Abstract
The activity at Aso Volcano was mainly defined as a sequence of ash emissions and occasional ejections of scoria fragments with ash. Ash emissions sometimes started without notable explosions. The measured porosity of scoriae was as high as 0.94. The scoriae had a flattened shape with a low-porosity outer rim. To elucidate the eruptive conditions causing such ash emission and generation of scoriae, we conducted three series of measurements. First, we heated the high-porosity scoriae from Aso Volcano at 900–1150 °C and found that the heated scoriae shrunk by losing the gas in the bubbles. At the highest temperature, 1150,^{circ } hbox {C}, bubbles segregated from the surrounding melt. Second, we conducted shear deformation experiments of scoriae and ash at 500–950 °C and found that the high-porosity scoriae easily fractured by low normal and shear stresses of sim 10^4 , hbox {Pa} at a low temperature of 500 °C. We also found that the fine ash at a high temperature of 950 °C was sintered. Third, we measured the permeability of the sintered ash plate and unheated powder-like ash layer. The permeability of the ash plate is less than 2.5 times 10^{-13} , hbox {m}^2, while that for the ash powder is greater than 10^{-11} , hbox {m}^2. The unheated ash particles could move in the container during the permeability measurements. This effect allowed the formation of pipe-like structures in the ash layer and increased its permeability. On the basis of these measurements, we infer the conditions inside the erupting conduit. There exists high-porosity magma foam in the conduit. The top of the magma foam is cold (<500 °C) and has a sufficiently high porosity (>0.7) to be fractured at a low stress level (sim 10^4 , hbox {Pa}). The fractured magma foam generates the ash layer above the magma foam. The gas flow from the underlying magma foam makes the high-permeability structure in the ash layer. Eventually, the bottom of the ash layer sinters to regulate the gas flow. The pressurized magma foam breaks the sintered ash layer. The breakage at the bottom of the ash layer may not cause a notable explosion but causes ash emission. The fragmented magma foam becomes high-porosity scoriae at a high temperature, which can generate the low-porosity outer rim by shrinkage and flatted shape.
Highlights
Strombolian eruptions are frequently associated with the release of low-viscosity magma with a composition of basalt to basaltic andesite (e.g., Houghton and Gonnermann 2008; Taddeucci et al 2015)
Oscillatory deformation of scoriae To confirm the importance of the temperature on fracturing, we show the results from oscillatory measurements, from which we can evaluate whether the sample behaves as solid-like elastic materials or dissipative viscous fluids
The throat radius for the powder-like ash layer with a permeability of 4 × 10−10 m2 and φb(Random) = 0.65 is rcr = 130 μm, one order of magnitude larger than that estimated for the sintered ash plate. This estimate is consistent with our interpretation; that is, in the powder-like ash layer, particles move to make a pipe-like structure and increase the permeability. These results show that heating reduces the permeability of the ash layer by reducing the mobility of ash particles
Summary
Strombolian eruptions are frequently associated with the release of low-viscosity magma with a composition of basalt to basaltic andesite (e.g., Houghton and Gonnermann 2008; Taddeucci et al 2015). In November 2014, Aso Volcano located in central Kyushu, Japan, resumed its volcanic activity from the Nakadake 1st crater (Fig. 1a–c), after an approximately 20-year quiescent period (e.g., Cigolini et al 2018; Marumoto et al 2017; Miyabuchi et al 2018; Shinohara et al 2018a; Yokoo and Miyabuchi 2015). The first half of this activity, from November 2014 to May 2015, was characterized by ash emissions and Strombolian eruption, which ejected scoriae with and without ash. The latter half, from September 2015 to February 2016, was characterized by phreatomagmatic and phreatic eruptions (Miyabuchi et al 2018). The Nakadake 1st crater usually has hot and acid water as a crater lake, but it dried during the first half of the eruption sequence
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