Recent developments in the understanding of magma chambers (reservoirs), pre-eruptive magma processes, and the conditions that lead to volcanic eruptions are reviewed mainly from a petrological point of view. A “magma reservoir” consists of inner “magma chamber (s)” filled with eruptible magma containing less than ca. 50% crystal, and outer “mush” region with more than ca. 50% crystal. Most of magma reservoirs are in a state of mush, so that “rejuvenation” or “remobilization” is necessary before eruption. Magma can erupt if its viscosity is less than ca. 10⁶ Pa s. More viscous magma can erupt only after a precursory eruption of less viscous magma, such as a hybrid magma between the viscous magma and a less viscous mafic magma. In this context, pre-eruptive magma viscosity, i.e. magma viscosity at the magma reservoir, is an important measure to evaluate magma eruptibility. Dating for whole mineral or even its local point (e.g., zircon age) and diffusion analysis for various types of minerals (e.g., magnetite, olivine, pyroxene, and plagioclase) have revealed timescales of pre-eruptive magma processes. Eruption triggers, such as injection of high-temperature magma, are inferred to occur days to months before the eruption in many cases. Magma residence times, during which the magmas are in eruptible conditions, are years to decades for typical magma systems, but may reach hundreds of thousand years for large caldera systems.

抄録全体を表示

Integrating multiple observations and mathematical modeling plays a key role in understanding dynamics of volcanic eruptions. In particular, this integration is important for magma ascent process in the conduit (i.e., conduit flow), because this process strongly controls diversity of eruption styles, whereas the direct observations of the process are lacking. In this paper, I investigated the relationship between the recent conduit flow model and multiple observations, especially focusing on how parameters and variables in the conduit flow model are constrained from the observations. Results show that most of the parameters and variables are well constrained by some observation methods, and that pressure changes in the magma chamber and the conduit obtained from the conduit flow model are important variables utilized for detecting eruption transitions from geodetic observations. In the 1995-2010 eruptions of Soufrière Hills volcano, conduit flow models were well combined with observation data, in which a key variable connecting models and observations was the period of cyclic behavior during lava dome eruptions. Recent observations for the 2011 eruptions of Kirishima-Shinmoe-dake volcano indicate that high-resolution measurements of subsurface and surface phenomena enable us to obtain more detailed information on eruption dynamics, which may promote development of a more advanced mathematical model.

抄録全体を表示

Pressurization, depressurization, or migration of magma or hydrothermal fluids can be measured by ground deformation. Recent progress of space geodetic techniques drastically increased the quantity of deformation data in active volcanoes. Recent emergence of Global Navigation Satellite System (GNSS) allows us to measure three dimensional displacements in higher spatial density than conventional measurements with tiltmeters, strainmeters, or leveling surveys because GNSS measurements are technically more tractable and much less costly. Recent emergence of Synthetic Aperture Radar (SAR) enables us to measure displacement field, without any ground-based instruments, in high spatial resolution on the order of meters if the condition allows. Tiltmeters and strainmeters, however, are still important tools to measure ground deformation of volcanoes because of their higher sensitivity than GNSS and SAR measurements. This article reviews recent progress and perspectives of ground deformation studies.

It is important to understand the error budget of measurements to assess the observed geodetic signals, although it does not seem to well taken care in many cases. This article thus reviews the error budget of geodetic measurements to demonstrate that the data from leveling surveys and SAR are spatially correlated and those from tiltmeters, strainmeters, and GNSS are temporally correlated.

The observed ground deformation field is capable of inferring location, shape and strength of sources causing the ground deformation. In volcanic regions, the deformation source is often pressurization or depressurization of various shapes of sources such as spheres, ellipsoids, conduits, dikes or sills. These parameters are often inferred with an assumption that they are embedded in an elastic, homogeneous, and isotropic halfspace because analytical solutions exist in many cases and this simple assumption often works reasonably well. However, this simple assumption is not always valid. To reflect more realistic features of a volcano, approximate and semi-analytical solutions have been provided to take irregular topography or heterogeneous structure of the crust into account. Also, recent progress of computational capability enables us to take complex material properties into account to model the deformation field through numerical simulations. However, excessive complication of the model is often unnecessary because of mathematical reasons and because material properties beneath volcanoes are not known in details in many volcanoes. This article reviews the effect of topography and material heterogeneity on the surface deformation field and discusses an appropriate complexity of the model in discussing the observed deformation field. Also this article deals with modeling deformation field through analog experiments. This method is especially useful in dealing with spontaneous faulting induced by magmatism. (View PDF for the rest of the abstract.)

抄録全体を表示

This paper reviews electromagnetic studies on volcanoes that were published between 2006 and 2015. Recent progress of the studies of resistivity structure, geomagnetic fields, electric self-potential, and volcanic lightning were summarized, respectively. Among these, a precise imaging of resistivity structure has substantially contributed to interpret volcanic activity in terms of the role of fluids. Moreover, it has been suggested that reliable resistivity structure is a key to understand the geomagnetic and self-potential variations on active volcanoes.

