Lake Rotomahana

Lake Rotomahana is an and the surrounds of the lake had become world famous following its first European written description in 1843.

Geology

thumb|Steaming cliffs on the shore of Lake RotomahanaLake Rotomahana is one of the most studied lakes in New Zealand, occupying the southwestern portion of a rift which formed during the 1886 eruption of Mount Tarawera. In the context of the geological discussion, a high standard hydrophonic survey took place in 2016 and the absolute measurements taken at this time, as lake level varies, are used for baseline. At this time the lake level was , not the geographically mapped height of , or the mean height of .  There is now an engineered surface channel to maintain maximum lake level.

Before the 1886 eruption, two small lakes were present in the current lake's basin and perhaps six smaller ponds. The other lake was called Lake Rotomakariri. After the eruption a new Lake Rotomakariri to its east briefly existed before being incorporated by lake level rise into the present lake. and it is possible to match these observations with current geolocated hydrophonic findings. Following the eruption, a number of craters filled over the course of 15 years to form today's Lake Rotomahana. As a result it is the most recently formed large natural lake in New Zealand, and at deep, separated by less than of terrain that is mostly material from the 1886 eruption. The original Lake Rotomahana has a slightly controversial level with respect to that of Lake Tarawera before 1886, partially because the eruption also changed the level of Lake Tarawera and there was a later lowering of Lake Tarawera's level around 1904. Perhaps the most likely deviation, looking at other nearby GNS stations, is only 0.5 m but for sense check purposes the biggest possible difference was allowed as GNS rate of change data was not available for one estimate. GNS datum rate of change at all Bay of Plenty sea side locations is less than a tenth of the value inland and sea level change due to ice cap melting is also not likely to be significant in the calculation since 1886. An historic assumption that the old Lake Rotomahana level was about higher than Lake Tarawera,

The original lake formed in an area of mostly rhyolytic eruptives and would have also been associated with lake sediments deposited at least in the time since the 1314 ± 12 CE Kaharoa eruption of Mount Tarawera. The eruption process which was basaltic deposited muddy material widely and many metres thick especially to the northeast. One or other of the lakes that existed before the eruption is the likely origin of shells of water-snails found, in a sample of fresh ash from Tauranga, and in an ash sample from Cape Runaway. The latest determination is 47 MW. The hydrothermal system that feeds still active geothermal features on the lake shore and had fed the Pink Terraces at the western side of the lake, has a heat flux of 21.3 W/m<sup>2</sup>. Recent research confirms Pātītī Island is the closest surviving pre-eruption feature on the old lake, i.e. to survive the 1886 eruption, being formerly known as Rangipakaru Hill. There is no public access to the lake, save for the Tourist Track, overland from Lake Tarawera.

A boat cruise on the lake, visiting hydrothermal features on the lake's shore, is available as an additional extra from the Waimangu Volcanic Valley tourism operation.

The nitrogen load on the lake is stable,

Scientists thought they had rediscovered the lower tiers of the Pink and White Terraces on the lake bed at a depth of in 2011. More recent research reports over 2016-2020 suggest the upper parts of both terraces lie on land and may therefore be accessed for physical evidence the terraces or sections of them survived in their original locations. Hochstetter's journals are the only known survey of the terraces before the eruption. Using Hochstetter's field diaries and compass data, a team of New Zealand researchers identified a location where they believe the Pink and White Terraces lie preserved at a depth of 10–15 metres (32–49 ft). The researchers were hoping to raise funds for a full survey of the area, but any work would first have to be approved by the local Māori tribe on whose sacred ancestral land the Pink and White Terraces are situated. Ground penetrating radar searches were undertaken in 2017 but the equipment failed to penetrate sufficiently deeply to show whether or not the terraces lay in their surveyed locations. Later Hochstetter survey research refined the Pink, Black and White Terrace locations. The issue of whether any of the terraces remain continues to remain unresolved. Other pre-eruption lakes and lakelets about Lake Rotomahana included Lakes Rotomakariri, Rangipakaru, Ruahoata and Wairake. The shape, location and orientation of Lake Makariri in Cole, 1970 (cited herein) is incorrect. He followed August Petermann's flawed map. Hochstetter shows the lake axis lay at an azimuth of 355 degrees. Recent research into these lake levels gave insight into changes at Lake Rotomahana in the lead-up to the eruption. In 2016 when a scuba team first dove the lake, they found no evidence of a sunken forest or trees, as reported by Fitzgerald off Moura. While there were forests over the pre-eruption Mt Tarawera, Tōtara trees were scattered and only recorded over the western and southern mountain flanks. Given Tōtara tree groves could hardly appear in the eruption craters, it appears likely any sunken forest lies in the north-east corner of the new lake.

The placement of the sunken forest would have been post-eruption, via the mechanism described by the US Forest Service after the Mt. St. Helens eruption. The trees were uprooted in the eruption and propelled into the crater during or after the eruption. As the new lake formed over decades, the trunks floated for a time, then tipped vertically; later descending into a vertical lie, and became embedded into the lake floor coming to resemble a sunken forest. Hence, another myth about the Tarawera eruption and Lake Rotomahana is explained.

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External links

Contains Figure 1 Hochstetter’s method-of-squares map 1859 of old Lake Rotomahana, which is only surveyed map pre-1886 not subject to other's artistic license
Pink & White Terraces photos
The 1886 eruption
The Pink and White terraces 2011