AN ignimbrite is a pumice-rich deposit that records the passage of a g
round-hugging ash flow (a 'pyroclastic flow') generated by the collaps
e of a volcanic eruption column(1). Geologists study such deposits to
reconstruct the parent eruptions and to predict the consequences of fu
ture eruptions. The 1,800-yr-old Taupo ignimbrite of New Zealand has b
een 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 current(3). We present an isothermal, hydraulic
model, the results of which, taken together with existing observation
s, suggest that the total flow rate of solids and gas was about 40 km(
3) s(-1) for around 15 minutes. This intense flux resulted in a flow w
hich 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 m s(-1). In view of the good agreement between predicted and ob
served radial trends in the Taupo deposit, we suggest that the origin
of other ignimbrites with similar characteristics should be reconsider
ed.