| Rotorua Caldera | |
| Map: | New Zealand#North Island |
| Elevation M: | 757 |
| Coordinates: | -38.08°N 176.27°W |
| Width: | [1] |
| Last Eruption: | < 25,000 years ago[2] |
The Rotorua Caldera is a large rhyolitic caldera that is filled by Lake Rotorua. It was formed by an eruption 240,000 years ago that produced extensive pyroclastic deposits. Smaller eruptions have occurred in the caldera since, the most recent less than 25,000 years ago. It is one of several large volcanoes in the Taupō Volcanic Zone on the North Island of New Zealand.
The major regional settlement of Rotorua city is located in the caldera. There is geothermal activity in the city, and the geothermal areas of Tikitere and Whakarewarewa are associated with the caldera. These areas are still associated with small hydrothermal eruptions.[3]
The caldera was formed in a single event paired major eruption, lasting only weeks, that is now dated to 240,000 ± 11,000 years ago.[4] It ejected more than of rhyolitic Mamaku ignimbrite giving it a Volcanic Explosivity Index of 7.[2] The eruption has been reinterpreted as a paired eruption, with a very slightly later, slightly smaller southerly eruption from the same mush body that also feed the Ohakuri Caldera. Ignimbrite, up to thick covering about 3100sqkm, was deposited in the surrounding area, particularly towards the west.[1] A small but rather thick outcrop named Mokai Ignimbrite exposed to the south-west, but beyond the known boundaries of the much thinner at these boundaries, Mamaku ignimbrite, was erupted at close to the same time. This is likely from a different source to either the Mamaku or Ohakuri ignimbrite. A different source would explain interlayered ash not present in northern Mamaku ignimbrite but there is close composition homogeneity, suggesting a similar magma melt source.[1] Perhaps rather than a very directional pyroclastic flow during the eruption events from a southern vent near Rotorua, this formation is explained by more complex pairing with an unknown vent in the area of the Kapenga Caldera. Whatever the Rotorua eruption was definitely paired with an eruption from the Ohakuri Caldera away, possibly through tectonic coupling, as paired events are being increasingly recognised. The ignimbrite from Ohakuri travelled at least 17 km towards Rotorua.[5] [4]
The outflow dense-rock equivalent (DRE) of the Mamaku ignimbrite Rotorua eruption alone was up to . The maximum DME of the Ohakuri eruption alone is .[4]
Caldera collapse occurred particularly during the eruption of middle layer of Mamaku Ignimbrite and in later stages of the eruption as the magma chamber underneath the volcano empted.[1] The circular depression left behind is now filled with Lake Rotorua but the current caldera is more like two ovoids offset from each other, about 22km (14miles) in maximum diameter. Mokoia Island, close to the centre of the lake, is a rhyolite dome that later erupted. There are other domes, including Hinemoa Point, Ngongotahā, Pohaturoa and Pukeroa.
The most recent magmatic eruption occurred less than 25,000 years ago, creating some of the smaller lava domes.[2] Mokoia Island has been assigned an age of less than 50,000 years.[1]