Lake Huka is a former lake whose waters, on its sudden explosive destruction, were a component in the creation of the largest phreatomagmatic eruption characterised to date and the largest volcanic eruption on earth in the past 70,000 years. This was the Oruanui eruption of the Taupō Volcano about 25,500 years ago. Presently the smaller Lake Taupō, currently the largest lake in New Zealand, occupies the area of the southern part of the former lake. Diatoms from sediments erupted from the former lake floor have been identified 850 km (530 mi) away on the Chatham Islands.
Geography The lake is named after the Huka Falls Formation, the Taupō Volcanic Zone's most widespread sedimentary lacustrine deposit. This is centred around the Huka Falls, in the Taupō Rift. The formation defines Lake Huka, and extends south from the Reporoa Caldera, to beyond the southern end of Lake Taupō with the furtherest south deposits in the drainage area of the Tongariro River. In length this is about 100 km (62 mi) and while the width of the Taupō-Reporoa basin deposits is about 20 km (12 mi), deposits identified onshore of Lake Taupō suggest a maximum width may have been about 30 km (19 mi) to the south. However, as Waiora Formation assigned deposits might contribute to this wider width, the definite lake may not have been this wide.
Geology The lake formed in a north-east oriented graben of the Taupō Rift that has a Mesozoic basement of greywacke. This basement is known to be in places more than 3 km (1.9 mi) deep presently. The massive Whakamaru ignimbrite eruption of 349,000 years ago, for example has deposits up to 50m thick typically at least 650 m (2,130 ft) below the present ground level. At its northern end in the Taupō-Reporoa Basin the eruptions that had formed the rhyolitic Reporoa Caldera some 280,000 years ago, were to define the lakes boundaries throughout its existence. At the southern end the current andesitic volcanoes commenced about 349,000 years ago as a stratovolcano that became Mount Tongariro. This massif's Tupuna and Haumata formations predate the lake, so that its high ground likely delimited the lake to the south. Volcanic deposits washed down from the Tongariro River drainage were to silt up its southern end as is the case with Lake Taupō today. Kakaramea has been dated to 229 ± 1 ka so may not have been active from the time of lakes formation but may have therefore defined a south-western initial shore. In the center and to the west the rhyolitic volcanic deposits associated with the Whakamaru caldera complex and the Taupō Volcano by 222,000 years ago defined the western shore. The lake's high stand remained fairly constant throughout its lifetime and was dictated by the emplacement of the last Waiora Formation eruptives in the north-west of the lake allowing its creation. This took place at 220,000 ± 31,000 BP. The high stand was about 400 m (1,300 ft) above present mean sea level, but given the rate of rift extension (subsidence) and historic much lower sea levels this relative level is almost meaningless to try to define further. Evidence for more than one high stand/lake terrace does exist. The lake was completely destroyed by the Oruanui eruption of the Taupō Volcano which was the largest phreatomagmatic eruption characterised to date, and occurred about 25,500 years ago. The reference zircon dating, as used to determine most eruption timings relevant to Lake Huka's evolution, has this, the largest eruption on earth in the past 70,000 years timed at 25,360 ± 160 BP. The Oruanui eruption can not be timed directly by zircon dating and has now been corrected by other means to 25,675 ± 90 years BP. The Huka Falls Formation was described first in 1965, and is mainly subsurface. It is found between 400–100 m (1,310–330 ft) above present mean sea level in this middle portion of the Taupō Volcanic Zone, and was only accurately dated recently. The Huka Falls Formation is above the mainly volcanic Waiora Formation, some of which is hard to distinguish from the Huka Falls Formation at its margins, and beneath the Oruanui Formation (Wairakei Breccia). The formation is most well characterised in its middle section due to well drilling for geothermal development and here there are three distinct units:
Upper with mudstones interbedded with volcaniclastic material in the Waireki area at a depth of 250–263 m (820–863 ft) below ground. between 25,360 ± 160 and 92,000 ± 11,000 years old Middle pumice-rich from a relatively deep-water pyroclastic eruption(s) The eruptions were under a Lake Huka water column that was 150–250 m (490–820 ft) deep Eruptions dated to between 168,000 and 92,000 years ago Lower with mudstones interbedded with volcaniclastic material Is younger than 220,000 ± 31,000 years ago. During the lake's existence its size, while never small, varied. Because of the length of its existence, relevant processes to such change, being structurally controlled subsidence, subsidence following explosive eruptions or by volcanic eruptions blocking water outflows and silting may have applied. Events that impacted on the lake are shown in the table.
Destruction The Oruanui eruption occurred in a ten-stage process with the main vents located under the southern Lake Huka system. There is a fair possibility on geological grounds that the southern section of Lake Huka, had recently separated from the northern section to create what could be called the first Lake Taupō, due to either or both of pre-eruption upwarping shortly before the eruption itself, or in a process that likely commenced about a thousand years earlier, due to eruptive activity of the Poihipi volcano adjoining Mount Tauhara whose magma chamber is under Wairakei and that had erupted at Trig 9471 and the Rubbish Tip Domes about 27,000 years ago.
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