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 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 and while the width of the Taupō-Reporoa basin deposits is about, deposits identified onshore of Lake Taupō suggest a maximum width may have been about 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 greywacke basement. This basement is known to be in places more than deep presently. The massive Whakamaru ignimbrite eruption of 349,000 years ago, for example has deposits up to 50m thick typically at least 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 massif 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 above present mean sea level, but given the rate of rift extension 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 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. 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 below ground.
*between 25,360 ± 160 and 92,000 ± 11,000 years old
Middle pumice-rich from a relatively deep-water pyroclastic eruption
*The eruptions were under a Lake Huka water column that was 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.

Date	Event	Location	Comment
220 to 25.36	Lake Huka's existence	-
220 ± 31	Emplacement Waiora Formation eruptives	North-west/Outlet	Outlet is Waikato River
198 ± 23	Tihia, andesitic deposits	South	The combined massive where Tihia is a parasite cone would have contributed to making the southern end of the lake shallower.
180 to 123	Pihanga, andesitic deposits	South	Would have contributed to defining the southern end of the lake.
150 to 60	Whakaroa ridge domes	Central	May have contributed to redefining central western part of lake shore.
140 to 45	K-trig basalts	Western Central	New central western shoreline. K-trig commenced this which did not finish until about 45 ka as defined by the Punatekahi cone and its Tihoi Tephra. These deposits have Huka Falls Formation under them and in places over them consistent with higher lake stands and erosion of the volcanics.
142 ± 7	Racetrack, rhyolite lava	Central	Made central portion lake shallower by about.
141 ± 8	Te Mihi 2, rhyolite lava	Central	Made central portion lake shallower by about.
122 ± 9	Te Mihi 1, rhyolite lava	Central	Made central portion lake shallower by about.
168 ± 24 to 92 ± 11	Middle Huka Falls Formation Eruption	Central	Made central portion lake shallower.
200-26	Tongariro volcanic centre	South	Volcanic sediments from multiple eruptions were washed down into the southern lake and the southern shore line retreated to the north.
~100	Illdefined Taupo Eruptions	South	Evidence of most of these was destroyed in the Oruanui eruption. The south east andesitic-basalt of the Karangahape Cliffs on the west side of Lake Taupō and observation that the Oruanui ignimbrite to the east of the lake comprises up to 50% andesitic lithology suggests that an andesitic cone of cubic kilometre-scale was destroyed during the Oruanui eruption
81.3 to 77.2	Motuoapa Peninsula, rhyolite	South	Two rhyolitic eruptions that changed south-western shoreline
58 ± 10	Emplacement Tauhara dacite	Eastern central	Created smaller new shore line eastern central region
34.5 ± 3.1	Motuoapa Peninsula, dacite	South	Possibly only local impact south-western shoreline
28.621 ± 1.428	Okaia	Central	This eruption's tephra deposits contained a diatom population distribution consistent with that of the later Oruanui eruption.
27	Trig 9471 and the Rubbish Tip Domes Emplacement	Central	Narrowed at least, if not blocked the shoreline in central region cutting Lake Huka in half about 27,000 years ago. These events included the phreatoplinian Poihipi eruption from Rubbish Tip Dome.
25.36 ± 0.16	Oruanui eruption	South	Destroyed lake

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.