Takayuki Kaneko

The outline of the 2011 eruption at Shinmoe-dake (Kirishima), Japan

The climactic phase of the 2011 eruption at Shinmoe-dake was a mixture of subplinian and vulcanian eruptive events, successive lava accumulation (lava dome) within the crater, and repetition of vulcanian events after the dome growth. It was preceded by inflation and elevated seismicity for about one year and by phreatomagmatic explosions of one week before. Small pyroclastic flows and ash-cloud surges formed during the subplinian events, when the eruption column reached the highest level and the vent was widened. A lava dome, which was extruded close to the vent of subplinian events, grew by swelling upward and filling the crater. After the vent was covered by the lava, an intense vulcanian event occurred from the base of the dome and the swelled dome became deflated. After that, vulcanian events were repeated for three months. Simultaneous eruption styles in the crater (vulcanian events, continuous ash emission and dome growth) and some phreatomagmatic events in the vulcanian stage probably are due to a complex upper-conduit system developed in water-saturated country rock.

Exogenous and endogenous growth of the Unzen lava dome examined by satellite infrared image analysis

Abstract Phase 1 of the most recent eruption of Unzen volcano (Japan) occurred between 1991 and 1993 and involved the endogenous and exogenous addition of magma to a growing summit lava dome. The temporal variation between endogenous and exogenous styles of dome growth has an important bearing on the hazard posed by dome collapse and explosive events. However, such data are difficult to acquire and have previously only been obtained in a fully quantitative manner for the Mount St. Helens dome, in that case using detailed photogrammetry. Here we investigate these competing dome growth processes at Unzen via analysis of their thermal emission signals present in shortwave infrared (SWIR) satellite imagery. We find that both the endogenous and exogenous growth styles exhibit identifiable SWIR signatures whose magnitude can be related to their respective magma supply rates. We find that the SWIR radiant area corresponding to endogenous growth is around four times larger than that corresponding to exogenous growth at the same magma discharge rate. Analysis of time series imagery indicates that, like Mount St. Helens, Unzen experienced a temporally decreasing magma discharge rate during phase 1. As time progressed and the discharge rate fell, the growth style of the Unzen dome changed from ‘exogenous only’, through ‘exogenous decreasing and endogenous increasing’ to ‘endogenous only’. Thus relative growth rates at the endogenous and exogenous areas of the Unzen dome appear to be at least partly controlled by the overall magma discharge rate. As with Mount St. Helens, the cause of this relationship was most likely the formation and strengthening of a brittle crust at the surface vent, which impeded exogenous growth more effectively as time elapsed and the discharge rate fell. These results suggest the similarity of the processes operating at the Unzen and Mount St. Helens domes, though important differences remain in that dome growth was basically continuous at Unzen, whilst at Mount St. Helens growth episodes were interspersed by significant periods of repose.

Satellite thermal analyses of lava dome effusion rates at Unzen Volcano, Japan

This paper presents analyses of infrared remote sensing data of the 1992–1994 lava dome growth episode of Unzen Volcano, Japan. The primary aim is to determine whether such data can indicate trends in the rate of magma supply to the dome, since this is one of the principal factors controlling the frequency of pyroclastic flow. Spaceborne, low-spatial-resolution infrared radiance data correctly delimit the two discrete phases of magma supply to the dome, and during phase 1 the short-wavelength data are found to be strongly correlated with ground-based estimates of lava effusion rate. The correlation is, however, significantly weaker during phase 2. Analysis of high-spatial-resolution data from airborne and satellite based systems allows us to determine the principal source of shortwave infrared flux as fumarolically heated dome surfaces, these being significantly hotter than the actively growing lava lobes. Variations in the low-spatial-resolution shortwave infrared signal are therefore directly attributed to fluctuations in the size and temperature of the fumarolically heated area. These fumarolic variations are largely dependent upon the flux rate of magmatic gas, which in turn is partly dependent upon the rate of magma supply. Gas fluxes measured using correlation spectrometer (COSPEC) measurements of SO2 concentration indicate that the relationship between fumarolic discharge and the effusion rate of lava was relatively stable during phase 1 but highly variable during phase 2. We suggest these variations as an explanation for the differing correlations observed between the emitted shortwave infrared flux and the effusion rate of lava during phases 1 and 2. We conclude that appropriate analysis of high- and low-spatial-resolution shortwave infrared remote sensing data may assist in elucidating trends in lava supply at volcanoes exhibiting similar dome growth episodes to that experienced at Unzen.

