The Kermadec-Tonga subduction zone is a convergent plate boundary that stretches from 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 country ...

northward. The formation of the Kermadec and Tonga Plates started about 4–5 million years ago. Today, the eastern boundary of the Tonga Plate is one of the fastest

subduction zones Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

, with a rate up to 24 cm/yr. The trench formed between the Kermadec-Tonga and Pacific Plates is also home to the second deepest trench in the world, at about 10,800 m, as well as the longest chain of submerged volcanoes.

Geological setting

The Kermadec-Tonga subduction zone is a convergent plate boundary that stretches from the southwest of the Kermadec Plate (northeast of

) to the northwest tip of the

Tonga Plate The Tonga Plate is a small southwest Pacific tectonic plate or microplate. It is centered at approximately 19° S. latitude and 173° E. longitude. The plate is an elongated plate oriented NNE - SSW and is a northward continuation of the Kermadec ...

, with the

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

being subducted under both the Kermadec and Tonga Plates. The Kermadec and Tonga Plates are micro oceanic plates in the Pacific Ocean, bounded by the

Australian Australian(s) may refer to: Australia * Australia, a country * Australians, citizens of the Commonwealth of Australia ** European Australians ** Anglo-Celtic Australians, Australians descended principally from British colonists ** Aboriginal Au ...

and Pacific Plates to the west and east respectively. The Kermadec Plate begins at the northeastern part of New Zealand and stretches northward to its contact with the Tonga Plate. The Tonga Plate begins 2500 km NNE of New Zealand and stretches northward, until the plate ends bounded by the Niuafo’ou Plate to the northwest and the Pacific Plate to the northeast. The Hikurangi Margin is an extension of the Kermadec-Tonga subduction zone further south down the east coast of the North Island.

Subduction

The southern end of the subduction zone transitions to a right lateral-moving

transform fault A transform fault or transform boundary, is a fault along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subduction ...

south of the North Island called the

Alpine Fault The Alpine Fault is a geological fault that runs almost the entire length of New Zealand's South Island (c. 480 km) and forms the boundary between the Pacific Plate and the Indo-Australian Plate. The Southern Alps have been uplifted on the f ...

. The subduction seems to be driven primarily by the excess weight of the cold/old oceanic plate entering the hot mantle of the Earth.

Trench

The eastern boundaries of the Tonga and Kermadec Plates constitute the

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...

zone of the Pacific Plate, characterized by a trench about 2,000 km in length. The trench is continuous, but has different names for different sections:

Hikurangi Trench The Hikurangi Trench, also called the Hikurangi Trough, is an oceanic trench in the bed of the Pacific Ocean off the east coast of the North Island of New Zealand, lying between the southern end of the Cook Strait and the Chatham Rise. It is the ...

, the

Kermadec Trench The Kermadec Trench is a linear ocean trench in the south Pacific Ocean. It stretches about from the Louisville Seamount Chain in the north (26°S) to the Hikurangi Plateau in the south (37°S), north-east of New Zealand's North Island. Togethe ...

and the Tonga Trench. The Tonga Trench is the second deepest trench in the world at about 10,800 m, with the deepest point, Horizon Deep, being the deepest point in the Southern Hemisphere and the second deepest point in the world, after the

Challenger Deep The Challenger Deep is the deepest-known point of the seabed of Earth, with a depth of by direct measurement from deep-diving submersibles, remotely operated underwater vehicles and benthic landers, and (sometimes) slightly more by sonar ba ...

in the

Mariana Trench The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maximum kno ...

. The eastern boundary of the Kermadec Plate is also the site of the Kermadec Trench, which is the fifth deepest trench in the world at about 10,000 m. The eastern boundary of the Tonga Plate is one of the fastest subduction zones with a rate of up to 24 cm/yr.

Creation

The Tonga and Kermadec Plates originated about 4-5 million years ago. Before their creation, the Pacific Plate was subducting under the Australian Plate, producing the Lau-Colville Ridge (now extinct). About 6 million years ago, this region underwent crustal extension and through a complicated series of spreading centers, ultimately leading to the separation of the Pacific and Australian Plate and the creation of what are now Tonga and Kermadec Plates. The Tonga and Kermadec Plates separated because the northern portion of the original plate was growing much more quickly (96 mm/year) than the southern portion (39 mm/year), eventually generating a transform fault between them, creating the Tonga and Kermadec Plates. Just as this phenomenon created the Tonga and Kermadec Plates, it was also the cause of the creation of the Niuafo’ou microplate to the northwest of the Tonga Plate because the Tonga's northern portion is still growing much faster than the southern counterpart.

Volcanism

There is extensive and currently active arc volcanism including the volcanoes of

Tonga Tonga (, ; ), officially the Kingdom of Tonga ( to, Puleʻanga Fakatuʻi ʻo Tonga), is a Polynesian country and archipelago. The country has 171 islands – of which 45 are inhabited. Its total surface area is about , scattered over in ...

, the

Kermadec Islands The Kermadec Islands ( mi, Rangitāhua) are a subtropical island arc in the South Pacific Ocean northeast of New Zealand's North Island, and a similar distance southwest of Tonga. The islands are part of New Zealand. They are in total are ...

, the South Kermadec Ridge Seamounts and the Taupō Volcanic Zone.

"Kermadec Sanctuary"

With the largest underwater volcano chain, the region surrounding the Kermadec-Tonga Subduction Zone is one of the most geologically diverse areas in the world. The Kermadec Sanctuary was proposed in 2015 by the Prime Minister of New Zealand,

John Key Sir John Phillip Key (born 9 August 1961) is a New Zealand retired politician who served as the 38th Prime Minister of New Zealand from 2008 to 2016 and as Leader of the New Zealand National Party from 2006 to 2016. After resigning from b ...

, at the United Nations in New York, which would create an area off limits to aquaculture, fishing, and mining. The sanctuary would be 620,000 square kilometers, making it the world's largest and most significant fully protected areas. The intention was to have the sanctuary in place, enacted by Parliament in November 2016. In September 2016, the enactment of the Kermadec Sanctuary was delayed due to failed negotiations over the Māori people's rights. As of June 2017, these issues have still not been resolved.

References

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External links

* https://web.archive.org/web/20110717144745/http://www.ifm-geomar.de/div/projects/zealandia/english/maps.html * http://www.teara.govt.nz/EarthSeaAndSky/OceanStudyAndConservation/SeaFloorGeology/2/en Plate tectonics Subduction zones Geographic areas of seismological interest Geology of New Zealand Seismic faults of New Zealand Volcanism of New Zealand