Browsing by Author "Geshi, Nobuo"
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- ItemMagmatic overpressures, volatile exsolution and potential explosivity of fissure eruptions inferred via dike aspect ratios(2020) Geshi, Nobuo; Browning, John; Kusumoto, ShigekazuBuoyant magmas abundant in exsolved volatiles (bubbles) drive the rapid upward-propagation of feeder dikes from magma chambers. The consequence of a feeder dike reaching the surface can result in an explosive volcanic eruption depending, partly, on the retention of volatiles. Therefore, timely detection of the vesicularity and overpressure of the magma during feeder dike ascent is critical for the prediction of the explosivity of any future eruption. In this study, we evaluated the explosivity of eruptions based on field investigations of the erupted products and the overpressure of magma in the conduit based on the dimensions of exposed feeder dikes. We found a positive correlation between the explosivity of eruptions and the magma overpressure generated in the conduit during recent fissure eruptions of Miyakejima volcano. Because the buoyancy of low-density magma produces positive overpressure at the dike’s top, feeder dikes with highly-vesiculated magmas possess high amounts of overpressure. An enlargement of the opening width of a feeder dike by magmatic overpressure results in a higher flux of vesiculated magma, which causes vigorous explosive activity. Our results suggest the possibility of forecasting the explosivity of an impending eruption if the width (or opening) of an ascending feeder dike is monitored in real-time through measurements of ground deformation and seismicity induced by the dike.
- ItemVolcanotectonics: the tectonics and physics of volcanoes and their eruption mechanics(2022) Gudmundsson, Agust; Drymoni, Kyriaki; Browning, John; Acocella, Valerio; Amelung, Falk; Bonali, Fabio L.; Elshaafi, Abdelsalam; Galindo, Ines; Geshi, Nobuo; Geyer, Adelina; Heap, Michael J.; Karaoğlu, Özgür; Kusumoto, Shigekazu; Marti, Joa; Pinel, Virginie; Tibaldi, Alessandro; Thordarson, Thorvaldur; Walter, Thomas R.The physical processes that operate within, and beneath, a volcano control the frequency, duration, location and size of volcanic eruptions. Volcanotectonics focuses on such processes, combining techniques, data, and ideas from structural geology, tectonics, volcano deformation, physical volcanology, seismology, petrology, rock and fracture mechanics and classical physics. A central aim of volcanotectonics is to provide sufficient understanding of the internal processes in volcanoes so that, when combined with monitoring data, reliable forecasting of eruptions, vertical (caldera) and lateral (landslide) collapses and related events becomes possible. To gain such an understanding requires knowledge of the material properties of the magma and the crustal rocks, as well as the associated stress fields, and their evolution. The local stress field depends on the properties of the layers that constitute the volcano and, in particular, the geometric development of its shallow magma chamber. During this decade an increasing use of data from InSAR, pixel offset and structure-from-motion, as well as dense, portable seismic networks will provide further details on the mechanisms of volcanic unrest, magma-chamber rupture, the propagation of magma-filled fractures (dikes, inclined sheets and sills) and lateral and vertical collapse. Additionally, more use will be made of accurate quantitative data from fossil and active volcanoes, combined with realistic numerical, analytical and machine-learning studies, so as to provide reliable models on volcano behaviour and eruption forecasting.