Takehiro Koyaguchi

On the formation of eruption columns following explosive mixing of magma and surface‐water

When magma vents into the sea or a crater lake, the ensuing magma-water interaction can affect the style of eruption dramatically. If the mass of surface water incorporated into the erupting material is small, ( 108 kg/s, the height progressively decreases with surface water content. This occurs when the magma and surface water begin to constitute a significant fraction of the mass at the top of the column, so that an increasing fraction of the initial magmatic thermal energy is converted to the surface water rather than the entrained air. The transitions in eruption style which result from changes in the mass of surface water mixing with the magma may account for observations of both buoyant plumes and wet surge during the eruptions of Taal in 1965 and Miyake-jima in 1983 and for the changes in the eruptive activity at Surtsey in 1963–1964 as the access of seawater to the vent became more restricted. We also present calculations which suggest that the accretionary lapilli, which are often found in wet flow deposits, may result from condensation of vapor in both the cold, wet collapsing fountains and in the flows themselves.

Tectonic stress controls on ascent and emplacement of magmas

Abstract The tectonic stresses can significantly affect the propagation of a magma-filled crack. It has been pointed out that the rheological boundaries control the emplacement of magmas through the effect of stress. However, it has not been clarified how the role of rheological boundaries depends on the regional tectonic and thermal states. We have evaluated the role of rheological boundaries under various tectonic and thermal conditions and found that the level of magma emplacement may jump according to the changes in the tectonic force or the surface heat flow. The stress profiles were estimated by a simple model of lithospheric deformation. We employed a three-layer model of the lithosphere; the upper crust, the lower crust and the upper mantle have different rheological properties. A constant horizontal force is applied to the lithosphere, and the horizontal strain is assumed to be independent of depth. When realistic tectonic forces (>10 11 N/m) are applied, the rheological boundaries mainly control the emplacement of magma. The emplacement is expected at the MOHO, the upper–lower crust boundary, and the brittle–ductile boundary. For lower tectonic forces ( 11 N/m), the tectonic stress no longer plays an important role in the emplacement of magmas. When the tectonic stress controls the emplacement, the roles of rheological boundaries strongly depend on the surface heat flow. When the surface heat flow is relatively high (>80 mW/m 2 ), the stress in the mantle is quite low and the MOHO cannot trap ascending magmas. For relatively low heat flow ( 2 ), on the other hand, the MOHO acts as a magma trap, and the upper–lower crust boundary acts as a magma trap only when the magma supply rate is sufficiently high. Our results suggest that the emplacement depth can change responding to the change in the tectonic force and/or that in the surface heat flow. This may provide us a key to understand the relation between the evolution of a volcanic region and its tectonic and/or thermal history.

Textural and compositional evidence for magma mixing and its mechanism, Abu volcano group, southwestern Japan

Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO*/MgO vs SiO2 diagram. One trend is characterized by FeO*/MgO-enrichment with a slight increase in SiO2 content (Fe-type trend), whereas the other shows a marked SiO2-enrichment with relatively constant FeO*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.The mode of mixing changes depending on the mixing ratio. In the mafic mixture, basalt and dacite magmas can mix in the liquid state (liquid-liquid mixing). In the silicic mixture, on the other hand, the basalt magma was quenched and formed inclusions (liquid-solid mixing). During mixing, the disaggregated basalt magma and the host dacite magma soon reached thermal equilibrium. Compositional homogenization of the mixed magma can occur only when the equilibrium temperature is sufficiently above the solidus of the basalt magma. The Si-type trend is chemically and petrographically similar to the calc-alkalic trend. Therefore, a calc-alkalic trend which is distinguished from a fractional crystallization trend (e.g. Fe-type trend) may be a product of magma mixing.

