Ioah Guyot

The guyot belongs to the Magellan Seamounts[2] which stretch from the Mariana Trench to Ita Mai Tai seamount.[3] It is also known as Fedorov and Ioan/[4] IOAN, which stands for "Institute of Oceanology of the Academy of Sciences of the USSR".[5] Ita Mai Tai guyot lies south-southeast of Ioah and Pallada guyot north-northwest,[6] other seamounts in the area east of Ioah are Changpogo, Gramberg, Zatonskii and Arirang.[7] There are about 1000 seamounts in the central western Pacific.[3]

Ioah Guyot is a shield volcano[8][9] with an arcuate shape; the two halves that make it up have dimensions of 110 by 66 kilometres (68 mi × 41 mi) and 83 by 65 kilometres (52 mi × 40 mi).[1] The guyot rises 4.5 kilometres (2.8 mi) from the seafloor to 1,420 metres (4,660 ft)[5]–1,380 metres (4,530 ft) depth, forming a summit plateau with a surface area of 1,380 square kilometres (530 sq mi)[10] that is covered by 25–75-metre (82–246 ft) high hills.[1] The rim of the summit plateau is formed by volcanic rocks on the eastern part of the seamount and by reefal limestones on the western;[11] the reef deposits form a sometimes 5 kilometres (3.1 mi) wide[12] and in total about 200 kilometres (120 mi) long ring around the summit platform of Ioah Guyot[13] and cover an area of about 315 square kilometres (122 sq mi), making it the largest limestone outcrop of the Magellan Seamounts.[14] The lower slopes are gentle but steepen between 1,700–2,000-metre (5,600–6,600 ft) depth. Only the lower slopes are covered with thick sediment layers; the upper slopes have sediment accumulations only in sheltered areas[5] and also feature step-like structures.[1] A number of volcanic cones grew on Ioah and form morphostructures, with a density of about 11.1 vents per 1,000 square kilometres (390 sq mi).[15][16] Some of these vents form alignments and Ioah lies at the intersection of two strike-slip faults.[17][9] The seamount rises from the East Mariana Basin over Jurassic seafloor, between the two Ogasawara Fracture Zones.[18][19][20]

The seamount developed about 87 million years ago on the Pacific Plate and is now extinct;[4] its estimated age is placed in a range between 88.5 and 86.2 million years. It was located in the Southern Hemisphere as it formed.[19][8] Some secondary volcanic cones may be much younger, of Miocene age.[32] The formation of Ioah and the other Magellan Seamounts has been explained by a hotspot that would now be located close to the Rarotonga hotspot, Samoa hotspot and Society hotspot were it still active.[22][33][34] Compositionally, volcanic rocks from Ioah resemble these of the Rarotonga hotspot.[35]

During the Aptian-Cenomanian, limestones and volcanic rocks formed sediments on Ioah Guyot which developed a reef system.[36] A secondary reefal phase occurred during Santonian to Maastrichtian times[21] and a third one during the Eocene.[37] Ioah developed the largest reefs of the Magellan Seamounts,[2] with coral material accumulating to thicknesses of 200–300 metres (660–980 ft).[23] During the Eocene, tuffs were emplaced on the seamount.[38] Ferromanganese crusts developed later during the Paleogene[39] and Miocene-Pleistocene,[40] and up to 150 metres (490 ft) sediments accumulated on the summit plateau.[11]

Species that lived on Ioah Guyot during the Cretaceous include ammonoids, belemnites,[41] bivalves, bryozoans, cephalopods, corals, crinoids, foraminifera, gastropods, rudists, sea pens,[23] [42][43] sea urchins[5] and sponges.[23] Presently, a rich fauna has been identified on Ioah Guyot, including scleractinian corals without zooxanthelles such as Fungiacyathus pliciseptus and Peponocyathus australiensis which is usually found in much shallower waters.[18][25][44]

• Pako Guyot
• Vlinder Guyot