Emplacement of the Taupo ignimbrite by a dilute turbulent flow

Abstract

AN ignimbrite is a pumice-rich deposit that records the passage of a ground-hugging ash flow (a 'pyroclastic flow') generated by the collapse of a volcanic eruption column¹. Geologists study such deposits to reconstruct the parent eruptions and to predict the consequences of future eruptions. The 1,800-yr-old Taupo ignimbrite of New Zealand has been interpreted to represent en masse emplacement by an avalanche-like flow with a volumetric solids concentration in excess of 30% (ref. 2). The evidence for this is equivocal, however, and here we propose the alternative view that the deposit was emplaced by a relatively dilute and turbulent density current³. We present an isothermal, hydraulic model, the results of which, taken together with existing observations, suggest that the total flow rate of solids and gas was about 40km³s^–1 for around 15 minutes. This intense flux resulted in a flow which had a near-vent solids concentration of 0.3% by volume, was about 1 km thick, and travelled outward from the vent with a typical speed of 200 ms^–1. In view of the good agreement between predicted and observed radial trends in the Taupo deposit, we suggest that the origin of other ignimbrites with similar characteristics should be reconsidered.