years ago), before the presence of Cretaceous reefs on many of them was noticed. About 6% of the Pacific seafloor is covered with almost a million seamounts. While there are some differences to present-day reef systems, many of these seamounts were formerly atolls. All these structures originally formed as volcanoes in the Mesozoic ocean. Fringing reefs may have developed on the volcanoes, which then became barrier reefs as the volcano subsided and turned into an atoll; the barrier reefs in turn surround a lagoon or tidal flat. The crust underneath these seamounts tends to subside as it cools, and thus the islands and seamounts sink. Continued subsidence balanced by upward growth of the reefs led to the formation of thick carbonate platforms. 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 developed. The formation of many such seamounts has been explained by the hotspot theory. According to this theory, an active volcano lies on a spot of the lithosphere heated from below; as the plate above this hotspot moves, the volcano is moved away from the heat source and volcanic activity ceases. The hotspot will then heat the area of the plate now above it, producing another active volcano. In this way, a chain of volcanoes that get progressively older away from the currently active one is generated. With some exceptions, radiometric dating of the Mid-Pacific Mountains has yielded evidence of an eastward movement of volcanism which is consistent with the hotspot theory; in the case of Horizon Guyot, volcanism may have migrated southwestward which is not entirely consistent with the hotspot theory. When it formed, Horizon Guyot may have been located close to a spreading centre. Composition Volcanic rocks dredged from Horizon Guyot are of basaltic composition and define a tholeiitic suite. Augite, labradorite, olivine, plagioclase and pyroxene form phenocrysts while pigeonite is found in the groundmass. Other guyots and samples from the Mid-Pacific Mountains have similar compositions to these found on Horizon Guyot. Some volcanic rocks occur in the form of hyaloclastite which contains palagonite and sideromelane. Dredged volcanic rocks are heavily altered; this has given rise to analcime, augite, calcite, clay, clinoptilolite, iddingsite, ilmenite, labradorite, magnetite iron oxides and talc. Carbonates are found as limestone and siltstone; some limestones were formed by living beings.