The Ontong Java Plateau (OJP) is a massive

oceanic plateau An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides. There are 184 oceanic plateaus in the world, covering an area of or about 5.11% of the ...

located in the southwestern

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

, north of the

Solomon Islands Solomon Islands is an island country consisting of six major islands and over 900 smaller islands in Oceania, to the east of Papua New Guinea and north-west of Vanuatu. It has a land area of , and a population of approx. 700,000. Its capit ...

. The OJP was formed around (Ma) with a much smaller volcanic event around 90 Ma. Two other southwestern Pacific plateaus, Manihiki and

Hikurangi Hikurangi is a settlement in Northland, New Zealand. The city of Whangārei is 17 km (11 miles) to the south, and Kawakawa is 39 km (24 miles) northwest. The Glenbervie Forest is southeast of the settlement. State Highway ...

, now separated from the OJP by

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

oceanic basin In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, ocean basins are large  geologic basins that are below sea level. Most commonly the ocean is divided into basins fol ...

s, are of similar age and composition and probably formed as a single plateau and a contiguous

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 formation ...

together with the OJP. When eruption of lava had finished, the Ontong Java–Manihiki–Hikurangi plateau covered 1% of Earth's surface and represented a volume of of

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...

. This "Ontong Java event", first proposed in 1991, represents the largest volcanic event of the past 200 million years, with a magma eruption rate estimated at up to per year over 3 million years, several times larger than the

Deccan Traps The Deccan Traps is a large igneous province of west-central India (17–24°N, 73–74°E). It is one of the largest volcanic features on Earth, taking the form of a large shield volcano. It consists of numerous layers of solidified flo ...

. The smooth surface of the OJP is punctuated by

seamount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise a ...

s such as the

Ontong Java Atoll Ontong Java Atoll or Luangiua, (formerly ''Lord Howe Atoll'', not to be confused with Lord Howe Island) is one of the largest atolls on earth. Geographically it belongs to a scattered group of three atolls which includes nearby Nukumanu Atol ...

, one of the largest atolls in the world.

Geological setting

The OJP covers , roughly the size of Alaska. It reaches up to below sea level but has an average depth closer to . It is bounded by Lyra Basin to the northwest, East Mariana Basin to the north, Nauru Basin to the northeast, and the Ellice Basin to the southeast. The OJP has collided with the Solomon Islands island arc and now lies on the inactive Vitiaz Trench and the

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

–

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 A ...

plate boundary. The high plateau, with a crustal thickness estimated to at least but probably closer to , has a volume of more than . The maximum extent of the event can, however, be much larger since lavas in several surrounding basins are closely related to the OJP event and probably represent dike swarms associated with the formation of the OJP.

Tectonic evolution

OJP formed quickly over a

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 hot ...

head, most likely the then newly formed

Louisville hotspot The Louisville hotspot is a volcanic hotspot responsible for the volcanic activity that has formed the Louisville Ridge in the southern Pacific Ocean. Location The Louisville hotspot is believed to lie close to the Pacific-Antarctic Ridge, alt ...

, followed by limited volcanism for at least 30 million years. The extant seamounts of the

Louisville Ridge The Louisville Ridge, also known as the Louisville Seamount Chain, is an underwater chain of over 70 seamounts located in the Southwest portion of the Pacific Ocean. As one of the longest seamount chains on Earth it stretches some Vanderkluysen, ...

started to form 70 Ma and have a different isotopic composition, and therefore a shift in intensity and magma supply in the plume must have occurred before that. The early, short-duration eruptions of OJP coincide with the global Early Aptian oceanic anoxic event (known as OAE1a or the Selli Event, 125.0–124.6 Ma) that led to the deposition of black

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

s during the interval 124–122 Ma. Additionally, isotopic records of seawater in sediments have been associated with the 90 Ma OJP submarine eruptions. About 80% of the OJP is being subducted beneath the Solomon Islands. Only the uppermost of the crust is preserved on the Australian Plate. This collision has lifted some of the OJP between above sea level. The construction of Pliocene stratovolcanoes in the western end of the convergence zone has resulted in the

New Georgia Islands The New Georgia Islands are part of the Western Province of Solomon Islands. They are located to the northwest of Guadalcanal. The larger islands are mountainous and covered in rain forest. The main islands are New Georgia, Vella Lavella, Ko ...

() and

Bougainville Island Bougainville Island (Tok Pisin: ''Bogenvil'') is the main island of the Autonomous Region of Bougainville, which is part of Papua New Guinea. It was previously the main landmass in the German Empire-associated North Solomon Islands, North Solo ...

(). Shortening, uplift, and erosion of the northern Melanesian arc and the Malaita

accretionary prism An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the do ...

at deep levels has produced

Guadalcanal Guadalcanal (; indigenous name: ''Isatabu'') is the principal island in Guadalcanal Province of Solomon Islands, located in the south-western Pacific, northeast of Australia. It is the largest island in the Solomon Islands by area, and the se ...

(),

Makira The island of Makira (also known as San Cristobal and San Cristóbal) is the largest island of Makira-Ulawa Province in the Solomon Islands. It is third most populous island after Malaita and Guadalcanal, with a population of 55,126 as of 2020. ...

(), and

Malaita Malaita is the primary island of Malaita Province in Solomon Islands. Malaita is the most populous island of the Solomon Islands, with a population of 161,832 as of 2021, or more than a third of the entire national population. It is also the se ...

Geological setting

Tectonic evolution

References

Notes

Sources

Further reading