roughly spherical in shape, has a radius of about 3 km, is centered at 6 km depth, and extends to 10 km depth (Sudo and Kong 2001). P and Q are hot regions, that is, reflector voids revealed by a 3-D seismic reflection analysis (Tsutsui and Sudo 2004). CC is a crack-like conduit with a dimension of 1 km at a depth of about 1.8 km (bsl) beneath the Nakadake first crater based on the spatial pattern of the observed long-period tremors amplitudes (Yamamoto et al. 1999). C1 and C2 are a magma chamber and a conduit, respectively, inferred from significant conductive block anomalies indicated by the three-dimensional modeling of the electrical resistivity structure (Hata et al. 2016). A portion with less than 10 O-m of the anomaly C1, whose most conductive cell at 4 km depth (bsl) indicated a melt fraction of 76–87%, was distributed from 8 to 2 km depth (bsl). The horizontal of the anomaly C1 is beneath Kishimadake, which is consistent with the estimate of Sudo and Kong (2001). The anomaly C2 is centered at 2 km depth (bsl) and ranges from 4 to 0 km. Gray areas indicate JMA earthquake hypocenters within 2 km of the profile line between June 2002 and April 2016 (Hata et al. 2016). These geophysical observations indicate the presence of a magma chamber at 2–10 km depth (bsl). b Density (solid curves) and volume of bubbles (broken curves) of the A1, A2, A3, B1, B2, and B3 magmas calculated in this study (Table 5; Additional file 3: Table S2). The symbols are the same as in Fig. 5. Black broken lines indicate the density structure beneath the Aso caldera investigated by Komazawa (1995). He estimated the density structure to be 2200–2400 kg m−3 at a depth less than 1 km (bsl) and 2700 kg m−3 at a depth greater than 1 km. LD is a low-density block with a density contrast of 150 kg m−3 extended from 6 to 17 km depth (bsl) proposed by Komazawa (1995). Comparison between the bulk densities of the A1, B1, A3, and B3 magmas and the density of the crust at different depths indicated. (1) At 200–600 MPa (6–21 km depth (bsl)), the A1, B1, A3, and B3 magmas had lower or similar bulk density (2586–2787 kg m−3) than the crust (Additional file 3: Table S2). The density contrast between the A1 and B1 magmas and crust at 200 MPa was 68–114 kg m−3, which was similar to the indication from the gravimetric analysis. (2) At pressures of 50–200 MPa (1–6 km depth (bsl)), the magmas had lower bulk densities than the crust (2345–2632 kg m−3 for A1 and B1 magmas and 2562–2692 kg m−3 for A3 and B3 magmas; Table 5; Additional file 3: Table S2), which would allow the magmas to ascend by buoyancy. (3) At a pressure of 25 MPa (0 km depth (bsl), the B1 magma had lower bulk density than the crust (2007 kg m−3 for B1 magma versus 2200–2400 kg m−3 for the crust), while the A1, A3, and B3 magmas had higher or similar bulk density (2400–2594 kg m−3) to the crust

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