Lake Huka
Location of Lake Huka
Location of Lake Huka
Lake Huka
LocationNorth Island
Coordinates38°37′S 176°06′E / 38.62°S 176.1°E / -38.62; 176.1
Lake typepaleolake
Primary outflowsWaikato
Basin countriesNew Zealand
First flooded220 ka[1]
Max. length100 km (62 mi)[2]
Max. width20 km (12 mi)+[2]

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 25,000 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 (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.[3] 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.[4] In length this is about 100 km (62 mi) and while the width of the Taupō-Repora 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.[2] However, as Waiora Formation assigned deposits might to contribute to this wider width,[2] 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.[5] This basement is known to be in places more than 3 km (1.9 mi) deep presently.[5] The massive Whakamaru ignimbrite eruption of 349,000 years ago,[5] 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,[6] 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[7], 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.[8][lower-alpha 1] 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.[9] The high stand was about 400 m (1,300 ft) above present mean sea level,[10] 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.[11]

The lake was completely destroyed by the Oruanui eruption of the Taupō Volcano which was the largest phreatomagmatic eruption characterised to date.[12] Zircon dating elsewhere, has this, the largest eruption on earth in the past 70,000 years timed at 25,360 ± 160 BP.[13][9][14][12]

The Huka Falls Formation was described first in 1965,[15] 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,[10] and was only accurately dated recently.[9][lower-alpha 2] 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).[16][3][lower-alpha 3] 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
    • in the Waireki area at a depth of 250–263 m (820–863 ft) below ground.[17]
    • between 25,360 ± 160[13] and 92,000 ± 11,000 years old[1]
  2. 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[15]
    • Eruptions dated to between 168,000 and 92,000 years ago[1]
  3. 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.[2] 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.[15] Events that impacted on the lake are shown in the table.

Events impacting on Lake Huka[lower-alpha 2]
Date (ka)EventLocationComment
220 to 25.36Lake Huka's existence-[lower-alpha 2][9][13]
220 ± 31Emplacement Waiora Formation eruptivesNorth-west/Outlet[lower-alpha 2] Outlet is Waikato River
198 ± 23Tihia, andesitic depositsSouth[8] [lower-alpha 1] The combined massive where Tihia is a parasite cone would have contributed to making the southern end of the lake shallower.
180 to 123Pihanga, andesitic depositsSouth[8] Would have contributed to defining the southern end of the lake.[lower-alpha 1]
150 to 60[18] Whakaroa ridge domesCentralMay have contributed to redefining central western part of lake shore.
140 to 45K-trig basaltsWestern CentralNew 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.[11]
142 ± 7Racetrack, rhyolite lavaCentral[lower-alpha 2]Made central portion lake shallower by about 0.5 m (1 ft 8 in).
141 ± 8Te Mihi 2, rhyolite lavaCentral[lower-alpha 2]Made central portion lake shallower by about 35 m (115 ft).
122 ± 9Te Mihi 1, rhyolite lavaCentral[lower-alpha 2]Made central portion lake shallower by about 155 m (509 ft).
168 ± 24 to 92 ± 11Middle Huka Falls Formation EruptionCentral[lower-alpha 2]Made central portion lake shallower.
200-26Tongariro volcanic centreSouthVolcanic sediments from multiple eruptions were washed down into the southern lake and the southern shore line retreated to the north.
~100Illdefined Taupo EruptionsSouthEvidence 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[19]
81.3 to 77.2Motuoapa Peninsula, rhyoliteSouthTwo rhyolitic erruptions that changed south-western shoreline[20]
58 ± 10Emplacement Tauhara daciteEastern central[lower-alpha 2]Created smaller new shore line eastern central region
34.5 ± 3.1Motuoapa Peninsula, daciteSouthPossibly only local impact south-western shoreline[20]
28.621 ± 1.428OkaiaCentralThis eruption's tephra deposits contained a diatom population distribution consistent with that of the later Oruanui eruption.[21]
27Trig 9471 and the Rubbish Tip Domes EmplacementCentralNarrowed 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.[22][23]
25.36 ± 0.16Oruanui eruptionSouth[lower-alpha 2]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, [22] 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.[22][23]

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.[24] 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.[25] 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.[24] This change was not immediate and only occurred permanently sometime after the eruption.[26]

Later lakes

Lake Taupō was formed and filled over a period of about a hundred years after the Oruanui eruption.[27] 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 (1,180 ft) and lake deposits at up to 400 m (1,300 ft) 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.[28] Much later, after the 232 CE Hatepe eruption two temporary Lake Reporoa's were created transiently in the Reporoa Basin.[29]

Ecology

Intact samples of erupted lake sediment from the Oruanui eruption in the form of lithic clasts in the ignimbrite contain diatoms.[30] 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.[31] 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.[32] Diatoms have been identified in tephra from the eruption 850 km (530 mi) away on the Chatham Islands.[33]

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.[21]

Notes

  1. 1 2 3 For discussion of K-Ar verus Ar-Ar age and the former's potential for inaccuracy in low potassium oxide andesite see Pure 2020
  2. 1 2 3 4 5 6 7 8 9 10 Recent accurate ages have been obtained by zircon dating (Rosenberg et al. 2020). This means that some historic literature age estimations could need correction. For example the lake may have existed for 100,000 years less than previously estimated and recent dating methods suggest the Oruanui eruption was a thousand years later than some earlier dating suggested. Indeed the consensus age of the Oruanui eruption (see article for references) is now 25675 ± 90 BP. The Zircon dates are within the error of this and so this article has tried to correct estimates published before 2020 to the more accurate zircon dates for consistency in relative times, and such corrected ages will be marked by this note. It has been assumed in this re-basing that the Waiora Formation pre-dates Lake Huka. No doubt other lakes existed in the area before Lake Huka, and may have been incorporated when it formed.
  3. ↑ The geological terms used are those of Rosenberg et al. 2020. Because a number of terms have been used to classify the local rock secession the following note may help in understanding this article in the context of other articles on Wikipedia. The Taupo Group (Rosenberg et al. 2020) contains the Oruanui Formation as its most recent deposited member but this is slightly different to how the term Oruanui Formation was defined when it replaced the term Wairakei Breccia(Chi & Brown 1991) due to later discoveries. The Taupo Group contains as we go older, the recent Taupo subaerial rhyolite lavas from the Aratiata, Trig 9471 and Rubbish tip domes, then the dacite Tauhara Formation lavas and pyroclastic deposits but there is overlap in time between these and the upper Huka Falls Formation. The oldest part of the Taupo Group that overlaps in time with the lower Huka Falls Formation is the K-Trig Formation. Finally the two Te Mihi and Racetrack rhyolite lavas are subclassified as part of the lower Huka Falls Formation.

References

Sources

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.