The Taupo eruption, New Zealand I. General aspects

Abstract

The ca . a.d. 186 Taupo eruption was the latest eruption at the Taupo Volcanic Centre, occurring from a vent, at Horomatangi Reefs, now submerged beneath Lake Taupo in the central North Island of New Zealand. Minor initial phreatomagmatic activity was followed by the dry vent 6 km 3 Hatepe plinian outburst. Large amounts of water then entered the vent during the 2.5 km 3 Hatepe phreatoplinian ash phase, eventually stopping the eruption, though large amounts of water continued to be ejected from the vent area, causing gullying of the ash deposits. After a break of several hours to weeks, phreatoplinian activity resumed, generating the 1.3 km 3 Rotongaio ash, notable for its fine grainsize and for containing significant quantities of non- or poorly-vesicular juvenile material. The vent area then became dry again, and eruption rates and power markedly increased into the 23 km 3 Taupo ‘ ultraplinian ’ phase, which is the most powerful plinian outburst yet documented. Synchronous with this ultraplinian activity, lesser volumes of non- to partially-welded ignimbrite were generated by diversion of ejecta from, or partial collapse of, the eruption column. The rapid rate of magma withdraw al during this phase removed support from the vent area, to trigger local vent collapse and initiate the catastrophic eruption of the ca. 30 km 3 Taupo ignimbrite. Finally, after some years, lava was extruded on to the floor of the reformed Lake Taupo, and floating fragments derived from the lava carapace were driven ashore. The known eruption volume is more than 65 km 3 , while additional volumes are represented by primary material now beneath Lake Taupo and layer 3 to the ignim brite phases; a total volume of more than 105 km 3 is likely, equivalent to more than 35 km 3 of magma plus more than 3 km 3 of lithic debris. Airfall deposits more than 10 cm thick blanketed 30000 km 2 of land east of the vent, while ignimbrite covers a near-circular area of 20000 km 2 . Widespread and locally severe ground shaking occurred during, but mostly after the eruption, associated with subsidence in the Lake Taupo basin. Secondary deposits are abundant above and extending beyond the Taupo ignimbrite, consisting of the products of surface water interacting with the still-hot ignimbrite and subsequent water reworking of the light, pumiceous materials. The complexity and size of this eruption preclude accurate forecasting of the size, nature and return period of the inevitable next eruption from the Taupo Volcanic Centre.