大規模火砕流と給源の陥没カルデラ

抄録

Recent progresses in researches on large-scale pyroclastic flows and their source calderas are reviewed. Subsurface structures for the Valles type caldera at Long Valley and Yellowstone are confirmed by means of seismic profiling, gravity, deformation measurements and drilling. Pyroclastic flows, lake deposits, moat rhyolites and slumped megabreccias fill the caldera floor. Basement formations are subsided up to 1-2 km tilting block-wise up to 10 degrees. No major rupture is confirmed on the basement formations. Existence of ring fracture is clearly proven based on geochronological and petrological data of moat rhyolite and component analysis of lithic clasts within pyroclastic flow deposit. Head of magma reservoir beneath Long Valley Caldera is located 4.5-7 km below surface and the reservoir size is comparable to the surface caldera depression. On the contrary, subsurface structure for Japanese funnel-shaped caldera is not yet confirmed clearly. Major interests on recent researches for large-scale pyroclastic flow are concentrated on grain size analysis and numerical or model experiments of fluidization to reveal flow and emplacement mechanisms. Ground surge, ground layer, fines-depleted ignimbrite and co-ignimbrite ash fall are defined based on grain size analysis. Co-ignimbrite lag breccia deposit is recognized as proximal facies. Low-aspect ratio ignimbrite is considered to be more turbulent than regular pyroclastic flows. Gravitational column collapse model is proposed. Fluidization experiments show that the pyroclastic material is not always completely fluidized compared with the case of well-sorted industrial material suggesting the semi-fluidized behavior of pyroclastic flows.