抄録全体を表示

This paper reviews recent (last 10 years) progress in petrological and textural studies on eruptive products relevant to syneruptive magma ascent in the conduit. Compared with the period when petrological and textural studies focused only on magma behavior in the reservoir, they came to have more potential to link various volcanological research fields including geophysics and geology.

The target of the present review is narrowed down to 1) analysis of groundmass crystals (microlites) for revealing eruption mechanism of specific eruption, and 2) decompression experiment for general understanding of crystallization kinetics and 3) accompanying techniques in collecting eruptive products in the field, in selecting representative samples for detailed textural analyses, in conducting textural analyses of ejecta and experimental products, and in running decompression experiments.

The basis in revealing magma ascent rates using microlites is that higher magma ascent rate results in larger magnitude of undercooling, resulting in nucleation-dominated crystallization. Its validity has been repeatedly confirmed in decompression experiments, as in the period before 2006. New trend in the last decade is that increasing number of experiments came to deal with basaltic and rhyolitic magmas, which is in contrast with the predominance of experiments for andesitic and dacitic magmas over the period before 2006. Similar trend is found for studies for specific eruption with or without experimental replication of microlite textures. The appearance of microlite number density water exsolution rate meter (Toramaru et al., 2008) made it easier to estimate absolute magma ascent rate, even when experimental facilities are not available.

The author proposes the method by which further development of this research field is possible. The method includes a) utilization of different types of eruption products (e.g. ash) and b) utilization of different sizes of groundmass crystal (e.g. nanolites) and c) acquisition of knowledge on crystallization kinetics at low pressure. The last two points are necessary when we try to infer final emplacement process of the ejecta at the surface (e.g. to solve the genetic problems of clastogenic lava flow and rootless cone), in conjunction with geological studies.

抄録全体を表示

The dimensions of volcanic plumes (i.e., heights and spreading area) are the key observable data to estimate the intensity of eruptions, and they are commonly used as an initial input for the calculation of tephra dispersal models. The relationship between these observed data and the eruption conditions such as mass eruption rate in the various atmospheric conditions has been investigated by analytical methods, simple numerical models, and large numerical simulations. In this paper, we have overviewed the recent progress of these numerical models. The latest three-dimensional fluid-dynamics model of eruption clouds has been tested from a viewpoint of comparison with the observed data of the Pinatubo 1991 eruption, the Shinmoe-dake 2011 eruption, and the Kelud 2014 eruption. To develop an operational model of eruption clouds, the effective values of the empirical constants used in the simple numerical models are estimated on the basis of the simulation results of the three-dimensional model. We propose the diagrams to quickly estimate the plume height when the eruption and atmospheric conditions (e.g., mass eruption rate and wind velocity) are given.

抄録全体を表示

In this article, recent progresses in modeling and forecasting of tephra transport phenomena are reviewed. First, we describe elementary processes of the Tephra Transport and Dispersion Models (TTDMs) especially from the Lagrangian point of view. The processes consist of advection, diffusion, gravitational fallout, dry deposition, wet scavenging, aggregation, resuspension and so on, which are taken into the operational models of Volcanic Ash Advisories and Volcanic Ash Fall Forecasts in the world. Second, we show examples of volcanic ash cloud and tephra fall forecasts on the basis of the case studies of the eruptions at Eyjafjallajökull in 2010 and Shinmoe-dake volcano in 2011, respectively. Finally, some representative numerical weather prediction models which drive the TTDMs and verifications of the TTDMs are also described. Other important parameters of eruption source which give initial condition of the TTDMs are briefly remarked as future works.

抄録全体を表示

We conducted detailed stratigraphic, petrographic and petrochemical investigations on the Benri subunit which is the products by one of the caldera-forming eruption cycles of Aso-4 pyroclastic flow, Aso volcano, central Kyushu, Japan. The Benri subunit is limitedly distributed in the outer side of the northwestern part of Aso volcano. It is mainly composed of scoria, pumice and banded pumice for juvenile fragments, and andesitic lithic fragments (accessory) set in a matrix. The Benri subunit is divided into seven units (Unit1∼Unit 7), based on the amount and shape of essential fragments, and on the characteristics of lithic fragments and matrix. As a whole, the upward transition of deposits from pumice-rich (non-welded) to scoria-rich (weakly-welded), and to pumice-rich again was recognized in the units. Petrographic observations and mineral chemistries, particularly for wide range of the anorthite content of plagioclase (An_40-90) in most scoriae, pumices and banded pumices, provide evidences for magma mixing with different amounts of mafic and felsic components. Pumices in some units are characterized by low anorthite content of plagioclase (An_30-50) which are similar to those in the silicic eruption products at the initial stage of the Aso-4 cycle. Whole-rock major and some trace element contents of scoriae, pumices and banded pumices show linear variation trends between two assumed (mafic and felsic) end members. These results suggest the magma mixing as an important process for producing the magmas of Benri subunit.

These observations and results suggest that, before the eruption producing the Benri unit, compositionally zoned (stratified) magma layers had been formed between mafic and felsic end member magma layers in the magma chamber. The eruption sequence of the composite magmas is assumed to have changed as from felsic magma dominant to mafic magma dominant, and then felsic magma increasing again.

抄録全体を表示