The 1800a Taupo eruption: “Ill wind” blows the ultraplinian type event down to Plinian

The most powerful category of explosive volcanic eruptions, called "ultraplinian," was proposed in 1980 on the basis of the distribution of a single pyroclastic fall product of one phase of the 1800a Taupo eruption in New Zealand. Dispersal data, a measure of the "footprint" of the deposit, were used subsequently to estimate eruption plume heights of 50–51 km, more than 10 km higher than observed or estimated for any historical Plinian eruption. Today, this unit remains the only deposit to have met the rigorous ultraplinian dispersal criteria, and it is an important and widely cited exemplar in physical volcanology. The earlier study was based on total thicknesses for the entire bed and grain-size data across that full thickness. We have now subdivided this bed into 26 subunits and measured their individual thicknesses and selected maximum clast sizes. Our data show that the apparent large footprint of this bed is an artifact of a previously unrecognized shift in the wind field during the eruption, rather than extreme eruptive vigor. Our study demonstrates the dangers of applying full-thickness approaches to even seemingly uniform fall deposits. The results throw into some doubt the need for the term ultraplinian, at least for this deposit. With the revision of plume heights for the Taupo ultraplinian, a height range of 35–40 km may be practical for use as an upper limit for source parameters in models of transport and dispersal of volcanic ash.