Abstract

A new type of ignimbrite, a “fines-depleted ignimbrite,” is largely depleted in the finer constituents and is clast-supported for all clasts exceeding about 2 mm in size. Its formation is attributed to the loss of fine (mostly submillimetre-sized) vitric material from the pyroclastic flow, by an amount equal to about half of the original mass of the flow. This loss, the large size and high content of lithic clasts, and the thorough intermixing of carbonized vegetation are believed to indicate that the pyroclastic flow, at least in part, traveled turbulently. Turbulent flow was partly a consequence of a high flow velocity, for which there is independent evidence (the same flow climbed 1,500 m up a mountain 46 km from source), and was partly caused by the ingestion of forest and the resulting high throughput of gas in the flow head. The implication is that normal ignimbrite is generated by laminar flow, whereas turbulent flows generate the significantly different, fines-depleted variant.

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