The Oruanui Supereruption
Lake Taupō’s surface hides a violent past. The lake fills a caldera formed during the Oruanui eruption, the world's most recent supereruption, which occurred around 25,500 years ago. The eruption registered an 8 on the Volcanic Explosivity Index (VEI), the highest possible rating. The eruption ejected an estimated 1,170 cubic kilometers of material, consisting of 430 km³ of pyroclastic fall, 320 km³ of pyroclastic density current deposits (ignimbrite), and 420 km³ of material that collapsed back into the caldera.
The sheer volume of ejected magma caused the ground to collapse, creating the vast depression that would become Lake Taupō. Ignimbrite—a scorching-hot flow of gas and rock fragments, buried the central North Island up to 200 meters deep. Ash from the eruption covered most of New Zealand. An 18-centimeter-thick ash layer was deposited on the Chatham Islands, 850 kilometers away. The eruption was phreatomagmatic, meaning it involved a violent interaction between magma and water from a large lake system that existed over the vents at the time. This interaction created distinctive deposits containing accretionary lapilli, which are small balls of clumped ash.