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 contine ...

, 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 ca ...

. The OJP was formed around (Ma) with a much smaller volcanic event around 90 Ma. Two other southwestern Pacific plateaus, Manihiki and Hikurangi, 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 basins, are of similar age and composition and probably formed as a single plateau and a contiguous large igneous province 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% of a ...

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 flood ...

. 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, 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 continen ...

– Australian 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, 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, especia ...

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 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 ...

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 () 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 Solomons. Its land area ...

(). Shortening, uplift, and erosion of the northern Melanesian arc and the Malaita accretionary prism 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 202 ...

(), 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