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The Oruanui eruption of New Zealand's Taupo Volcano was the world's largest known eruption in the past 70,000 years, with a Volcanic Explosivity Index of 8. It occurred around 26,500 years ago in the Late Pleistocene and generated approximately 430 km3 (100 cu mi) of pyroclastic fall deposits, 320 km3 (77 cu mi) of pyroclastic density current (PDC) deposits (mostly ignimbrite) and 420 km3 (100 cu mi) of primary intracaldera material, equivalent to 530 km3 (130 cu mi) of magma. 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.
Modern Lake Taupo partly fills the caldera generated during this eruption; a 140 km2 (54 sq mi) 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 falls and pyroclastic flows.
Tephra from the eruption covered much of the central North Island with ignimbrite up to 200 metres (660 ft) deep. Most of New Zealand was affected by ashfall, with even an 18 cm (7 in) ash layer left on the Chatham Islands, 1,000 km (620 mi) away. Later erosion and sedimentation had long-lasting effects on the landscape, and caused the Waikato River to shift from the Hauraki Plains to its current course through the Waikato to the Tasman Sea.