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.
The Hatepe Eruption of 232 AD
The Oruanui event was larger, the Hatepe eruption in 232 AD was the most violent eruption on Earth in the last 5,000 years. Though it ranked a 7 on the VEI scale, its intensity was exceptional. The eruption began with smaller phreatomagmatic explosions before culminating in a climactic phase that unleashed a devastating pyroclastic flow. This flow traveled at speeds between 600 and 900 kilometers per hour, blanketing 20,000 square kilometers of land. It was so powerful that it surmounted mountain ranges over 1,500 meters high.
The eruption column reached an estimated height of 40 to 50 kilometers. The ash produced likely responsible for historical accounts of red and darkened skies recorded by observers in both Rome and China. The eruption altered the landscape dramatically, blocking the lake's previous outlet and raising its level by 35 meters. Eventually, this natural dam failed, releasing a catastrophic flood down the Waikato River. After this event, smaller eruptions formed rhyolitic lava domes, which now exist as the submerged Horomatangi Reefs on the lake floor.