Horizon Guyot is a presumably Cretaceous guyot (tablemount) in the Mid-Pacific Mountains, Pacific Ocean. It is an elongated ridge, over long and high, that stretches in a northeast-southwest direction and has two flat tops; it rises to a minimum depth of . The Mid-Pacific Mountains lie west of Hawaii and northeast of the Line Islands. It was probably formed by a hotspot, but the evidence is conflicting. Volcanic activity occurred during the Turonian-Cenomanian eras 100.5–89.8 million years ago and another stage has been dated to have occurred 88–82 million years ago. Between these volcanic episodes, carbonate deposition from lagoonal and reefal environments set in and formed limestone. Volcanic islands developed on Horizon Guyot as well and were colonised by plants. Horizon Guyot became a seamount during the Coniacian-Campanian period. Since then, pelagic ooze has accumulated on the seamount, forming a thick layer that is further modified by ocean currents and by various organisms that live on the seamount; sediments also underwent landsliding. Ferromanganese crusts were deposited on exposed rocks. Name and research history The seamount is named after the research vessel and is also known as Horizon Ridge, Horizon Tablemount, Gora Khorayzn and . During the Deep Sea Drilling Project, the drill cores called Site 44 and Site 171 were taken on Horizon Guyot in 1969 and 1971, respectively; a further drill core was obtained north of the seamount at Site 313 in 1973. This seamount is the best studied seamount of the Mid-Pacific Mountains and more is known about its morphology than any other seamount of the Mid-Pacific Mountains. Geography and geology Local setting Horizon Guyot lies west of Hawaii and is part of the Mid-Pacific Mountains. Unlike conventional island chains in the Pacific Ocean, the Mid-Pacific Mountains feature an oceanic plateau with guyots (also known as tablemounts) which become progressively younger towards the east. Other guyots in the Mid-Pacific Mountains are Sio South, Darwin, Thomas, Heezen, Allen, Caprina, Jacqueline, Allison and Resolution. South of Horizon Guyot, deep water in the "Horizon passage" leads into the Line Islands and Horizon Guyot is sometimes considered to be a member of that chain. The seamount rises - to a minimum depth of –, and is a ridge wide and over long; Horizon Guyot is the largest seamount in the Mid-Pacific Mountains. It trends in a southwest-northeast direction with an orientation matching that of other structures in the region such as fracture