Discrimination of lava dome activity styles using satellite-derived thermal structures

Abstract Thermal imaging from satellites is one methodology used for the monitoring and scientific investigation of volcanoes, including those characterised by active lava domes. To be most effective, the remote sensing techniques employed must allow the cause of any observed thermal anomaly to be identified, ideally using information contained within the remote sensing data itself, whilst using any ancillary field data to guide analysis and the necessary assumptions. This study investigates a method by which such discrimination maybe accomplished for activity at lava domes. For this purpose we use three Landsat Thematic Mapper (TM) scenes of Unzen Volcano (Japan) to determine the temperature structure of the active lava dome existing during 1991–1993. These data are of particularly high quality since the TM scenes were obtained during night-time overpasses and a large amount of supplementary data are available to parameterise the model used to retrieve the subpixel temperature structure of the dome surface. The results are matched to near-contemporaneous geological sketch maps of the then current situation in order to identify the TM pixel groupings corresponding to the different styles of dome activity, namely fumarolic degassing, exogenous and endogenous dome growth, and collapse deposits resulting from block and ash flows, small scale pyroclastic flows and rockfalls. The spatial and statistical characteristics of the resultant TM-derived temperature distributions are then investigated to determine rule-based criteria that may be used to differentiate the activity styles of lava domes based on their thermal structure within the remote sensing data. Results indicate that fumarolically active locations, regions of active dome growth, and areas of collapse deposition on the Unzen dome can be differentiated using only the statistical distribution of the hotspot temperatures and fractional areas identified via analysis of TM imagery. The statistics derived for hotspots at the fumarolically active areas show them to be, in general, significantly hotter (and smaller) than those found at the areas of active dome growth, whereas hotspots retrieved on the areas of collapse deposition are cooler (and larger). We find that the frequency distributions of hotspot temperatures extracted for the areas of endogenous and exogenous dome growth exhibit statistically inseparable means, but that discrimination between these two dome growth styles is possible using analysis of the spatial arrangement of the retrieved hotspots. Specifically the exogenous lobes are characterised by structures interpreted to be the effusing vent of new magma and the collapsing lobe front. These features are absent at locations showing only endogenous growth. It is hoped that the criteria developed here will prove useful during future quantitative analysis of the extended TM time-series available for Unzen, and for identification of thermal anomalies of uncertain origin at other active domes observed via satellite remote sensing.

Landsat infrared analysis of fumarole activity at Unzen Volcano : time-series comparison with gas and magma fluxes

Abstract We analyse shortwave infrared thermal data of the phase 1 eruption of Unzen Volcano (Japan) extracted from eight nighttime Thematic Mapper (TM) images taken from the Landsat 5 satellite between October 1991 and November 1992. We identify two discrete regions of the dome that were heated to high temperature by the ongoing eruptive activity; a fumarolically heated region and an area associated with the effusion of new lava. We concentrate analysis on the fumarolically heated region and investigate the relationships between parameters derived from the infrared radiance data and the nature of the fumarolic gas and magma fluxes. Temporal variations in the parameters derived from the radiance data closely follow those observed in the measured rate of magma effusion. The positive correlation observed between the fumarolically driven shortwave infrared flux and the magma discharge rate ( r 2 =0.64) indicates that degassing occurred efficiently and in proportion to the amount of magma supplied. Over our monitoring period, the data suggest that gas accumulation within the edifice did not occur, this conclusion agreeing with a previous finding obtained using correlation spectrometer (COSPEC) analysis of SO 2 flux rates. A positive correlation ( r 2 =0.56) was also found between the mean radiance of the pixels in the fumarolically heated region and the overall size of that region. This suggests a potential mechanism whereby, when gas pressure within the edifice increased, excess gas escaped through additional pathways to the surface as well through an increased flux at the main fumarolic vents.

Precursory activity and evolution of the 2011 eruption of Shinmoe-dake in Kirishima volcano—insights from ash samples

After a precursory phreatic stage (2008 to 2010), the 2011 Shinmoe-dake eruption entered a phreatomagmatic stage on January 19, a sub-Plinian and lava accumulation stage at the end of January, a vulcanian stage in February–April, and a second phreatomagmatic stage in June–August. Component ratio, bulk composition, and particle size of the samples helped us define the eruptive stages. The juvenile particles were first found in the January 19 sample as pumice (8 vol%) and were consistently present as scoria and pumice particles thereafter (generally ~50 vol%, decreasing in weaker events). The January 19 pumice has water-quench texture. After the lava accumulation, particles of that lava origin came to account for 30~70 vol% of the ash. The second phreatomagmatic stage is proposed because of fine ash and long eruption period. The SiO2 contents of bulk ash are lower in post-January 19, 2011 eruptions, reflecting lower average SiO2 contents in 2011 ejecta than in past ejecta. The free-crystal assemblages were two pyroxenes + plagioclase + Fe-Ti oxides until 2010; olivine joined the assemblage in 2011, when juvenile ash was erupted. This change is consistent with the absence or smaller sizes of olivine phenocrysts in past ejecta.

