The Waimihia and Hatepe plinian deposits from the rhyolitic Taupo Volcanic Centre

Abstract

A new study has been made of the Waimihia and Hatepe pumice deposits. Grainsize variation used in conjunction with isopach maps reveals that both deposits came from a vent or vents within the present Lake Taupo, probably situated in the Horomatangi Reef area; for the Waimihia pumice the isopach map alone is inadequate to give the vent position since there is a roughly uniform maximum thickness along 30 km of dispersal axis. The upper half of the Waimihia pumice contains streaky mix pumice and some mafic scoria, and it was dispersed in a slightly different direction from the lower rhyolitic part. Various indicators show that the dispersive power, and hence eruptive column height, in the Waimihia eruption was only slightly less than in the Taupo ultraplinian eruption; in the Hatepe eruption, an “average” plinian event, they were much less. In magnitude, the Waimihia eruption is one of the biggest plinian events yet studied. Application of a crystal concentration method yields volumes of 29 km³ and 6 km³ respectively for the Waimihia and Hatepe deposits, significantly higher than in previous estimates, and some 60% of these volumes is dispersed outside the mapped area. The whole-deposit grainsize populations have also been calculated; they are quite similar to the population of the Taupo plinian deposit in containing about 80% of sub-millimetre material, an unexpectedly high value in view of the coarseness of accessible parts of the deposits.

Keywords:

• Taupo Volcanic Zone
• Waimihia Lapilli
• Hatepe Lapilli
• explosive volcanism
• pumice falls
• volume
• grainsize
• vent position