The Ōkāreka Embayment (also spelled Okareka or Ōkareka) is a volcanic feature in

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

of New Zealand. Its most significant recent volcanic eruption was about 15,700 years ago and this deposited the widespread Rotorua

tephra Tephra is fragmental material produced by a Volcano, volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, ...

that reached beyond Auckland.

Geography

The Ōkāreka Embayment extends from the western margin of

Lake Tarawera Lake Tarawera is the largest of a series of lakes which surround the volcano Mount Tarawera in the North Island of New Zealand. Like the mountain, it lies within the Ōkataina Caldera. It is located to the east of Rotorua, and beneath the peak ...

to include to its north

Lake Ōkāreka A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from t ...

and in the west Lake Tikitapu and the eastern half of

Lake Rotokākahi Lake Rotokākahi or Green Lake, is one of four small lakes lying between Lake Rotorua and Lake Tarawera in the Bay of Plenty Region of New Zealand's North Island The North Island ( , 'the fish of Māui', historically New Ulster) is one of ...

Geology

Both the Ōkāreka Embayment and the Tarawera volcanic complex are inside the old

Ōkataina Caldera Ōkataina Caldera (Ōkataina Volcanic Centre, also spelled Okataina) is a volcano, 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 ...

, often termed the Ōkataina volcanic centre. The caldera forming its eastern boundary has been called the

Haroharo Caldera The Haroharo Caldera (Haroharo volcanic complex) is a postulated volcanic feature in Taupō Volcanic Zone of the North Island, New Zealand within the larger and older Ōkataina Caldera. Since 2010 further studies have tended to use the terms H ...

, but as no major single event formed it, this is perhaps better regarded as a general categorisation term, explaining why some maps of the Horahora Caldera include the embayment. Citing It is now regarded as a subsidiary volcanic part of the Ōkataina Caldera related to a mechanism of collapse and subsidence at the edges of a major event caldera due to lateral movement of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

. The Ōkataina Caldera has in the last 21,000 years contributed a total magma eruptive volume greater than about .

Eruptions

The Northern Dome, just to the east of Lake Tikitapu formed 25,171 ± 964 years ago in the Te Rere rhyolite eruption which also had other vents in the Ōkataina Volcanic Complex. Such domes typically form over a vent that have an initial pyroclastic eruption and the vent(s) for this eruption lies under the Northern and Eastern domes. The Te Rere

deposits from the initial pyroclastic eruption had a volume of and are widely distributed. The DRE volume of all the Te Rere eruptions totals about . All the vents in the Ōkareka Embayment lie on the Haroharo linear vent zone's western end. The Rotorua eruption now dated at 15,635 ± 412 cal.yr BP, was a two phase eruption commencing with a

plinian eruption Plinian eruptions or Vesuvian eruptions are volcanic eruptions characterized by their similarity to the eruption of Mount Vesuvius in 79 AD, which destroyed the ancient Roman cities of Herculaneum and Pompeii. The eruption was described in a le ...

that deposited of material to the north-west from a vent now under the Trig 7693 dome and that lasted no more than 4 days. Significant ash cover was towards the Rotorua area (hence the name) but ash fall was as far away as

Auckland Auckland ( ; ) is a large metropolitan city in the North Island of New Zealand. It has an urban population of about It is located in the greater Auckland Region, the area governed by Auckland Council, which includes outlying rural areas and ...

with a total ash volume of including later minor ash deposits. The largely degassed magma body then in a dominantly effusive rhyolite dome forming process built up Trig 7693 and Middle Dome to the south east of the Ōkāreka Embayment over several years, but no more than 6, to a total volume of .

Tephra Context

As far back as 1839, a German explorer Dr Ernst Dieffenbach described near Rotoroa the first recorded description of layered

s from ash fall in New Zealand. However correlation between eruption years and what has been coined as tephrostratigraphy is not straightforward where there is no historical written record.

Radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for Chronological dating, determining the age of an object containing organic material by using the properties of carbon-14, radiocarbon, a radioactive Isotop ...

was later used in the vicinity, to date recent eruptions, as deposition of each tephra was followed by a period of quiescence and soil formation. Such a series as published in 1990 (so the dates may have been modified by scientific discourse since) reads (with some translation from original jargon) : #Rotomahana Mud 1886 CE ( Tarawera) #Kaharoa 1314 CE ( Tarawera) #Taupo 232 CE ( Taupo) #Rotokawau 3600 years before 1950 ( Rotokawau craters) #Whakatane 5500 years before 1950 ( Haroharo) #Mamaku 8000 years before 1950 ( Haroharo) #Rotoma 9500 years before 1950 ( Haroharo) #Waiohau 14,000 years before 1950 ( Tarawera) #Rotorua 15,600 years before 1950 (Okareka/Haroharo) #Rerewhakaaitu (pale layer within grayish loess deposits) 17,500 years before 1950 ( Tarawera). The impact on the Waikato region must have been marked as lake sediment from near

Hamilton, New Zealand Hamilton (, ) is an inland city in the North Island of New Zealand. Located on the banks of the Waikato River, it is the seat and most populous city of the Waikato, Waikato region. With a territorial population of , it is the country's List of c ...

shows evidence of very active plant turnover just before almost of tephra is deposited from the Rotorua event. The soils between the tephra layers in the Ōkāreka Embayment have been analysed and are consistent with a cold dry climate between 25.2 ka to 14 ka and a more wet and warm climate since. Okareka Tephra was produced from vents in an eruption of

Mount Tarawera Mount Tarawera is a volcano on the North Island of New Zealand within the older but volcanically productive Ōkataina Caldera. Located 24 kilometres southeast of Rotorua, it consists of a series of rhyolitic lava domes that were fissured ...

23,535 ± 300 years BP, so does not have an origin in the embayment.

Risk

A repeat of the Rotorua eruption with its ash distribution against the prevailing winds but towards the major population centres of Rotoroa,

Hamilton Hamilton may refer to: * Alexander Hamilton (1755/1757–1804), first U.S. Secretary of the Treasury and one of the Founding Fathers of the United States * ''Hamilton'' (musical), a 2015 Broadway musical by Lin-Manuel Miranda ** ''Hamilton'' (al ...

and

would be very destructive and disruptive. The town of Rotorua would be made uninhabitable by a ash fall, as happened in the Rotorua eruption. This would collapse all normally built homes and of land would be denuded of all vegetation.

References

{{Reflist, 32em Taupō Volcanic Zone Calderas of New Zealand Rift volcanoes Okataina Volcanic Centre VEI-5 volcanoes Holocene calderas