CLASSIFICATION OF PYROCLASTIC FLOWS

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Abstract
Pyroclastic flow is defined as the flow of high-temperature, essential, fragmental materials. This is synonymous with the nuée 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 recognized: 1) Projection of fragments from the crater by explosive expansion of gas within the crater; 2) descent of fragments or liquid magma from the crater caused only by the action of gravity; and 3) swift downflow of the mixture of gas and fragments. This last is intermediate between the first two and corresponds to pyroclastic flow. A new classification of pyroclastic flows is proposed based upon viscosity of the materials, which ls inferred from the nature of the deposit. The volume of the deposit increases as the viscosity decreases. 1) Nuée ardente in the strict sense: Represented by the nuée ardente of Mt. Pelée, Merapi, etc. The fragments are less porous, which indicates the high viscosity. The volume of the deposit is small, generally less than 0.01 km3. 2) Pyroclastic flow of the intermediate type: Represented by certain pyroclastic flows of Asama, Hakone, and Myoko volcanoes. Both viscosity and volume (0.1 - 1 km3) are 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, and Aso volcanoes. Low viscosity leads -to full vesiculation into pumice. Many of them are larger in volume ( > 10 km3) than 1) and 2), and calderas of the Krakatau type are often formed after the eruption of larger pumice flows.--Auth. English summ.