The 26.5 ka Oruanui eruption, New Zealand: an introduction and overview

The 26.5 ka Oruanui eruption, from Taupo volcano in the central North Island of New Zealand, is the largest known 'wet' eruption, generating 430 km(3) of fall deposits, 320 km(3) of pyroclastic density-current (PDC) deposits (mostly ignimbrite) and similar to420 km(3) of primary intracaldera material, equivalent to similar to530 km(3) of magma. Erupted magma is >99% rhyolite and <1% relatively mafic compositions (52.3-63.3% SiO2). The latter vary in abundance, at different stratigraphic levels from 0.1 to 4 wt%, defining three 'spikes' that are used to correlate fall and coeval PDC activity. The eruption is divided into 10 phases on the basis of nine mappable fall units and a tenth, poorly preserved but volumetrically dominant fall unit. Fall units 1-9 individually range from 0.8 to 85 km(3) and unit 10, by subtraction, is 265 km(3); all fall deposits are of wide (plinian) to extremely wide dispersal. Fall deposits show a wide range of depositional states, from dry to water saturated, reflecting varied pyroclast:water ratios. Multiple bedding and normal grading in the fall deposits show the first third of the eruption was very spasmodic; short-lived but intense bursts of activity were separated by time breaks from zero up to several weeks to months. PDC activity occurred throughout the eruption. Both dilute and concentrated. currents are inferred to have, been present from deposit characteristics, with the latter being volumetrically dominant (>90%). PDC deposits range from mm- to cm-thick. ultrathin veneers enclosed within fall material to >200 m-thick ignimbrite in proximal areas. The farthest travelled (similar to90 km), most energetic PDCs (velocities >100 m s(-1)) occurred during phase 8, but the most voluminous PDC deposits were emplaced during phase 10. Grain size variations in the PDC deposits are complex, with changes seen vertically in thick, proximal accumulations being greater than those seen laterally from near-source to most-distal deposits. Modern Lake Taupo partly infills the caldera generated during this eruption: a similar to140 km(2) structural collapse area is concealed beneath the lake, while the lake outline reflects coeval peripheral and volcano-tectonic collapse. Early eruption phases saw shifting vent positions, development of the caldera to its maximum extent (indicated by lithic lag breccias) occurred during phase 10. The Oruanui eruption shows many unusual features; its episodic nature, wide range of depositional conditions in fall deposits of very wide dispersal, and complex interplay of fall and PDC activity. (C) 2001 Elsevier Science B.V. All rights reserved.