Lake Huka

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. This was the Oruanui eruption of the Taupō Volcano about 25,500 years ago, which was the largest eruption on earth in the past 70,000 years. 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 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 and while the width of the Taupō-Repora basin deposits is about 20 km, deposits identified onshore of Lake Taupō suggest a maximum width may have been about 30 km to the south. However, as Waiora Formation assigned deposits might to 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 greywacke basement. This basement is known to be in places more than 3 km 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 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 stratovolcano Mount Tongariro massiff had started forming about 349,000 years ago, with its Tupuna and Haumata formations predating 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 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 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:
 * 1) Upper with mudstones interbedded with volcaniclastic material
 * 2) *in the Waireki area at a depth of 250 - 263 m below ground.
 * 3) *between 25,360 ± 160 and 92,000 ± 11,000 years old
 * 4) Middle pumice-rich from a relatively deep-water pyroclastic eruption(s)
 * 5) *The eruptions were under a Lake Huka water column that was 150 – deep
 * 6) *Eruptions dated to between 168,000 and 92,000 years ago
 * 7) Lower with mudstones interbedded with volcaniclastic material
 * 8) *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.

Outflow
Outflow of Lake Huka was always via the Waikato River, but had major downstream implications to the evolving geology of the Hamilton Basin as now found in the Waikato Basin, and Hauraki Rift as now found in the Hauraki Plains. The predominant pattern was an ancestral Waikato River that drained the Taupo Rift through the Ōngāroto Gorge and reached the Pacific Ocean to the north at the Hauraki Gulf via the Hauraki Plains. By the time of the Oruanui eruption the lower reaches of the Waikato River were a mature river system. It is unknown if the evidence for some historic drainage well before the Oruanui eruption, into the Hamilton Basin by the Waikato River relates to a period when Lake Huka existed. Whatever the destruction of Lake Huka was associated ultimately by the breakdown of a volcanic dam located over the central portion of the former Lake Huka and then a change in the Waikato River course so that it now flowed through the Waikato Basin, into the Tasman Sea. This change was not immediate and only occurred permanently sometime after the eruption.

Later lakes
Lake Taupō was formed and filled over a period of about a hundred years after the Oruanui eruption. In the former northern region of Lake Huka there is evidence for a temporary lake in the Reporoa Basin with a shoreline terrace at about 360 m and lake deposits at up to 400 m above today's sea level but this was either drained before the main break-out flood from Lake Taupō, or was destroyed during the break-out flood. Much later, after the 232 CE Hatepe eruption two temporary Lake Reporoas were created transiently in the Reporoa Basin.

Ecology
Intact samples of erupted lake sediment from the Oruanui eruption in the form of lithic clasts in the ignimbrite contain diatoms. In particular Cyclostephanos novaezelandiae is found which is much rarer in the current volcanic lakes of the region, and this is believed to be because a Cyclostephanos novaezelandiae ecosystem became less likely due to reorganisation of the watershed in the aftermath of the eruption, and climate warming following the Last Glacial Maximum. Further as Cyclostephanos novaezeelandiae, is endemic to New Zealand's North Island it would serve if identified in tephra to confirm the eruptive source region. Diatoms have been identified in tephra from the eruption 850 km away on the Chatham Islands.

The earlier Taupō Volcano Okaia eruption, that erupted through Lake Huka, also dispersed diatom specimens from lake sediments, with a very similar ecological population to the Oruanui eruption.