The caldera has seen six eruptions in the past 10,000 years, most recently the 1886 Mount Tarawera eruption in the caldera's southeastern corner. The caldera contains two major lava dome complexes, the Haroharo vent alignment in the north and Tarawera vent alignment in the south. These two vent alignments are associated with current subsidence in the last 20 years of about 1.5 cm/year (0.59 in/year) which is assumed to be because of mainly cooling and contraction of previous magma melt. Other volcanoes connected with the caldera include Putauaki (Mount Edgecumbe) and the maar crater of Lake Rotokawau which is most likely to have formed from a basaltic dike extrusion associated with the common magma mush body.

Threat

While most currently active New Zealand volcanoes produce small eruptions relatively frequently, Ōkataina's volcanoes tend to erupt very violently after intervals of centuries. As such, they pose significant potential threats to the Bay of Plenty Region but are also the most significant volcanic risk in New Zealand. During the last 20,000 years, pyroclastic and lava eruptions have occurred of several types; low-silicate basalt eruptions, high-silicate rhyolite eruptions, and the rarer intermediate andesite and dacite eruptions. The most common magma type at Ōkataina is rhyolite. The warning time before eruptions is currently suspected to be potentially hours as volcanic unrest signals are very non specific, historic composition analysis is consistent with this speed from magma reservoir to surface and this was all the warning given by the only rhyolitic eruption of the modern era.

Eruption mechanism

The underlying arc volcanism is driven initially by large inputs of basaltic melt from the subducted Pacific Plate. These basaltic melts often never reach the surface due to a relatively high density of the magma compared to the surrounding Australian Plate crust, but may trigger earthquake swarms. Usually, these intrusions cool in the crust and then either solidify to a gabbroic igneous intrusion (also known as a pluton) at depth or are associated with the generation of more evolved magmas with higher silicate content that separate. They may then as evolved intrusions, cool further without erupting to form a felsic intrusion or can ascend to then erupt as rhyolite, dacite, or andesite. Sometimes such eruptions are believed to be primed by a basaltic melt predecessor. In the case of the Ōkataina Caldera the sub-surface architecture is known to be made up of discrete melt-mush pockets, and with one dacite exception already mentioned, these are rhyolitic. The melt-mush pockets are mainly between 5 and 8 km (3.1 and 5.0 mi) in depth but one has been characterised at 3 km (1.9 mi) depth. The pockets have erupted compositionally distinct magmas in single eruptions. The composition is related to heat and volatiles transferred between the parent basalts and such rhyolites over the time the sub pocket has been maturing. Basaltic-rhyolitic magma interaction definitely happens from local and world wide studies, and will also be a factor in the many different eruption styles that have occurred. Sometimes basalt appears to lead the eruption, at other times it has been postulated that tectonic earthquakes are the final enabler of an eruption.

Any basaltic magmas that do reach the surface will have traversed this complicated crustal region and may erupt as a dyke. This is believed to have happened with the 1886 Mount Tarawera eruption.

History

It is likely that the volcanic history of the area began some 625,000 years ago. The caldera was formed by at least five huge eruptions between 400,000 and 50,000 years ago.

The oldest eruptive sub caldera is called the Utu caldera and is located in the south central portion. The basement of this sub caldera is about 5 km (3.1 mi) below present ground level.

The most significant collapse event, with an eruptive volume of 150 cubic kilometres (36 cu mi) was 280,000 years ago. This collapse was associated with eruption of the Matahina Ignimbrite which covers over 2,000 km (770 sq mi) of the central North Island. The second major phase Matahina sub caldera is to the south east and its basement is also about 5 km (3.1 mi) below present ground level. The original shape of the Matahina caldera has been modified (and buried/destroyed) by various events including at least eight smaller eruptions between 70,000 and 24,000 years ago. For example the dacite Puripuri basin/embayment is a subsidence related feature. This subsidence is related to the lateral movement of the underlying magma towards the eastern caldera margins.

The paired eruptions approximately 50,000 years ago of Rotoiti and at Earthquake Flat at far northern and southern ends of the caldera respectively had eruptive volumes of 120 cubic kilometres (29 cu mi) and 10 cubic kilometres (2.4 cu mi). The resulting Rotoiti sub caldera is to the north of the Utu Caldera.

Between this eruption and 21,000 years ago over 81 km (19 cu mi) of Mangaone silicic plinian tephras or pyroclastic flow deposits occurred but it is unknown where the eruptions were centered. One of these events can be assigned to the Kawerau ignimbrite eruption of 33,000 years ago, with its location within the central part of the Matahina Caldera at level of the Puhipuhi Basin. An area of low gravity on gravimetric studies is consistent with the fourth phase Kawerau Caldera being here and its basement being about 2 km (1.2 mi) below present ground level.

Although the latest caldera models include the Haroharo vent alignment they do not allow for the separate existence of a Haroharo caldera as many had historically postulated existed.

More recently volcanoes within the caldera are known to have erupted eleven times in the last 21,000 years, with all but two of those eruptions being rhyolite. The Rotoma eruptions occurred in a north eastern embayment, and again like with the case of the Puripuri basin, the magma erupted from a lateral reservoir is associated with subsidence back to the eastern Rotoiti caldera margin. The Ōkareka Embayment to the west is also associated with caldera rim subsidence, this time the western shared rims of the Utu, Matahina and Rotoiti calderas.

Two of these eruptions, both at Tarawera, occurred within the last 2000 years (in 1886 and c. 1314AD). The most explosive of the eruptions in the last 21,000 years is likely to have been on the Haroharo vent alignment at about 5500 BCE. This ejected some 17 km (4.1 cu mi) of magma. During the last 21,000 years the Ōkataina volcano has contributed a total magma eruptive volume of about 80 km (19 cu mi) in all its eruptions.

In summary the more significant eruptions have been:

Tectonics

Faults are not defined under this very active caldera. The active Paeroa Fault terminates at the caldera edge and the active Ngapouri-Rotomahana Fault is just to the south. The two recently active main vent alignments in the Ōkataina Caldera, the Horahora and Tarawera vents, are parallel with these identifiable faults outside the caldera, although the faults are not on the exact vent line. In the last 9,500 years, four of the seven major ruptures of the Manawahe Fault have been associated in time with a volcanic eruption of the Okataina volcanic centre. This fault is just to the east of Lake Rotoma at the boundary between the tectonic Whakatāne Graben and the magmatic Ōkataina segments of the Taupō Rift. These are the Whakatane eruption of about 5500 years ago, the Mamaku eruption of about 8000 years ago and at least two fault ruptures in before or during the Rotoma eruption of 9500 years ago. Similarly the Ngapouri-Rotomahana Fault and Paeroa Fault have multiple ruptures associated in time with volcanism including immediately prior to the Mamaku and Rotoma rhyolite eruptions in the case of the Paeroa Fault and of the Ngapouri-Rotomahana Fault immediately prior to the Kaharoa eruption. At least 30% of major Taupō Volcanic Zone eruptions have now been associated with significant local fault ruptures within 30 km (19 mi) of the eruption.