Ōkataina Caldera (Ōkataina Volcanic Centre, also spelled Okataina) is a volcanic caldera and its associated volcanoes located in Taupō Volcanic Zone of New Zealand's North Island. It has several actual or postulated sub calderas. The Ōkataina Caldera is just east of the smaller separate Rotorua Caldera and southwest of the much smaller Rotomā Embayment, which is usually regarded as an associated volcano. It shows high rates of explosive rhyolitic volcanism, although its last eruption was basaltic. The postulated Haroharo Caldera contained within it has sometimes been described in almost interchangeable terms with the Ōkataina Caldera or volcanic complex or centre and by other authors as a separate complex defined by gravitational and magnetic features. Since 2010 other terms such as the Haroharo vent alignment, Utu Caldera, Matahina Caldera, Rotoiti Caldera and a postulated Kawerau Caldera are often used, rather than a Haroharo Caldera classification.
Geography The caldera covers an area of about 450 square kilometres (170 mi2), stretching from Lake Rotoehu in the north to Lake Rotomahana in the south. The north east boundary bisects Lake Rotoiti and the north east includes all of Lake Rotomā. The south west corner is defined by the domes of the Ōkareka Embayment and the Waimangu Volcanic Rift Valley while the south east aspect is dominated by Mount Tarawera and the volcanic badlands of the Puhipuhi Basin. The caldera also contains several lakes, including part or all of Lake Ōkareka, Lake Ōkataina, Lake Rotoehu, Lake Rotomā, Lake Rotoiti, Lake Rotomahana, Lake Tarawera and Lake Tikitapu.
Geology The surface volcanic deposits are mostly rhyolite, with some basalt and one area of dacite. The caldera is now thought to contain the Utu Caldera, the Matahina Caldera, the Rotoiti Caldera, and the Kawerau Caldera, with the Matahina major event about 322,000 years ago and Rotoiti events about 55,000 years ago being characterised as caldera forming. There are three associated geologically collapse structure embayments, being the Rotomā Embayment or Caldera, the Ōkareka Embayment as another now in-filled structure, and the most recently formed (44–31,000 years ago) Puhipuhi Embayment. The oldest parts of the caldera basement are now over 5 km (3.1 mi) deep and the younger Rotoiti and Kawerau calderas are still 2.5 km (1.6 mi) deep and largely infilled by eruptives.
Eruptions
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 the currently most 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, and are the most significant volcanic risk in New Zealand. During the last 20,000 years, several types of pyroclastic and lava eruptions have occurred: 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 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.
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