Testing the accuracy of solar‐reflected radiation corrections applied during satellite shortwave infrared thermal analysis of active volcanoes

Shortwave infrared (SWIR) satellite imagery is frequently used to study thermal emission from active volcanoes. When using daytime SWIR observations it is necessary to first isolate the thermally emitted signal component, principally by subtracting contributions due to solar-reflected radiation. Two differing approaches have been used: (1) A “mean” approach based on the average SWIR reflectance of the nonthermally anomalous volcanic background, and (2) a “per-pixel” approach which estimates the SWIR reflectance of each pixel using a measure of their near infrared reflectance. We assess the accuracy of these using near-coincident daytime and nighttime Landsat Thematic Mapper (TM) imagery of the active lava dome of Unzen Volcano. Using the nighttime data as a reference we find the most commonly applied mean daytime correction approach to be seriously inadequate, returning results for thermally radiant area 81–94% smaller than the nighttime values. The per-pixel correction method appears somewhat more effective but still provides values 47–61% smaller than the reference data set. Daytime underestimation of total SWIR thermal spectral radiance is less severe than that of total thermally radiant area because it is primarily pixels having low thermal signals that pose a problem for the daytime approaches. Total thermal spectral radiance is underestimated by 52–79% using the daytime data with the mean correction approach, and by 28–59% with the per-pixel approach. When applying the TM “dual-band” technique to determine subpixel hot spot temperatures from the SWIR thermal signals, we find that surfaces cooler than 400°C are poorly represented in the daytime retrievals. However, when using the per-pixel daytime data correction method the distribution of retrieved hot spots >400°C corresponds quite well to the pattern derived via nighttime imagery. Due to the significant parameter underestimation found to be inherent when using daytime data, this study further indicates the value of nighttime observations to volcanic SWIR thermal studies. If daytime data are used, then we recommend use of the per-pixel solar-reflected radiation correction method but would advise caution when quantitatively interpreting the derived thermal parameters, at least at andesitic and dacitic volcanoes. It is possible that daytime retrievals may be more robust at basaltic volcanoes due to their characteristic higher magmatic temperatures and lower surface reflectances.

Geometric considerations for the remote monitoring of volcanoes: Studies of lava domes using ATSR and the implications for MODIS

A lock-off switch comprises a lock plate rotatably disposed on a slider and having a projection at one side and a hole elongated in the lengthwise direction and a pin secured to the slider and loosely fitted in the elongated hole of the lock plate thereby to hold the lock plate rotatably. In the elongated hole of the lock plate a compression spring stretches between the pin and one end of the elongated hole, thus urging the lock plate toward a switch case. The slider has a stopper engageable with the projection of the lock plate thereby to stop the slider. An urging means, such as a compression spring, is provided at the other side of the slider and urging the projection of the lock plate into engagement with the stopper, thereby to prevent the slider from being pushed. To push the slider, the lock plate is rotated against the urging force of the urging means thereby to release the projection of the lock plate from the stopper.

Low-altitude remote sensing of volcanoes using an unmanned autonomous helicopter: an example of aeromagnetic observation at Izu-Oshima volcano, Japan

The results of aeromagnetic observations at Izu-Oshima, Japan using an unmanned autonomous helicopter are reported. A practical observation system was assembled, adopting a bird-type magnetometer installation, and dense observations of the northern half of the caldera area including the central cone were made from a very low altitude. In the detailed magnetization intensity mapping deduced from the collected data, low magnetization intensity at the vent and three rows of high magnetization intensity on the caldera floor were found. The former is interpreted as the presence of high-temperature materials, such as magma or hot rock, or vacant space in the conduit. The latter is considered to be due to solidified dykes. Low magnetization intensity suggesting a magma body (or subsidiary magma chamber) was not detected below the caldera floor. The observation results confirmed that aeromagnetic observation using an unmanned autonomous helicopter had sufficient performance for volcanic observations, and could also be utilized as a low-altitude platform for other sensors.

Geochemistry of Quaternary basaltic lavas in the Norikura area, central Japan: Influence of the subcontinental upper mantle on the trace elements and Sr isotope compositions

Abstract The trend of across-arc major- and trace-element compositions and 87Sr/ 86Sr ratios for Quaternary lavas from the western part of central Japan are different from those from NE Japan. Basalts from the Norikura area, located at the backarc-side of the western part of central Japan are characterized by lower K/Zr and higher 87Sr/ 86Sr (~0.705–0.706) than those of the backarc-side of NE Japan. Beneath the Norikura area, the Pacific plate is subducting at the depth where dehydration of phlogopite in the down-dragged peridotite occurs. Through dehydration, fluid is added to the mantle wedge which consists of a N-MORB-like source, triggering partial melting, and producing uprising mantle diapirs. These diapirs have high K/Zr and low 87Sr/ 86Sr as indicated by basalts in the backarc-side of NE Japan. Subcontinental mantle materials which have low LIL/HFS and high 87Sr/ 86Sr exist in the uppermost portions of the mantle wedge beneath the Norikura area, and interact geochemically with the uprising diapirs. The geochemical characteristics of the Norikura area may be attributed to the involvement of the subcontinental upper mantle.