Seismic reflection profiling of the Taupo Caldera, New Zealand

Abstract

Previously published estimates of eruptive volumes suggest that the Taupo volcano, in the centre of New Zealand's North Island, is one of the most active rhyolitic volcanoes anywhere in the world. The last eruption from this volcano in about 180 AD is the most violent eruption yet documented. The estimates of eruptive volume are based upon exposures outside of the collapse caldera presently occupied by Lake Taupo. The development of a digital seismic reflection recording system suitable for surveying from a small boat has dramatically improved seismic images obtained from Lake Taupo, enabling the identification of eruptive structures observed onshore with seismic units in the lake. Three seismically distinctive units occur to the west of Horomatangi Reefs, the interpreted eruption vent of the 180 AD Taupo eruption. The units from lakefloor downwards consist of a featureless unit 100 m to 150 m thick (unit A), a sequence of laminated reflectors up to 700 m thick (unit B) and an underlying unit characterised by mounded and discontinuous reflectors (unit C). The structure of these units suggests they were derived from the area of the 180 AD eruption. Two alternative interpretations of the observed seismic sequence are possible. Unit A may correspond to the entire 180 AD Taupo eruption deposit within the lake or alternatively this eruption may have formed all three units. The corresponding eruption volumes implied for the 180 AD Taupo eruption beneath Lake Taupo are estimated to be >70 km3 or >130 km3, respectively. Both of these volumes are greater than previous estimates. The deeper northeastern corner of Lake Taupo has a contrasting seismic reflection character and is interpreted as a collapse structure formed by the removal of underlying magma during the Taupo eruption.