No abstract

[1] During explosive eruptions, a mixture of pyroclasts and volcanic gas forms a buoyant eruption column or a pyroclastic flow. We systematically investigate how the condition that separates these two eruption styles (column collapse condition) depends on crater shape and magma chamber conditions by integrating the theoretical models for conduit flow, flow inside a crater, and eruption column dynamics. The results show that previous model predictions of column collapse condition based on the relationship between magma discharge rate ( _ m) and water content (n f ) strongly depend on crater shape (depth D and opening angle ). When a crater is present, the decompression and/or compression of gas-pyroclast mixture inside and just above the crater result in two distinct types of column collapse: collapse with increasing _ m (HM-side collapse) and that with decreasing _ m (LM-side collapse). HM-side collapse is caused by an increase in conduit radius during the waxing stage of an eruption. LM-side collapse is associated with a decrease in magma chamber pressure during the waning stage. The value of _ m for HM-side collapse varies by two orders of magnitude depending on crater shape for fixed n f . This estimate also depends on assumed models for decompression into the atmosphere. The value of _ m for LM-side collapse is <10 6 kg s −1 for a shallow crater with a small opening angle, whereas it can be >10 8 kg s −1 when D tan > 10 2 m. These results are consistent with the field observations from the St. Helens 1980 and Pinatubo 1991 eruptions.

[1] We investigated the coupled effects of gas escape and crystallization on the dynamics of lava dome eruptions using a one-dimensional conduit flow model. The relationship between chamber pressure p ch and mass flow rate q for steady conduit flow commonly has a regime of negative differential resistance (i.e., dp ch /dq < 0), which causes a transition from lava dome to explosive eruption. Two positive-feedback mechanisms that result in negative differential resistance have been identified. First, effective magma viscosity decreases with increasing q because of a delay of crystallization, leading to reduced viscous wall friction (feedback 1). Second, magma porosity increases with increasing q because of less efficient gas escape, leading to reduced gravitational load (feedback 2). For high-phenocryst-content magma (volume fraction >0.5), feedback 1 is the main mechanism that forms negative differential resistance. In this case, the transition from lava dome to explosive eruption occurs when the magma supply rate exceeds a fixed critical value. For low-phenocryst-content magma (volume fraction <0.5), feedback 2 plays a key role so that the transition is controlled by the permeability of the surrounding rocks; the critical magma supply rate remarkably decreases with decreasing permeability. Transition due to feedback 2 is associated with a change in the chemical composition of volcanic gas, a drastic increase in magma porosity from nearly 0 to greater than 0.8, and overpressure at a shallower level, which can be detected from geochemical and geophysical field observations. Citation: Kozono, T., and T. Koyaguchi (2012), Effects of gas escape and crystallization on the complexity of conduit flow dynamics during lava dome eruptions,

The National Research Institute for Earth Science and Disaster Prevention (NIED) developed volcano observation stations at the Kirishima volcanic group in 2010. The stations observed remarkable crustal deformation and seismic tremors associated with the Shinmoe-dake eruption in 2011. The major eruptive activity began with sub-Plinian eruptions (January 26) before changing to explosive eruptions and continuous lava effusion into the summit crater (from January 28). The observation data combined with GEONET data of GSI indicated a magma chamber located about 7 km to the northwest of Shinmoe-dake at about 10 km depth. The tiltmeter data also quantified detailed temporal volumetric changes of the magma chamber due to the continuous eruptions. The synchronized tilt changes with the eruptions clearly show that the erupted magma was supplied from the magma chamber; nevertheless, the stations did not detect clear precursory tilt changes and earthquakes showing ascent of magma from the magma chamber just before the major eruptions. The lack of clear precursors suggests that magma had been stored in a conduit connecting the crater and the magma chamber prior to the beginning of the sub-Plinian eruptions.

The physical condition of the 1 February, 2011, vulcanian explosion at Shinmoedake volcano, Japan, is estimated based on the size of impact craters created by ballistic ejecta, using a ballistic trajectory model and a scaling law for impact crater formation. The initial velocity, impact velocity and mass of ejecta were estimated at 240-290 m/s, 140 ± 20 m/s and 1-3 ton, respectively. The gas mass fraction at the source was calculated to be 0.04-0.1, using the initial velocity and a theoretical model of vulcanian explosion. This gas mass fraction is higher than the petrologically estimated value for pre-eruptive magma. Low-angle jets from the explosion and the estimated depth and size of a pressurized gas region suggest a shallow source inside the lava dome. The observation and results imply that segregation and accumulation of gas in a shallow conduit played a role in an increase of excess pressure immediately below the dome surface, prior to the vulcanian explosion.