zones on the seafloor. Faulting has been observed on the western side of the seamount. Two summit platforms lie on the ridge. The eastern one is the larger of these platforms and the western oval-shaped platform lies close to the western end of the ridge. These platforms are relatively flat and are surrounded by a slope break beyond which the guyot falls off steeply to the surrounding abyssal plain. This appearance characterises Horizon Guyot as a guyot although the elongated shape is unlike that of most guyots in the region which have one circular summit platform. At the margin of the platform, lie terraces which are up to wide and up to high and that discontinuously surround the summit platform; the flat surfaces of the terraces may be former fringing reefs. On the eastern summit platform there are buried terraces beneath the sediment cover. Sediment layers cover almost the entire summit of Horizon Guyot, and consist mainly of sand, with clay and silt making up a minor part. Features on the sedimented seafloor are flat areas, hummocks, ripples and sediment waves. Seismic transects have revealed a relief of about in the underlying basement and the presence of a central peak. Material obtained in drill cores includes chalk, chert, hyaloclastite, limestone, ooze and sandstone; basalt and chert outcrop in some places. In certain areas boulders and cobbles cover the seafloor; ferromanganese crusts cover exposed rocks. The seamount shows evidence of repeated mass failures; including hummocky terrain, scarps and slump blocks, which are on average thick. Landsliding is probably triggered by earthquakes; after the failure the landslides either stay coherent and do not travel far but some advance quickly and far. Talus blocks up to in size cover the seafloor around Horizon Guyot. Regional setting The West Pacific Ocean seafloor contains many guyots of Mesozoic age (251.902 ± 0.024 – 66 million years ago) that developed in unusually shallow seas. 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). 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