Lake Rotomahana is an lake in northern New Zealand, located 20 kilometres to the south-east of Rotorua. It is immediately south-west of the dormant volcano Mount Tarawera, and its geography was substantially altered by a major 1886 eruption of Mount Tarawera. Along with the mountain, it lies within the Ōkataina Caldera. It is the most recently formed larger natural lake in New Zealand, and the deepest in the Rotorua district.
History
The New Zealand Ministry for Culture and Heritage gives a translation of "warm lake" for , following Hochstetter. and the surrounds of the lake had become world famous following its first European written description in 1843.
Geology
Lake 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 it is important to note a high standard hydrophonic survey took place in 2016 and the actual absolute measurements taken at this time, as lake level varies, are used for geological baseline. At this time the lake level was , not the geographically mapped height of , or the mean height of . Lake Rotomahana has no natural surface outlet, and its water level varies by about one meter in response to rainfall and evaporation. There is now an engineered surface channel to maintain maximum lake level. Hydrogeologic models of the catchments in the Ōkataina Caldera predict that the Lake Rotomahana catchment is contributing subsurface to the Lake Tarawera catchment with an outflow, depending upon the size of the catchments so defined, of between .
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. The now lake floor was mostly dry when surveyed in the period 1886 to 1888 after the eruption, and it is possible to match these observations with current geolocated hydrophonic findings. High quality pictures of the old Lake Rotomahana and associated tourist attractions were widely available in Europe by 1875. 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 larger natural lake in New Zealand, and at deep, the deepest in the Rotorua district. The former official depth was previously , at a lower lake level and with less precise equipment. The lake bottom currently has up to of sediments, which means the Rotomahana crater bottom is above sea level.
The lake's northern shore lies close to the lower Lake Tarawera, 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 about 1904.
The consensus range of difference with current water level between the old and current Lake Rotomahana appears to be .
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 meters thick especially to the north-east. 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. Essentially the new crater evacuated material to a depth of at least where the pre-eruption lakes had been and possibly as much as in places.
The current average conductive heat flux is at least three times higher than that either beneath Lake Rotorua or Lake Taupō. The latest determination is 47 MW. The hydrothermal system that feeds still active geothermal features on the lake shore and had feed the Pink Terraces at the western side of the lake, has a heat flux of 21.3 W/m2. There is an area southwest of Pātītī Island that has a heat flux averaging 13 W/m2 that appears to be in a lake floor crater created in 1886.