Muography. Группа авторов
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Название: Muography
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Физика
isbn: 9781119723066
isbn:
Figure 1.4 Muographic image of Satsuma‐Iwojima volcano, Japan. The arrow indicates the location of the bubbly low‐density magma.
An example of how a muographic image can be used to understand the eruption dynamics is shown in Fig. 1.4. Statistical errors ranged from 0.02 to 0.2 g/cm3 above 250 m a.s.l. with the variation depending on the location in the volcano. Satsuma‐Iwojima, Japan, continuously emits large amounts of magmatic gases without a significant output of magma. The density gradually decreases up the conduit, and the top of the magma column at 400 m a.s.l. has the lowest density, indicating the presence of magma degassing, in agreement with the magmatic convection model (Shinohara & Tanaka 2011; Tanaka et al., 2009a). In this convection model, a magma conduit is connected to a deep magma chamber (Fig. 1.5), and in the upper part of the conduit, the gas escapes from the magma and exits the volcano. The degassed magma sinks, and, at the same time, new low‐density non‐degassed magma ascends from the bottom of the conduit and the cycle continues.
Degassed magma, which has a high proportion of bubbles, has been interpreted as being the low‐density region, and its dimensions (location and diameter) were compared with the following results of field measurements and laboratory and numerical modeling studies (Shinohara & Tanaka 2011). (i) The depth of the magma head observed 200 m below the crater floor was consistent with the degassing pressure of the magma and had a value of 0.5–3.0 MPa in Satsuma‐Iwojima (Kazahaya et al., 2002). (ii) High‐temperature (> 900 oC) volcanic gasses were continuously emitted from the vent (Shinohara & Tanaka, 2011). (iii) The oversaturation of volatiles in the melt was found, i.e., the degassing had been occurring under relatively low‐pressure conditions (Hedenquist et al., 1994). (iv) Gravity mapping of Satsuma‐Iwojima (with a residual profile derived from Bouguer anomalies assuming a density of 2.0 g/cm3) revealed that a low‐gravity region was located within the volcanic cone, which reached its lowest value in the crater region (Komazawa et al., 2005). The overall features of these results are consistent with the interpretation of this muographic result. Shinohara and Tanaka (2011) compared their muographic results with the results of gravity surveys of Masaya volcano, Nicaragua, and concluded that the low‐density region at the top of the magma column is a common feature of conduit magmatic convection. Conduit magmatic convection is often considered to be the degassing mechanism of basaltic volcanoes (Aiuppa et al., 2009; Burton et al., 2007; Oppenheimer et al., 2009; Shinohara & Witter, 2005; Shinohara et al., 2008). Significant and persistent degassing also occurs in silicic volcanoes during quiescent periods between eruptions (Delgado‐Granados et al., 2001; Edmonds et al., 2003; Kazahaya et al., 1994).
Figure 1.5 Conduit magmatic convection model of Satsuma‐Iwojima volcano.
The magma supply rate will be underestimated if only the geodetic modeling of the expansion rate of the volcanic body is applied. If the magmatic convection process is assumed as a model of magma degassing, the rate of the magma supply to the shallow part of the volcano will be estimated as more than one order of magnitude larger than the surficial expansion rate of the volcano (Kazahaya & Mori, 2016).
1.3.3 Phreatic Explosions and Magmatic Eruptions
The first successful nuclear emulsion‐based muographic observation system was set up in Asama volcano, Japan, in 2006 (Tanaka et al., 2007a). An image was reconstructed from the muon trajectories that were recorded in the nuclear emulsion, which was placed in a vault created 1 km east of the center of the crater. Topography of the volcanic plug located at the crater floor was estimated from airborne synthetic aperture radar (SAR) measurements both before and after the 2004 eruption (Urabe et al., 2006), which were used for comparison with the high‐density region that indicated a volcanic plug in the muographic image (Fig. 1.6a). The statistical error for the bulk density of this region was 0.04 g/cm3.
Additionally, the 2006 measurement imaged a low‐density region that indicated a vacant magma pathway underneath the volcanic plug, as shown in Fig. 1.6a. The statistical error for the bulk density of this region was 0.2 g/cm3. This low‐density region was interpreted as a porous magma pathway that was plugged by magma deposited on the crater floor, which was created by the following process. After the eruption process was completed in 2004, the magma deposit on the crater floor cooled and solidified, and the magma in the pathway drained away, leading us to speculate that it left a vacant or highly porous pathway. The muographic image captured a structure consistent with this interpretation in the anticipated region below the crater floor. If this space is over‐pressured by future volcanic activities, the plug may explode, rapidly releasing fragments of this magma deposit.
Additional observational data for the Asama volcano has become available since 2008 after the installation of a scintillation detector in a vault created 1.2 km east of the center of the crater with a real‐time reading device (Tanaka et al., 2009b). This muographic observation system was accessible from a remote personal computer (PC) via a wireless local area network (LAN) and the internet. During the observation, Asama erupted on Feb 2, 2009. The system was stable and continued to operate before and after the eruption. Fig. 1.6b shows data averaged over a month before the eruption compared with data averaged over the same time period after the eruption. To clarify the difference between its condition before and after the eruption, a dashed line has been added to indicate the shape of the crater before the eruption. On the other hand, the density of the pathway underneath the crater floor did not show a statistically significant change. Since a petrological study of the 2009 eruption ejecta indicated that the chemical composition of the magma matched that of the volcanic ejecta observed in the 2004 eruption, a plausible scenario for the 2009 eruption has been proposed in the following way (Tanaka et al., 2009b). Magma did not flow up the pathway in the 2009 eruption, and high‐pressure vapor simply blasted through the volcanic plug generated in the 2004 eruption.
Figure 1.6 Muographic image of Asama volcano, Japan (a) and images before and after 2009 eruption (b). The dashed line in (b) indicates the shape of the crater before the eruption. The green‐yellow‐red region in (a) indicates the magma deposit in the crater СКАЧАТЬ