Pyroclastic Flowの分類

抄録

The pyroclastic flow is defined as the flow of high-temperature, essential, fragmental materials. This is a synonym of the nuee ardente in the broad sense. Three modes of emplacement of high-temperature, essential, solid (or liquid) materials after the ejection from the crater may be recongized: 1) Projection of the fragments from the crater by the explosive expansion of gas which occurs within the crater. 2) Descent of the fragments or liquid magma from the crater which is caused only by the action of gravity. 3) Swift downflow of the mixture of gas and fragments. This is intermediate between 1) and 2) and to this corresponds the pyroclastic flow. A new classification of the pyroclastic flows is proposed. The principle is based upon the viscosity of the materials, which is inferred from the nature of the deposit. The volume of the deposit increases as the viscosity decreases. 1) Nuee ardente in the strict sense. Represented by the nuees ardentes of Mt. Pelee, Merapi, etc. The fragments are less porous, which indicates the high viscosity. The volume of the deposit is small, generally less than 0.01km³. 2) Pyroclastic flow of the intermediate type. Represented by certain pyroclastic flows of Asama. Hakone. Myoko Volcanoes. The viscosity and volume (0.1～1km³) are both intermediate between 1) and 3). 3) Pumice flow. Represented by pumice and tuff flows of all sizes, such as those of Crater Lake, Hakone, Katmai. Aso Volcanoes. Low viscosity leads to the full vesiculation into pumice. Many of them are larger in volume (＞10km³) than 1) and 2), and the caldera of Krakatau type is often formed after the eruption of larger pumice flows.