The eruption went through several stages, with six distinct marker horizons identified. Despite the uniform composition of the erupted magma, a wide variety of eruptive styles were displayed, including weak phreatomagmatism, Plinian eruptions, and a huge pyroclastic flow. Rhyoliticlava domes were extruded some years or decades later, forming the Horomatangi reefs and Waitahanui bank. The main pyroclastic flow devastated the surrounding area, climbing over to overtop the nearby Kaimanawa Ranges and Mount Tongariro, and covering the land within with ignimbrite. Since New Zealand was not settled by the Māori until more than a thousand years later, the area had no known human inhabitants when the eruption occurred. Tsunami deposits of the same age have been found on the central New Zealand coast, evidence that the eruption caused local tsunamis, but much more widespread waves may have been generated. The Hatepe eruption further expanded the lake that had formed after the much larger Oruanui eruption around 26,500 years ago. The previous outlet was blocked, raising the lake 35 metres above its present level until it broke out in a huge flood, flowing for more than a week at roughly 200 times the Waikato River's current rate.
Dating the event
For many years it was believed, following research by Colin J. N. Wilson and others, that the eruption could be dated to exactly 186 AD. This meant that it coincided with meteorological phenomena described by Fan Ye in Han China and by Herodian of Antioch in the Roman Empire. This is now questioned. Estimated dates previously suggested for the Taupo eruption have ranged from AD 131 to 233. The earlier date was based on from carbonized vegetation enclosed in the eruption products. However, the 22 selected samples used to obtain an average date of 1,819 ± 17 years BP had much larger standard deviations than the average date itself. Most if not all geologists now accept that the tephra or pumice fall from the eruption was far greater than previously thought, approximately instead of. The later date of 233 AD ± 13 was a result of radiocarbon dating by R. Sparks.. The presence of contaminating 14C-less magmatic carbon in pre-eruption groundwaters has been proposed as skewing towards older dates, leading to suggested dates for the Hatepe eruption decades or even centuries younger than previously thought. Humans had not settled in New Zealand at the time of the eruption, and would not for more than 1,000 years. The nearest human populations at the time were in Australia, more than to the west.
