The Tonga–Kermadec Ridge is an

oceanic ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a div ...

in the south-west

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

underlying the

Tonga Tonga, officially the Kingdom of Tonga, is an island country in Polynesia, part of Oceania. The country has 171 islands, of which 45 are inhabited. Its total surface area is about , scattered over in the southern Pacific Ocean. accordin ...

–

Kermadec Kermadec or de Kermadec may refer to: Geography * Kermadec Islands, a subtropical island arc in the South Pacific Ocean northeast of New Zealand * Kermadec Plate, a long narrow tectonic plate located west of the Kermadec Trench * Kermadec Trench, ...

island arc Island arcs are long archipelago, chains of active volcanoes with intense earthquake, seismic activity found along convergent boundary, convergent plate tectonics, tectonic plate boundaries. Most island arcs originate on oceanic crust and have re ...

. It is a result of the most linear, fastest converging, and seismically active

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

boundary on Earth, the Kermadec–Tonga subduction zone, and consequently has the highest density of

submarine volcano Submarine volcanoes are underwater vents or fissures in the Earth's surface from which magma can erupt. Many submarine volcanoes are located near areas of tectonic plate formation, known as mid-ocean ridges. The volcanoes at mid-ocean ridges ...

es. The Tonga–Kermadec Ridge stretches more than north-northeast from

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

North Island The North Island ( , 'the fish of Māui', historically New Ulster) is one of the two main islands of New Zealand, islands of New Zealand, separated from the larger but less populous South Island by Cook Strait. With an area of , it is the List ...

. The Pacific Plate

subducts Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second pla ...

westward beneath the

Australian Plate The Australian plate is or was a major tectonic plate in the eastern and, largely, southern hemispheres. Originally a part of the ancient continent of Gondwana, Australia remained connected to India and Antarctica until approximately when Indi ...

along the ridge. It is divided into two segments, the northern Tonga Ridge and southern Kermadec Ridge, by the

Louisville Seamount Chain The Louisville Ridge, often now referred to as the Louisville Seamount Chain, is an underwater chain of over 70 seamounts located in the Southwest portion of the Pacific Ocean. One of the longest seamount chains on Earth, it stretches some Vander ...

. On its western side, the ridge is flanked by two

back-arc basin A back-arc basin is a type of geologic Structural basin, basin, found at some convergent boundary, convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found ...

s, the

Lau Basin The Lau Basin is a back-arc basin (also addressed as "interarc basin") at the Australian-Pacific plate boundary. It is formed by the Pacific plate subducting under the Australian plate. The Tonga-Kermadec Ridge, a frontal arc, and the Lau-Colvill ...

and

Havre Trough The Havre Trough (Havre Basin) is a currently actively rifting back-arc basin about to wide, between the Australian Plate and Kermadec microplate. The trough extends northward from New Zealand's offshore Taupō Volcanic Zone commencing at Zea ...

, that began opening at 6 and 2 Ma respectively. Beyound the basins is the Lau-Colville Ridge. Together with these seafloor structures the ridge forms the eastward-migrating, 100 million year old Lau–Tonga–Havre–Kermadec arc/back-arc system or complex. The extension in the Lau–Havre basin results in a higher rate of subduction than convergence along the Australian–Pacific plate boundary. The rates of extension, subduction, and convergence all increase northwards in this complex, subduction at a rate of and extension at a rate of . As a result, the Tonga–Kermadec Ridge moves independently of both tectonic plates and forms the Tonga–Kermadec plate, in its turn fragmented into the Niuafo'ou,

, and

microplates. The

Samoa Samoa, officially the Independent State of Samoa and known until 1997 as Western Samoa, is an island country in Polynesia, part of Oceania, in the South Pacific Ocean. It consists of two main islands (Savai'i and Upolu), two smaller, inhabited ...

and

Louisville Louisville is the most populous city in the Commonwealth of Kentucky, sixth-most populous city in the Southeast, and the 27th-most-populous city in the United States. By land area, it is the country's 24th-largest city; however, by populatio ...

mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic ho ...

s both contribute to the lavas of two of the northern Tonga islands,

Tafahi Tafahi is a small () island in the north of the Tonga archipelago, in fact closer to Savaii (Samoa) than to the main islands of Tonga. It is only north-northeast away from Niuatoputapu, and fishermen commute in small outboard motorboats almost ...

and

Niuatoputapu Niuatoputapu (''The Sacred Niua'') is a volcanic island in the island nation of Tonga, Pacific Ocean. Its highest point is , and its area is . Its name means ''sacred island''. Older names for the island are Traitors Island or Keppel Island. ...

;

ocean island basalt Ocean island basalt (OIB) is a volcanic rock, usually basaltic in composition, erupted in oceans away from tectonic plate boundaries. Although ocean island basaltic magma is mainly erupted as basalt lava, the basaltic magma is sometimes modified ...

(OIB) from the Samoa plume were introduced from 3–4 Ma when subduction in the

Vitiaz Trench The Vityaz Trench (also known as the Vitiaz or East Melanesian Trench) is an oceanic trench tectonic feature of the South West Pacific Ocean floor. History It is named after the Russian research vessel '' Vityaz'' that discovered it in 1958. ...

(north-west of Tonga) ceased. The lavas of the Louisville Seamount Chain were generated 80–90 Ma but began to subduct under the Tonga–Kermadec Ridge at 8 Ma. The

Hikurangi Hikurangi is a settlement in Northland Region, Northland, New Zealand. The city of Whangārei is to the south, and Kawakawa, New Zealand, Kawakawa is northwest. The Glenbervie Forest is southeast of the settlement. New Zealand State Highway ...

and

Manihiki 250px, Map of Manihiki Atoll Manihiki is an atoll in the northern group of the Cook Islands known informally as the "Island of Pearls". It is located in the Northern Cook Island chain, approximately north of the capital island of Rarotonga, ...

plateaux, north and south of the Tonga–Kermadec Ridge respectively, form part of the

Ontong Java Ontong Java Atoll or Luangiua (formerly Lord Howe Atoll, not to be confused with Lord Howe Island) is an atoll in the Solomon Islands, and one of the largest atolls on earth. It is inhabited by a Polynesian community of about people, who spe ...

-Hikurangi-Manihiki

large igneous province A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive ( sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The format ...

(LIP), the largest volcanic event on Earth during the past 200 million years. The

Osbourn Trough The Osbourn Trough, is a -long extinct mid-ocean ridge, that may have stopped spreading as recently as 79 million years ago. It is a west-to-east oriented sea floor feature, located to the east of the present Tonga-Kermadec Ridge where the presen ...

, located just north of the Tonga–Kermadec and Louisville intersection, is the palaeo-spreading centre between the Hikurangi and Manihiki plateaux away from which the age of the Pacific Plate increases from 85 Ma to 144 Ma. The subduction of the Hikurangi Plateau beneath New Zealand and the southern part of the Kermadec Arc has resulted in large volumes of lava and a high density of volcanoes in the arc. The initial Hikurangi-Kermadec collision, however, occurred to the north where a missing piece of the Ontong Java-Hikurangi-Manihiki LIP has already been subducted.

References

; Notes ; Sources * * * * * * {{Oceanic features of Zealandia, state=collapsed Underwater ridges of the Pacific Ocean Geography of the New Zealand seabed Subduction zones Seismic zones of Oceania

See also

References