Ōkataina Caldera

Ōkataina Caldera 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, 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 Puhipuhi Embayment. The oldest parts of the caldera basement are now over deep and the younger Rotoiti and Kawerau calderas are still 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, which is assumed to be because of mainly cooling and contraction of previous magma melt. Other volcanoes connected with the caldera include Putauaki 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.
File:Charles-Blomfield-Mount-Tarawera-in-eruption-June-10-1886.jpg|thumb|centre|260px|The 1886 eruption of Mount Tarawera, depicted in this contemporary painting by Charles Blomfield, is the most recent major eruption from the Ōkataina Caldera.

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 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 in depth but one has been characterised at 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 below present ground level.
The most significant collapse event, with an eruptive volume of was 280,000 years ago. This collapse was associated with eruption of the Matahina Ignimbrite which covers over of the central North Island. The second major phase Matahina sub caldera is to the south east and its basement is also about below present ground level. The original shape of the Matahina caldera has been modified 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 and. The resulting Rotoiti sub caldera is to the north of the Utu Caldera.
Between this eruption and 21,000 years ago over 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 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. 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 of magma. During the last 21,000 years the Ōkataina volcano has contributed a total magma eruptive volume of about in all its eruptions.
In summary characterised eruptions have been:

Years ago	Calendar date	Eruptive name	Vent / Vent alignment / Caldera	VEI	Volume erupted	Notes
	16 May 1981 CE	Waimangu	Tarawera	1
	23 February 1978 CE	Waimangu	Tarawera	1
	22 February 1973 CE	Waimangu	Tarawera	1
	16 June 1951 CE	Rotomahana	Tarawera	1
	17 November 1926 CE	Rotomahana	Tarawera	1
	1924 CE	Waimangu	Tarawera	1
	1918 CE	Waimangu	Tarawera	1
	24 March 1917 CE	Waimangu	Tarawera	1		eruption continued into April
	5 November 1915 CE	Waimangu	Tarawera	1
	4 February 1915 CE	Waimangu	Tarawera	1
	28 January 1914 CE	Waimangu	Tarawera	1
	27 January 1913 CE	Waimangu	Tarawera	1
	April 1912 CE	Waimangu	Tarawera	1
	24 July 1910 CE	Waimangu	Tarawera	1
	1 October 1908 CE	Waimangu	Tarawera	1
	21 February 1906 CE	Waimangu	Tarawera	1
	17 June 1905 CE	Waimangu	Tarawera	1
	18 February 1905 CE	Waimangu	Tarawera	1
	January 1900 CE	Waimangu Geyser	Tarawera	1
	1896 CE	Waimangu	Tarawera	1
	10 June 1886 CE	Tarawera	Tarawera	5	DRE	Basaltic eruption Wahanga-Waimangu fissure
± 12 cal.yr	1314 ± 12 CE	Kaharoa tephra	Tarawera	5	DRE	This eruption was immediately preceded by a rupture on the Edgecumbe fault.
	180 CE		Tarawera	-		Te Kopia thermal area which is on Tawera alignment
3710 ± 10 cal.yr	± 10 BCE	Rotokawau	Rotokawau	4		Basaltic eruption Rotokawau to Rotoatua
5526 ± 145 cal.yr	± 145 BCE	Whakatane	Haroharo	5	DRE	Makatiti and other domes
7940 ± 257 cal.yr	± 257 BCE	Mamaku	Haroharo	5	DRE
9423 ± 120 cal.yr	± 120 BCE	Rotoma	Haroharo	5	DRE
cal.yr	8050 BCE	Rotoma	Haroharo	-		West Rerewhakaaitu fissures
14,009 ± 155 cal.yr	± 155 BCE	Waiohau tephra	Tarawera	-	DRE
15,635 ± 412 cal.yr	±412 BCE	Rotorua tephra	Haroharo	-	DRE
17,496 ± 462 cal.yr	± 462 BCE	Rerewhakaaitu tephra	Tarawera	-	DRE
23,525–370+230 cal.yr	BCE	Okareka	Tarawera	-	DRE
25,171 ± 964 cal.yr	BCE	Te Rere	Kawerau Caldera	-	DRE	33,000 years ago Kawerau, now corrected to 25,171 years ago
31,500 cal.yr	BCE	Unit L	Unknown	-	Tephra
32,500 cal.yr	BCE	Omataroa	Unknown	-	Tephra
32,800 cal.yr	BCE	Awakeri	Unknown	-	Tephra
33,000 cal.yr	BCE	Mangaone	Unknown	-	Tephra
34,500 cal.yr	BCE	Unit H	Unknown	-	Tephra
35,000 cal.yr	BCE	Unit G	Unknown	-	Tephra
36,100 cal.yr	BCE	Hauparu	Unknown	-	Tephra
36,700 cal.yr	BCE	Te Mahoe	Unknown	-	Tephra
36,800 cal.yr	BCE	Maketu	Unknown	-	Tephra
38,000 approx. cal.yr	BCE	Unit C	Unknown	-	Tephra
39,000 approx. cal.yr	BCE	Ngamotu	Unknown	-	Tephra
40,000 approx. cal.yr	BCE	Unit A	Unknown	-	Tephra
49,000 approx. cal.yr	BCE	Earthquake Flat	Earthquake Flat	-
about 50,000 cal.yr	BCE	Rotoiti/Rotoehu tephra	Rotoiti Caldera '	6+	DRE	Basalt was emplaced on the floor of the rhyolitic reservoir. This was a complex eruptive sequence and sources assign either VEI of 6 or 7. VOGRIPA assigns a magnitude of 7.4.
50,000 + cal.yr	BCE	Matahi Scoria	Suspected to be Rotoiti Caldera	-		Basaltic immediately pre-Rotoiti
about 51,000	BCE	Puhipuhi Dacite	Puhipuhi Embayment	-		48,000+, i.e., is definitely before Rotoiti but age depends on actual Rotoiti age.
96,000 approx. cal.yr	BCE	Moerangi	Moerangi Dome	-
188,000 approx. cal.yr	BCE	Tutaeheke/Hap-Kapenga	Tutaeheke Dome	-
240,000 + cal.yr	BCE	Pokopoko pyroclastics	Unknown	-
240,000 + cal.yr	BCE	Onuku pyroclastics	Unknown	-
280,000 cal.yr	278000 BCE	Matahina	Matahina Caldera	7	DRE	Recharging basalt found on top ignimbrite layer. The latest age is claimed at 322,000 ± 7,000 which appears to be a reversion to the initial uncorrected timing. Also previously timed 230,000. - large as caldera collapse
280,000 + cal.yr	BCE	Matawhaura	Matawhaura Dome	-
280,000 + cal.yr	BCE	Murupara pyroclastics	Unknown	-
280,000 + cal.yr	BCE	Wairua	Wairua Dome	-
280,000 + cal.yr	BCE	Maunawhakamana	Maunawhakamana Dome	-
280,000 + cal.yr	BCE	Whakapoungakau	Whakapoungakau Dome	-		Lost volume with Matahini eruption
557,000 cal.yr	555000 BCE	Utu	Utu Caldera	-	DRE	VOGRIPA assigns a magnitude of 7.
625,000 cal.yr	623000 BCE	Ōkataina	Ōkataina	-