Vlinder Guyot

The Western Pacific Ocean contains a large number of mountains, including underwater guyots and emergent atolls and volcanic islands, all of which appear to originate from volcanic processes.[4] The guyot lies between Guam and Wake Island[5] and is part of the northwestern Magellan Seamounts;[6] the Magellan Seamounts include Pako Guyot, Ioah Guyot and Ita Mai Tai. These seamounts appear to relate to hotspot volcanism although the age progression is not perfectly linear and there are discrepancies to other Pacific Ocean hotspots such as the Musicians Seamounts.[7] Vlinder Guyot lies close to the Ogasawara Fracture Zone and this fracture zone may have influenced the development of the guyot;[8] Pako Guyot is located southeast of Vlinder.[1]

Vlinder Guyot rises 3.5 kilometres (2.2 mi) to a mean depth of 1,500 metres (4,900 ft) and its flat top has dimensions of 40 by 50 kilometres (25 mi × 31 mi)[9] with a trapezoid shape[1] and sometimes a cover consisting of volcanic rocks and pelagic ooze.[10] A post-erosional cone lies on the summit platform of Vlinder Guyot[1] and rises about 0.5 kilometres (0.31 mi) above it. The northern rim of the summit platform is cut by a 10.7 by 4.8 kilometres (6.6 mi × 3.0 mi) notch that appear to have formed through a mass failure; similar mass failures have been observed on Kilauea and Piton de la Fournaise in Hawaii and Reunion respectively[9] and in the case of Vlinder Guyot has involved over 10 cubic kilometres (2.4 cu mi) of rocks, which are now deposited over 6 kilometres (3.7 mi) away from the collapse scar.[11] The pedestal of the seamount lies at a depth of 5,100 metres (16,700 ft) and covers an area of 90 by 126 kilometres (56 mi × 78 mi). The existence of 2.2–5.1 kilometres (1.4–3.2 mi) wide[2] Miocene volcanic cones reaching a depth of 551 metres (1,808 ft) has been reported,[10] which makes Vlinder Guyot the shallowest among the Magellan Seamounts.[12] The volcanic cones form an irregular group of cones in the northeastern corner of the summit platform[13] and they feature reefal deposits.[14] The slopes of Vlinder Guyot feature benches and terraces as well as rectilinear grabens;[10] one such graben coincides with the young cones.[15]

Coinciding with the corners of the trapezoid are northeastern, south-southeastern, southwestern and north-northwestern protrusions that appear to be rift zones[1] and have lengths of 15–50 kilometres (9.3–31.1 mi).[9] The two eastern protrusions feature additional seamounts, especially the south-southeastern one where Oma Vlinder seamount lies.[1] Oma Vlinder rises to a depth of 1,520 metres (4,990 ft).[10] A more diffuse volcanic centre lies on the northwestern extension and has three rift zones as well[1] that are covered with volcanic cones[9] up to 400 metres (1,300 ft) high and 9 kilometres (5.6 mi) wide.[10] This centre appears to be older and apparently never rose above sea level, it is now located about 1,750 metres (5,740 ft) deeper.[9]

The seafloor under Vlinder is 155-190 million years old.[16] Remotely operated vehicle observations have found that the slopes of Vlinder Guyot are covered by sand and rocks. The sand is probably derived from pelagic sediments and also from the summit platform, while the rocks appear to be of both sedimentary and volcanic origin and are often covered by manganese crusts. [17]

Vlinder Guyot has erupted alkali basalt, basanite and hawaiite containing hornblende and plagioclase,[18] oceanite, tholeiite and trachybasalt,[19] while Oma Vlinder has erupted hawaiite.[18] Isotope data show some affinity to rocks recovered at Pitcairn and Rarotonga.[20] Other materials encountered include pelagic chalks, ferromanganese crusts up to 12.2 centimetres (4.8 in) thick, hyaloclastite,[21] limestone of foraminiferal[22] and reefal origin, mud, phosphorite,[21] turbidites,[23] volcaniclastic rocks[21] as well as lithified clays, gravelstones, sandstone, siltstone and tuffites.[19]

Presumably, tholeiites form the base of the seamount and alkaline-subalkaline rocks its summit, while basanites occur in the secondary cones.[2] Meanwhile, sediments such as limestone and silt and ferromanganese crusts cover the summit plateau.[24] Weathering of volcanic rocks has produced iddingsite and palagonite, with calcite, chlorite and phyllipsite.[25]

Based on argon-argon dating, the northwestern edifice appears to be 102.4 - 100.2 million years old while the various dates obtained on samples from Vlinder and Oma Vlinder cluster around 95.1 ± 0.5 million years ago. Oma Vlinder and the main Vlinder Guyot appear to have the same ages and drowned at the same time, while the post-erosional cone is about 20-30 million years younger than Vlinder.[26] The northwestern volcanic centre is too old to have been formed by the Magellan hotspot, while the post-erosional cone may relate to the Samoa hotspot that passed close to Vlinder Guyot between 75 - 65 million years ago[27] or to plate tectonic processes.[28] Miocene volcanic rocks have been found as well,[29] and Cretaceous clays have been reported.[23]

During the Aptian to Turonian, limestone deposits formed on Vlinder Guyot which are recognizable on the rift zones, Oma Vlinder and in parts of the main guyot. These limestones formed in lagoon and reef environments and contain fossils of bivalves, bryozoans, corals, echinoderms, foraminifera, gastropods, molluscs and rudists;[19] rudists and corals were among the most important reef builders when at the time Vlinder Guyot was an atoll. Its drowning commenced in the Albian to Cenomanian times although evidence of continued emergence exists until the Paleocene; shallow areas may have been formed by late-stage volcanic eruptions[30] that formed new cones on the flat top; they may have been emplaced on reefs[31] and above sea level.[32] The youngest volcanic rocks are 15 ± 2 million years old.[24]