The Rotorua Caldera, now in filled with

Lake Rotorua , image = Lake Rotorua.jpg , caption = Lake Rotorua , alt = Lake Rotorua , image_bathymetry = , pushpin_map=New Zealand#North Island , pushpin_map_alt = Location of Lake Rotorua , pushpin_relief=yes , caption_bathymetry = , location = R ...

, is a large

rhyolitic Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

. It is one of several large

volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates are ...

es located in the

Taupō Volcanic Zone The Taupō Volcanic Zone (TVZ) is a volcanic area in the North Island of New Zealand that has been active for the past two million years and is still highly active. Mount Ruapehu marks its south-western end and the zone runs north-eastward thro ...

on the

North Island The North Island, also officially named Te Ika-a-Māui, is one of the two main islands of New Zealand, separated from the larger but much less populous South Island by the Cook Strait. The island's area is , making it the world's 14th-largest ...

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

Geography

The major regional settlement of

Rotorua Rotorua () is a city in the Bay of Plenty region of New Zealand's North Island. The city lies on the southern shores of Lake Rotorua, from which it takes its name. It is the seat of the Rotorua Lakes District, a territorial authority encompass ...

is located in the caldera. There is geothermal activity in the city of Rotorua and the geothermal areas of

Tikitere Tikitere, also known as "Hell's Gate", is a suburb in Rotorua's most active geothermal area on State Highway 30, between Lake Rotorua and Lake Rotoiti in the Bay of Plenty, New Zealand. It includes many geothermal features such as steaming ...

and

Whakarewarewa Whakarewarewa (reduced version of Te Whakarewarewatanga O Te Ope Taua A Wahiao, meaning ''The gathering place for the war parties of Wahiao'', often abbreviated to Whaka by locals) is a Rotorua semi-rural geothermal area in the Taupo Volcanic ...

are associated with the caldera. These areas are still associated with small hydrothermal eruptions.

Geology

Eruption history

The caldera was formed in a single event paired major eruption, that lasted only weeks, is now dated about 240,000 years ago, and which ejected more than of rhyolitic Mamaku

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...

giving it a Volcanic Explosivity Index of 7. 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.

Ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...

, up to thick covering about , was deposited in the surrounding area, particularly towards the west. 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. Perhaps rather than a very directional

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of bu ...

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 away, possibly through tectonic coupling, as paired events are being increasingly recognised and ignimbrite from Ohakuri has travelled at least 17km towards Rotorua. The outflow

dense-rock equivalent Dense-rock equivalent (DRE) is a volcanologic calculation used to estimate volcanic eruption volume. One of the widely accepted measures of the size of a historic or prehistoric eruption is the volume of magma ejected as pumice and volcanic ash, k ...

(DRE) of the Mamaku

Rotorua eruption alone was up to . The maximum DME of the Ohakuri eruption alone is . Caldera collapse occurred particularly during the eruption of middle layer of Mamaku Ignimbrite and in later stages of the eruption as the

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it upw ...

underneath the volcano empted. A circular depression left behind is now filled with

but the current

is more like two ovoids offset from each other, about in maximum diameter.

Mokoia Island __NOTOC__ Mokoia Island is located in Lake Rotorua in New Zealand. It has an area of 1.35 square kilometres. The uninhabited island is a rhyolite lava dome, rising to 180 metres above the lake surface. It was formed after the Rotorua caldera c ...

, close to the centre of the lake, is a

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

dome that later erupted. There are other domes like Hinemoa Point, Ngongotahā, Pohaturoa and Pukeroa. The most recent magmatic

eruption Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often ...

occurred less than 25,000 years ago, creating some of the smaller lava domes. Mokoia Island has been assigned an age of less than 50,000 years.

240,000 years ago Ohakuri paired eruption

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 to the south.

, up to thick was deposited in the surrounding area to the south of Rotorua. 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). The pairing was possibly through tectonic coupling of separate magma bodies that co-evolved from a lower in the

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

common mush body, as paired events are being increasingly recognised. The maximum outflow dense-rock equivalent (DRE) of the Ohakuri ignimbrite is 100 cubic kilometres (24 cu mi) which means the combined eruptions produced of material. It has been postulated that the drainage of the linked deep magma mush body between

and Ohakuri resulted in more than 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. 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 /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. Understanding that there is volcanotectonic interrelationship lead to a complete reinterpretation of events in the

in the last 250,000 years.

References

External links