Adams Seamount, Forty Mile Reef
Summit depth	39 m (128 ft)[1]
Height	3,500 m (11,500 ft)
Location
Location	Pacific Ocean, SW of Pitcairn Island
Coordinates	25°22′S 129°16′W / 25.367°S 129.267°W / -25.367; -129.267
Geology
Last eruption	50 BCE ± 1000 years

Adams Seamount (also known as Forty Mile Reef^[2]) is a submarine volcano above the Pitcairn hotspot in the central Pacific Ocean about 100 kilometres (62 mi) southwest of Pitcairn Island.

Adams is part of a field of about 90 seamounts 90 kilometres (56 mi) east-southeast away from Pitcairn Island, and the largest of these.^[3] Adams lies southeast of another large seamount, Bounty Seamount.^[4] Most of these seamounts except for Adams and Bounty are less than 0.5 kilometres (0.31 mi) high.^[5] They were discovered in 1989 by the RV Sonne research ship.^[6]

It is a conical^[7] seamount rising 3,500 metres (11,483 ft) from the sea floor to about 39 metres (128 ft)^[1] or 59–75 metres (194–246 ft) below the surface of the ocean.^[2] The total volume of the seamount, whose base has a diameter of about 30 kilometres (19 mi), is about 858 cubic kilometres (206 cu mi).^[8] Adams has two summits,^[9] and coral and sand derived from coral has been found on Adams.^[5] Given its height, during the last glacial maximum Adams was likely an island.^[10]

Its slopes are covered by recent lava flows, volcanic debris and hyaloclastite.^[9] Lava flows feature aa lava characteristics and lava tubes, while deeper parts of the edifice are covered with lapilli and scoria.^[11] Parasitic vents form cones and mounds on its flanks.^[12]

Adams and the other seamounts were created by the Pitcairn hotspot, and these seamounts are its present-day location.^[3] This hotspot is one among several hotspots in the Pacific Ocean, along with the Austral hotspot, Hawaii hotspot, Louisville hotspot, Samoa hotspot and Society hotspot.^[6] The seamounts rise from a 30 million years old crust.^[13]

Alkali basalt, trachyte^[9] and tholeiite have been dredged from Adams Seamount.^[14]

The fresh appearance of samples and the lack of sedimentation indicates that Adams Seamount is a recently active seamount.^[3] Potassium-argon dating of rocks dredged from Adams Seamount has yielded Holocene ages, including one age of 3,000 ± 1,000 years before present.^[13] Other ages range from 4,000 - 7,000 years before present.^[15] Unlike Bounty, Adams Seamount displays no active hydrothermal system.^[5]

Adams seamount features a coral reef, one of the deepest tropical reefs in the world. It is mainly formed by Pocillopora sp. and Porites deformis corals, but also many reef fish and sharks; it is used as a fishing ground by Pitcairn.^[2] Adams seamount is part of the Pitcairn Islands Marine Reserve.^[16]