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Ignimbrite types and ignimbrite problems

Journal of Volcanology and Geothermal Research
Publication Date
DOI: 10.1016/0377-0273(83)90062-8


Abstract A spectrum of ignimbrite emplacement types exists, ranging from the “conventional” high-aspect ratio (H.A.R.I.) type, emplaced relatively quietly and passively in valleys, to the low-aspect ratio (L.A.R.I.) type, emplaced cataclysmically. Features of the L.A.R.I., such as a remarkable ability to scale mountains and cross open water and a strong fines-depletion of part of their deposits, stem from a high flow velocity which may result from an extremely high magma discharge rate. Being less rare than large-volume H.A.R.I. eruptions covering the same area, L.A.R.I. eruptions are a much more immediate volcanic hazard. Being thin and inconspicuous, a L.A.R.I. may easily be overlooked when determining the past record of a volcano. Another ignimbrite spectrum depends on variations in particle viscosity during emplacement and extends from the low-grade (water-cooled?) ignimbrite which is totally non-welded even if >50 m thick, to the high-grade (superheated?) one which is densely welded even if <50 m thick. Problems of air-cooling and water-cooling of ash flows need to be tackled, and it may be necessary to recognize strongly cooled ash flows which were emplaced in part at <100°C. One problem of ignimbrite eruptions is the origin of the extensive associated ash fall, comparable in volume to the ignimbrite. This ash may be: (a) pre-ignimbrite Plinian pumice; (b) co-ignimbrite ash, containing material lost from both the eruptive column and ash flow; (c) phreatoplinian, due to the entry of significant amounts of water into the vent; (d) phreatoplinian co-ignimbrite, due to explosions at rootless vents where ash flows enter water from land. Another problem is the origin of associated well-sorted and sometimes wavy-bedded deposits. These deposits may be from: (a) base surges, related to either the collapsing column or entry of water to the vent; (b) base surges, due to explosions at rootless vents where ash flows enter water from land; (c) fines depletion in, and deposition from, the strongly fluidized head of the ash flow; (d) standing waves in a high-velocity ash flow; (e) pyroclastic surges springing from the ash flow; (f) superficial turbulence in the topmost fractions of the ash flow as it comes to rest. Major problems concern the relationship between pyroclastic surges and flows, the ability of one to change into the other, and the distinction between their deposits. Thus, the May 18th 1980 “directed blast” of Mount St. Helens is widely regarded as a surge, yet produced deposits having many characteristics of a L.A.R.I.. Understanding the behaviour of the fine ash and dust fraction is thought to be critical to the solution of these problems.

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