Rotorua Caldera

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Rotorua Caldera
Rotorua.Caldera.Mount.Tarawera.JPG
NASA image identifying the Rotorua Caldera. South of the lake that fills much of the apparently circular caldera is the town of Rotorua. The caldera is a more complex shape with areas of collapse and the Tikitere Graben at its outlet. Mount Tarawera is in the lower right corner south east of the caldera and it and the lakes to the east are features of the adjacent active Ōkataina Caldera. The eruption products are thickest towards the north east.
Highest point
Elevation757 m (2,484 ft)
Coordinates38°05′S 176°16′E / 38.08°S 176.27°E / -38.08; 176.27
Dimensions
Width22 kilometres (14 mi)[1]
Geography
Rotorua Caldera is located in New Zealand
Rotorua Caldera
Rotorua Caldera
Rotorua Caldera is located in North Island
Rotorua Caldera
Rotorua Caldera
Rotorua Caldera (North Island)
CountryNew Zealand
RegionWaikato
Geology
Age of rockMamaku ignimbrite approximately 240,000 years[2]
Mountain typeCaldera
Volcanic arc/beltTaupo Volcanic Zone
Last eruption< 25,000 years ago[3]

The Rotorua Caldera, now in filled with Lake Rotorua, is a large rhyolitic caldera. It is one of several large volcanoes located in the Taupo Volcanic Zone on the North Island of New Zealand.

Geography[edit]

Downtown Rotorua, Lake Rotorua, and Mokoia Island

The major regional settlement of Rotorua is located in the caldera. There is geothermal activity in the town of Rotorua and the geothermal areas of Tikitere and Whakarewarewa are associated with the caldera. These areas are still associated with small hydrothermal eruptions.[4]

Geology[edit]

Eruption history[edit]

The caldera was formed in a single event major eruption, that lasted only weeks about 240,000 years ago, and which ejected more than 340 cubic kilometres (82 cu mi) of rhyolitic Mamaku ignimbrite giving it a Volcanic Explosivity Index of 7.[3] The eruption has been reinterpreted as a paired eruption with a very slightly later, slightly smaller southerly eruption from the same mush body feeding the Ohakuri Caldera.[2] Ignimbrite, up to 145 metres (476 ft) thick covering about 3,100 km2 (1,200 sq mi), 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 much thinner at these boundaries, Mamaku ignimbrite, was erupted at close to the same time but likely from a different source. A different source would explain interlayered ash not present in northern Mamaku ignimbrite but there was 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 paired with one from the Ohakuri Caldera 30 kilometres (19 mi) away, possibly through tectonic coupling, as paired events are being increasingly recognised and ignimbrite from Ohakuri has travelled at least 17km towards Rotorua.[5][6][2] The outflow dense-rock equivalent (DRE) of the Mamaku ignimbrite Rotorua eruption alone was up to 145 cubic kilometres (35 cu mi).[2] The maxiumum DME of the Ohakuri eruption alone is 100 cubic kilometres (24 cu mi).[6]

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] A circular depression left behind is now filled with Lake Rotorua but the current caldera is more like two ovoids offset from each other, about 22 km (14 mi) in maximum diameter. Mokoia Island, close to the centre of the lake, is a rhyolite dome that later erupted. There are other domes like 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.[3] Mokoia Island has been assigned an age of less than 50,000 years.[1]

240,000 years ago Ohakuri paired eruption[edit]

Paired eruptions Rotorua(Mamaku) and Ohakuri
Approximate location and outlines (white) of the paired single event Rotorua and Ohakuri calderas with approximate known surrounding Mamaku (blue)[2] and Ohakuri ignimbrites (yellow phase 1 and dark yellow phase 2).[7] The relationship to the inactive southern portion (red dots) and currently active northern portion (red line) of the Horohoro Fault is shown. Relationships also shown to old Taupō Rift (light yellow shading), modern Taupō Rift (light red shading) and Hauraki Rift (light purple shading). The present landmarks of Lake Taupō and Mount Tarawera are labelled for orientation