Evidence for two-stage mixing in magmatic inclusions and rhyolitic lava domes on Niijima Island, Japan

Abstract Fine-grained inclusions are found in the Achiyama and the Mukaiyama rhyolitic lava domes on Niijima Island, Japan. The existence of fine-grained rims in the inclusions indicates that these inclusions are a quenched mafic magma incorporated into their rhyolite host. The chemical composition and the phenocryst assemblage of the inclusions in the Achiyama dome are identical with those of the effusive basaltic lava on Niijima Island (Wakago basalt). The inclusions of the Mukaiyama dome are dacitic and have a disequilibrium phenocryst assemblage (olivine, clinopyroxene, orthopyroxene, hornblende, biotite, plagioclase, quartz and opaque minerals); whole-rock compositions lie on a mixing line between the host rhyolite and Wakago basalt. The dacitic magma is, therefore, a product of mixing of the basaltic and rhyolitic magmas. The occurrence of two different inclusions (basaltic and dacitic) in two different domes can be explained by a two-stage magma mixing model. The dacite is formed in the first stage by mixing in a vertically stratified magma chamber caused by an interfacial instability between rhyolitic and basaltic layers. A small amount of rhyolitic magma is entrained into the lower basaltic magma layer, and two magmas can mix essentially in a liquid state. The mixed dacitic magma is formed in the lower layer. The second stage is mixing caused by forced convection during the ascent through a conduit just before eruption. The stably stratified silicic and mafic magmas in a magma chamber can turn over and efficiently mix because of the formation of an unstable flow in a conduit. Under this condition, the system reaches thermal equilibrium immediately after the mafic magma is disaggregated and dispersed in the silicic host. The mafic magma solidifies when the silicic endmember is predominant in the mixture; hence the equilibrium temperature is below the solidus of the mafic magma. Mafic inclusions are formed at this stage. Whether the inclusions are basaltic or dacitic depends on whether the final ascent through a conduit (eruption) occurred before or after the formation of the layer of a mixed dacitic magma in a magma chamber.

Origin of modal and rhythmic igneous layering by sedimentation in a convecting magma chamber

EXPERIMENTAL investigations of convecting, particle-laden fluids show two regimes for convection driven by cooling from above1. In very dilute suspensions, convection will maintain a homogeneous distribution of particles throughout the convecting layer provided that particle fall velocities are small compared with turbulent fluid velocities. Above a critical concentration, convection is unable to keep the particles suspended, so the particles settle, leaving behind a layer of convecting fluid virtually free of particles. Here we apply these results to cooling magma chambers, in which crystallization leads to an increase in suspended crystal content with time. Discrete sedimentation events are predicted each time the concentration exceeds the critical value. For common igneous minerals, critical concentrations are very small (typically 0.002–0.03 wt%) and layers of the order of centimetres to a few metres thick will result. Because minerals of different density and size have different critical concentrations and settling velocities, complex fluctuations in sedimentation rate and mineral proportions can occur in a multi-component melt. This may lead to either regular repetitive cycles or more complex fluctuations. The process is confined to low-viscosity magmas, such as basalts, in which the crystals are able to separate from the active thermal boundary layer during convection.

The dynamics of magma mixing in a rising magma batch

AbstractThe conditions under which two magmas can become mixed within a rising magma batch are investigated by scaling analyses and fluid-dynamical experiments. The results of scaling analyses show that the fluid behaviours in a squeezed conduit are determined mainly by the dimensionless number

Magma discharge variations during the 2011 eruptions of Shinmoe-dake volcano, Japan, revealed by geodetic and satellite observations

I = \mu _1 U/g\Delta \rho R_{}^2

Reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits: 1. Methods

where μ1 is the viscosity of the fluid, U is the velocity, g is the acceleration due to gravity, Δρ is the density difference between the two fluids, and R is the radius of the tube. The parameter I represents a balance between the viscous effects in the uppermost magma which prevent it from being moved off the conduit walls, and the buoyancy forces which tend to keep the interface horizontal. The experiments are carried out using fluid pairs of various density and viscosity contrasts in a squeezed vinyl tube. They show that overturning of the initial density stratification and mixing occur when I>order 10-1; the two fluids remain stratified when I<order 10-3. Transitional states are observed when 10-3<I<10-1. These results are nearly independent of Reynolds number