Physical characteristics of scoriae and ash from 2014–2015 eruption of Aso Volcano, Japan

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 • 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 ∼ 10 4 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 × 10 −13 m 2 , while that for the ash powder is greater than 10 −11 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 ( ∼ 10 4 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. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Combined approach to estimate the depth of the magma surface in a shallow conduit at Aso volcano, Japan

“…The temperatures of the gas emissions from the vent and the bombs that landed around the vent were ∼100 • C and 130-200 • C, respectively. Although molten magma is hot (1000-1100 • C), the temperature of the magma's top cools in the air and can drop to <500 • C, according to the deformation experiments of scoriae collected at Aso volcano (Namiki et al 2018). If the gas-magma mixture in the resonant space of the conduit is as high as 100-500 • C, the sound velocity is 350-600 m/s given that the gas mass fraction is 0.67-0.94 (Patrick 2007).…”

Gas flow dynamics in the conduit of Strombolian explosions inferred from seismo-acoustic observations at Aso volcano, Japan

“…Both situations are satisfied with the condition of Eo > 40. Considering that N f at Aso is 3.01 × 10 3 when ρ l = 2720 kg/m 3 (Namiki et al 2018), D = 50 m (the maximum size; Fig. 2a) and μ = 10 3 Pa s (Giordano et al 2008), the Froude number is 0.3397.…”

“…The definition of U g is the ratio of U sg to the gas volume fraction (void ratio ε): U g = U sg /ε. Pushkina and Sorokin (1969) introduced the U sg conditions corresponding to the transition from churn flow to annular flow as where σ, ρ l , and ρ g represent the surface tension of magma (~ 0.1 N/m; Walker and Mullins 1981), the magma density (2720 kg/m 3 ; Namiki et al 2018), and the gas density (0.17-14 kg/m 3 ; H 2 0 at 1300 K from 1 atm to 8 MPa), respectively. As shown in Eq.…”

Special issue “Advancement of our knowledge on Aso volcano: current activity and background”

“…The total erupted tephra mass was 2.1 × 10 6 tons (1.2 × 10 4 tons/day), which was less than the tephra deposits of previous activities that have occurred within the past few decades. Namiki et al (2018) conducted three series of measurements for high-porosity scoriae and ash ejected from the Nakadake crater during magmatic eruption in 2014-2015. From their results of measurements, Namiki et al (2018) elucidated the eruptive conditions causing such ash emission and generation of scoriae and inferred the sequence of an eruption as follows.…”

“…Namiki et al (2018) conducted three series of measurements for high-porosity scoriae and ash ejected from the Nakadake crater during magmatic eruption in 2014-2015. From their results of measurements, Namiki et al (2018) elucidated the eruptive conditions causing such ash emission and generation of scoriae and inferred the sequence of an eruption as follows. Cooling of the uppermost part of the high-porosity foam in the conduit causes fracturing of the magma foam, which creates the glassy brown ash to be ejected.…”