The first major volcanic event 240,000 years ago was the initial Mamaku eruption followed within a hours/days/weeks of a smaller eruption (phase 1) from the same mush body feeding the Ohakuri Caldera about 30 km (19 mi) to the south.[2] Ignimbrite, up to 180 metres (590 ft) thick was deposited in the surrounding area to the south of Rotorua.[1] Between Rotorua and Ohakuri crosssections of the ash and ignimbrite from the two eruptions have been able to be sequenced completely and have relationships that can only be explained by a sequence of eruptions separated on occasions by days or less (e.g. no rainfall between eruptions).[7] The pairing was possibly through tectonic coupling of separate magma bodies that co-evolved from a lower in the mantle common mush body, as paired events are being increasingly recognised.[6] The maximum outflow dense-rock equivalent (DRE) of the Ohakuri ignimbrite is 100 cubic kilometres (24 cu mi) which means the combined eruptions produced 245 cubic kilometres (59 cu mi) of material.[2] It has been postulated that the drainage of the linked deep magma mush body between Rotorua and Ohakuri resulted in more than 250 metres (820 ft) of vertical displacement on the Horohoro Fault scarp and formed the Paeroa Graben, coincident to the north with the Kapenga Caldera between it and the Paeroa Fault to the east.[7] This is an area known as the Horohoro Cliffs escarpment and displaced Mamaku ignimbrite from the Rotorua Caldera eruption by this amount, presumably shortly after at least the initial the eruption. This fault, in the present day, while active has a much lower displacement rate of the order of 0.14 millimetres (0.0055 in)/year and has been assigned by some as the outer western fault of the modern Taupō Rift although most think this is further to the east.[8] Understanding that there is volcanotectonic interrelationship lead to a complete reinterpretation of events in the Taupō Volcanic Zone in the last 250,000 years.[6]

See also[edit]

References[edit]

  1. ^ a b c d e f Milner, David M (2001). The structure and eruptive history of Rotorua Caldera, Taupo Volcanic Zone, New Zealand (Thesis).
  2. ^ a b c d e f g Bégué, F.; Deering, C. D.; Gravley, D. M.; Kennedy, B. M.; Chambefort, I.; Gualda, G. A. R.; Bachmann, O. (2014). "Extraction, Storage and Eruption of Multiple Isolated Magma Batches in the Paired Mamaku and Ohakuri Eruption, Taupo Volcanic Zone, New Zealand". Journal of Petrology. 55 (8): 1653–1684. doi:10.1093/petrology/egu038.
  3. ^ a b c "Rotorua". Global Volcanism Program. Smithsonian Institution.
  4. ^ "Steam Explosions Rock New Zealand's Rotorua Caldera".
  5. ^ Loame, Remedy Charlotte (2016). Using a tephrostratigraphic framework to determine the past 40,000 yrs of fault rupture and paleohydrothermal activity on the east strand of the Whirinaki Fault, Ngakuru Graben, central Taupo Volcanic Zone (PDF) (Thesis).
  6. ^ a b c d Gravley, D.M.; Wilson, C.J.N.; Leonard, G.S.; Cole, J.W. (2007). "Double trouble: Paired ignimbrite eruptions and collateral subsidence in the Taupo Volcanic Zone, New Zealand". GSA Bulletin. 119 (1–2): 18–30. doi:10.1130/B25924.1.
  7. ^ a b c Gravley, Darren MClurg (2004). "The Ohakuri pyroclastic deposits and the evolution of the Rotorua-Ohakuri volcanotectonic depression" (PDF). Retrieved 17 August 2022.
  8. ^ Zachariasen, Judith; Van Dissen, Russ (2001). "Paleoseismicity of the northern Horohoro Fault, Taupo Volcanic Zone, New Zealand". New Zealand Journal of Geology and Geophysics. 44 (3): 91–40. doi:10.1080/00288306.2001.9514946.

External links[edit]