[15] Other guyots in the Mid-Pacific Mountains are Sio South, Darwin, Thomas, Heezen, Allen, Caprina, Jacqueline, Allison[16] and Resolution.
[32] Sediment layers cover almost the entire summit of Horizon Guyot,[23] and consist mainly of sand, with clay and silt making up a minor part.
[34] Seismic transects have revealed[32] a relief of about 150 metres (490 ft) in the underlying basement[31] and the presence of a central peak.
[38] The seamount shows evidence of repeated mass failures; including hummocky terrain, scarps and slump blocks,[39] which are on average 30 metres (98 ft) thick.
[21] The West Pacific Ocean seafloor contains many guyots of Mesozoic age (251.902 ± 0.024[a] – 66 million years ago[42]) that developed in unusually shallow seas.
[16] These are submarine mountains which are characterised by a flat top and usually the presence of carbonate platforms that rose above the sea surface during the middle Cretaceous (c. 145–66 million years ago[42]).
[44] During the Second World War, it was discovered that the seafloor of the Western Pacific Ocean was dotted with numerous flat-topped seamounts.
These were promptly identified as sunken islands; at first, it was believed that they had sunk below the water in the Precambrian (over 541 ± 1 million years ago[42]), before the presence of Cretaceous reefs on many of them was noticed.
[51] Continued subsidence balanced by upward growth of the reefs led to the formation of thick carbonate platforms.
[52] Sometimes volcanic activity continued even after the formation of the atoll or atoll-like structure, and during episodes where the platforms rose above sea level erosional features such as channels and blue holes[c] developed.
[59] Volcanic rocks dredged from Horizon Guyot are of basaltic composition and define a tholeiitic suite.
[27] Dredged volcanic rocks are heavily altered;[2] this has given rise to analcime, augite, calcite, clay, clinoptilolite,[62][63] iddingsite,[64] ilmenite, labradorite, magnetite[62] iron oxides and talc.
[65] At one point in the drill core, carbonates were found mixed with volcanic rocks; presumably this is a place where hyaloclastite accumulated and was reworked by sea currents.
[63] The limestone contains fossils of algae,[63] bryozoans, echinoids, foraminiferans, molluscs and ostracods;[66] dinoflagellates, pollen and scolecodonts are also found.
[37] Other materials found on Horizon Guyot are analcime,[63][11] barite,[71] calcite, celadonite,[11][63] cristobalite,[71] glauconite,[72] gypsum,[73] ironstone,[74] kaolinite, mica, montmorillonite, mudstone, quartz,[71] sapropel,[35] smectite and zeolite.
[77] Before this volcanic phase, Horizon Guyot had emerged from the sea and erosion had reworked some older rocks;[87] also, plants grew on the now exposed island.
[93] Pelagic sedimentation commenced in the Campanian[78] (83.6 ± 0.2 – 72.1 ± 0.2 million years ago[42]) when Horizon Guyot had already sunk to a depth of 1,500 metres (4,900 ft).
[94] Since the Miocene (23.03 – 5.333 million years ago[42]), sedimentation rates appear to have decreased as the guyot moved away from waters with high biological productivity and at some point in the last 10 million years erosion increased due to bottom currents[28] linked to the glaciation of Antarctica.
[103] The seamount induces its own semidiurnal tide and the sea currents reach their maximum at the margin of the summit platform where 20 centimetres per second (7.9 in/s) have been measured.
[106] Most of the sediments however are transported upslope;[29] those which do end up at the bottom of the seamount form talus deposits around Horizon Guyot.
[101] Fish found on Horizon Guyot include batfish, bathypteroids, chimeras, morids, sharks and synaphobranchid eels.
[107] Brittle stars, chaetognatha (arrow worms), copepods, corals, crustaceans, hydroids, loricifera, molluscs, nematodes, nemertinea, ophiuroids, ostracods, polychaetes, sipuncula, squat lobsters, vermes[37][108] and xenophyophorans make up the bulk of the fauna on Horizon Guyot today.