previous volcano and reefs started growing when volcanic activity was still underway; Horizon Guyot featured lagoonal environments with algal reefs. Prior to 1973 there was no evidence that Horizon Guyot had ever formed an island but later a stage of emergence was postulated. The seamount was an island for at least 6 million years. During the late Cretaceous, a second volcanic episode took place on Horizon Guyot and produced volcanites and volcanic sediments which buried older limestones. At that time, volcanic activity was underway not only on this seamount but also in the Line Islands; on Horizon Guyot this phase occurred perhaps as much as 30 million years after the previous volcanic stage. Before this volcanic phase, Horizon Guyot had emerged from the sea and erosion had reworked some older rocks; also, plants grew on the now exposed island. Shallow water deposition in Coniacian (89.8 ± 0.3 – 86.3 ± 0.5 million years ago) or Santonian (86.3 ± 0.5 – 83.6 ± 0.2 million years ago) to Maastrichtian (72.1 ± 0.2 – 66 million years ago) time has been inferred from the presence of unstable coccoliths of such age in drill cores. Drowning and sedimentation Horizon Guyot reached above sea level at least until the Cenomanian, unlike other Mid-Pacific Mountains guyots which sank below sea level during the Albian. Plant remnants are found in rocks of Turonian and Coniacian age, implying that Horizon Guyot was still emergent at that time; but by the Coniacian, Horizon Guyot was submerging. It is not known why Horizon Guyot drowned but the burial of the reefs by volcanic activity may have played a role. Pelagic sedimentation commenced in the Campanian (83.6 ± 0.2 – 72.1 ± 0.2 million years ago) when Horizon Guyot had already sunk to a depth of . Since the Miocene (23.03 – 5.333 million years ago), 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 linked to the glaciation of Antarctica. Pelagic sediments accumulate on some guyots after they have drowned. A dome-shaped cap of pelagic ooze accumulated on top of Horizon Guyot during the Tertiary, reaching a maximum thickness of - in some places. In the saddle between the summit platforms it is about thick; an unconformity separates it from Cretaceous deposits. The sediment layers