Hatepe eruption

The Hatepe eruption, named for the Hatepe Plinian pumice tephra layer, sometimes referred to as the Taupō eruption or Horomatangi Reef Unit Y eruption, is dated to 232 CE ± 10 and was Taupō Volcano's most recent major eruption. It is thought to be New Zealand's largest eruption within the last 20,000 years. The eruption ejected some 45-105 km3 of bulk tephra, of which just over 30 km3 was ejected in approximately 6–7 minutes. This makes it one of the largest eruptions in the last 5,000 years, comparable to the Minoan eruption in the 2nd millennium BCE, the 946 eruption of Paektu Mountain, the 1257 eruption of Mount Samalas, and the 1815 eruption of Mount Tambora.

Stages of the eruption
The eruption went through several stages, with six distinct marker horizons identified, although phase 5 has at least 26 deposit subunits. Most of the stages only affected the immediate surrounds of the caldera and regions to its east due to prevailing wind patterns. 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. Rhyolitic lava domes were extruded some years or decades later, forming the Horomatangi Reefs and Waitahanui Bank.

The main extremely violent pyroclastic flow travelled at close to the speed of sound and devastated the surrounding area, climbing over 1500 m to overtop the nearby Kaimanawa Ranges and Mount Tongariro, and covering the land within 80 km with ignimbrite.Only Ruapehu was high enough to divert the flow. The power of the pyroclastic flow was so strong that in some places it eroded more material off the ground surface than it replaced with ignimbrite. There is evidence that it occurred on an autumn afternoon and its energy release was about 150 megatons of TNT equivalent. The eruption column penetrated the stratosphere as revealed by deposits in ice core samples in Greenland and Antarctica. As New Zealand was not settled by the Māori until more than 1,000 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 (like those observed after the 1883 Krakatoa eruption). The stages as reclassified from 2003 are:

After
It is estimated that it might have taken as much as 30 years to refill the emptied lake in the caldera. There were massive changes in the landscape for 40 km around with all life sterilised and prior landforms evened out, with beyond the ignimbrite sheet likely forest fires and ash associated die back especially to the west. In 1937 it was recognised that the deposit from the Hatepe eruption had been so hot to burn the forest over a 160 km (99 mi) distance, but this was not recognised as being due to a pyroclastic flow until 1956. Valleys had been filled with ignimbrite, evening out the shape of the land. The Waikato River had been blocked by ignimbrite deposits with the lowest being at Orakei Korako and the temporary lake that was formed over perhaps 2 to 3 years after the eruption in the older Reporoa Caldera had an area of about 90 km2 and a volume of about 2.5 km3. This broke through in a massive flood with peak flow believed to be 17,000 m3/s, over 100 times the current river maximum flood flow.

In due course 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 meters above its present level until it broke out after about 20 years in a huge flood. Over 20 km3 of water escaped down river in less than 4 weeks, with peak discharge of the order of 30,000 m3/s so flowing for more than a week at roughly 200 times the Waikato River's current rate.

Following the eruption rhyolitic lava domes were extruded, these smaller eruptions of unknown total size also created large pumice rafts that were later discovered deposited on the lake shoreline. The volcano continues to be classified as active with periods of volcanic unrest.

Dating the eruption
Early radiocarbon dating effort on 22 selected carbonized samples yielded an uncalibrated average date of 1,819 ± 17 years BP (131 CE ± 17). Research by Colin J. N. Wilson and others remarked that ongoing calibration pushes the radiocarbon result to a more recent date, and they proposed 186 CE as the exact year of eruption based on ancient Chinese and Roman records of unusual atmospheric phenomena in about this year.

In an effort led by R.S.J. Sparks and others to investigate interhemispheric calibration offset in 1995, the team analyzed the uncalibrated ages of tree rings of a single tree killed in Taupo eruption, cross-matched the uncalibrated tree ring chronology to Northern Hemisphere calibration curve, and extrapolated the calibrated tree ring dates to obtain the outermost ring date of 232 CE ± 15, i.e. the last moment the tree was alive.

In 2012, to circumvent interhemispheric calibration offset, the uncalibrated dates of tree rings of a single tree killed in Taupo eruption were wiggle-matched to New Zealand-derived calibration data set to obtain the currently most precise eruption date of 232 CE ± 8 (95.4% confidence). This date is statistically indistinguishable from that of 1995 study and is the currently accepted date. It is suggested that the presence of magmatic carbon in pre-eruption groundwaters may have contaminated radiocarbon ages. However, rhyolitic shards derived from the Taupo eruption have been identified in the Roosevelt Island ice core and are independently dated to 230 CE ± 19, thus refuting propositions of a potential age bias. These dates are also within a wider range of 205 CE to 373 CE determined by paleomagnetic dating but the age continues to be slightly controversial, for the reasons mentioned.

Post-eruption soil deficiencies
The tephra soils associated with the eruption were deficient in several essential minerals, with cobalt deficiency being the cause of bush sickness in animals that precluded productive livestock farming until this issue was identified and addressed. This identification by New Zealand government scientists in 1934 was probably the most significant single advance in New Zealand agriculture ever, but was not able to be fully exploited until the 1950s with the deployment of cobalt-ion-containing superphosphate fertiliser from aircraft.