The White River Ash: largest Holocene Plinian tephra

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The White River Ash is a bi-lobate tephra in eastern Alaska, Yukon Territory, and western Northwest Territories. Plinian-type eruptions produced the north lobe ~1900years BP and the larger east lobe ~1250years BP (14C years). Present evidence favors the vent for the east lobe to be beneath the Klutlan Glacier. East lobe pumice is not present atop Mt. Churchill, so the pumice there must belong to the north lobe and is also likely to have come from a vent beneath the Klutlan Glacier. Isopachs of the east lobe, now known to stretch as far east as Great Bear Lake, indicate an east lobe volume of ~47km3. Thickness and grain size of the east lobe decay in exponential fashion, producing straight line plots when the thickness half-distance and clast half-distance are plotted against the square root of the isopach area, the proximal slope being steeper than the distal. The east lobe eruption is indicated to have been into a wind of about 10m/s and to have produced an eruptive cloud height of ~45km. The eruption rate was at least 2.8× 108 kg/s.

Le White River Ash est un tephra à deux lobes situé dans l’Est de l’Alaska, le Territoire du Yukon et l’Ouest des Territoires du Nord-Ouest. Des éruptions de type plinien ont produit le lobe Nord ~1900 ans avant le présent et le lobe Est, le plus gros, ~1250 ans avant le présent (années 14C). Les données actuelles favorisent l’hypothèse que la cheminée du lobe Est était sous le glacier Klutlan. Il n’y a pas de pierre ponce du lobe Est sur le mont Churchill, la pierre ponce doit donc appartenir au lobe Nord et proviendrait aussi d’une cheminée sous le glacier Klutlan. Les isopaches du lobe Est, lequel, on le sait maintenant, s’étend aussi loin que le Grand lac de l’Ours, indiquent un volume de ~47km3 pour le lobe Est. L’épaisseur et la granulométrie du lobe Est diminuent de manière exponentielle produisant des lignes droites lorsque la demi-distance de l’épaisseur et la demi-distance des clastes sont mises en graphique par rapport à la racine carrée de la superficie de l’isopache, la pente proximale étant plus abrupte que la pente distale. L’éruption du lobe Est se serait produite dans un vent d’environ 10m/s et aurait produit un nuage éruptif d’une hauteur de ~45km. Le taux d’éruption était d’au moins 2,8 × 108 kg/s.

Document Type: Research Article

Publication date: June 1, 2008

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