TheOntong Java Plateau (OJP) is a massiveoceanic plateau located in the southwesternPacific Ocean, north of theSolomon Islands.The OJP was formed around116 million years ago (Ma),^[1] with a much smaller volcanic event around 90 Ma. Two other southwestern Pacific plateaus,Manihiki andHikurangi, now separated from the OJP byCretaceousoceanic basins, are of similar age and composition and probably formed as a single plateau and a contiguouslarge igneous province together with the OJP.^[2]When eruption of lava had finished, the Ontong Java–Manihiki–Hikurangi plateau covered 1% of Earth's surface and represented a volume of 80 million km³ (19 million cu mi) ofbasalticmagma.^[3]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 22 km³ (5.3 cu mi) per year over 3 million years, for a total several times larger than theDeccan Traps.^[4]The smooth surface of the OJP is punctuated byseamounts such as theOntong Java Atoll, one of the largest atolls in the world.^[5][6]

Ontong Java Plateau
Stratigraphic range: Cretaceous125–90 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Type	Igneous
Area	1.5 million km2 (580,000 sq mi)
Lithology
Primary	Basalt
Location
Coordinates	3°03′S160°23′E / 3.050°S 160.383°E /-3.050; 160.383
Region	South Pacific Ocean
Type section
Named for	Ontong Java Atoll
Ontong Java Plateau class=notpageimage| Location of the Ontong Java Plateau in the Pacific Ocean

The OJP covers 1.5 million km² (580,000 sq mi), roughly the size of Alaska. It reaches up to 1,700 m (5,600 ft) below sea level but has an average depth closer to 2,000–3,000 m (6,600–9,800 ft). 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 inactiveVitiaz Trench and thePacific–Australian plate boundary.^[5][6]

The high plateau, with a crustal thickness estimated to at least 25 km (16 mi) but probably closer to 36 km (22 mi),^[7] has a volume of more than 5 million km³ (1.2 million cu mi). 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.^[5][6] These swarms or eruptions involved the Ontong Java-Manihiki-Hikurangi plateaus.^[2]

The OJPbasaltic basement is fourtholeiitic magma series called the Kwaimbaita, Kroenke, Singgalo, and Wairahito.^[8] All except the Singgalo are isotopically identical so they are likely from the samemantle source.^[8] It can be assumed that the Singgalo basalts have a different mantle source to the rest. The Kwaimbaita basalts are dominant and in stratigraphic series the oldest.^[8][9]

OJP formed quickly over amantle plume head, most likely the then newly formedLouisville hotspot, followed by limited volcanism for at least 30 million years. The extant seamounts of theLouisville Ridge started to form 70 Ma and have a differentisotopic composition, and therefore a shift in intensity and magma supply in the plume must have occurred before that.^[10]

The early, short-duration eruptions of OJP were thought to coincide with the globalEarly Aptian oceanic anoxic event (known as OAE1a or the Selli Event, 125.0–124.6 Ma) that led to the deposition of blackshales during the interval 124–122 Ma. However, dating of the basalts to between 117 and 108 Ma makes this unlikely.^[1] There are still unresolved dates by two other research groups that are 4 Ma older so the potential association is not totally eliminated,^[11] but all OJP lavas are normally magnetized so are presumably dated after theCretaceous Normal Superchron (C34, CNS) began at 120.964 Ma^[12] Additionally, isotopic records of seawater in sediments have been associated with the 90 Ma OJP submarine eruptions.^[13]About 80% of the OJP is beingsubducted beneath the Solomon Islands. Only the uppermost 7 km (4.3 mi) of the crust is preserved on the Australian Plate.^[14]This collision haslifted some of the OJP between 200 and 2,000 m (660–6,560 ft) above sea level. The construction of Pliocene stratovolcanoes in the western end of the convergence zone has resulted in theNew Georgia Islands (1,768 m, 5,801 ft) andBougainville Island (2,743 m, 8,999 ft). Shortening, uplift, and erosion of the northern Melanesian arc and the Malaitaaccretionary prism at deep levels has producedGuadalcanal (2,447 m, 8,028 ft),Makira (1,250 m, 4,100 ft), andMalaita (1,251 m, 4,104 ft).^[15]

• Fitton, J. G.; Mahoney, J. J.; Wallace, P. J.; Saunders, A. D. (2004).Origin & Evolution of the Ontong Java Plateau: introduction(PDF). Vol. 229. Geological Society, London, Special Publications. p. 1.Bibcode:2004GSLSP.229....1F.CiteSeerX 10.1.1.560.1753.doi:10.1144/GSL.SP.2004.229.01.01.ISBN 978-1-86239-157-4.S2CID 129614562. Retrieved17 December 2016.(Ocean Drilling Program:Volume 192 Scientific Results)