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. 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. The limestone contains fossils of algae, bryozoans, echinoids, foraminiferans, molluscs and ostracods; dinoflagellates, pollen and scolecodonts are also found. Some limestones have been modified by silicification and phosphatisation. Clinoptilolite, pyrite, radiolarian fossils and yellow glass shards are found in the ooze, and some volcanic rocks and manganese rocks are cemented by indurated ooze. Ferromanganese and phosphorite crusts coat rocks. These ferromanganese crusts consist of iron oxides and manganese oxides and are related to manganese nodules and might become targets for future mining efforts. Other materials found on Horizon Guyot are analcime, barite, calcite, celadonite, cristobalite, glauconite, gypsum, ironstone, kaolinite, mica, montmorillonite, mudstone, quartz, sapropel, smectite and zeolite. Horizon Guyot is at least of Albian (c. 113–100.5 million years ago) age and is perhaps as much as 120 million years old. Radiometric dating has yielded ages of 88.1 ± 0.4 million years and more recently of 82.5 ± 0.4 million years; this may reflect either prolonged volcanism or that the older date is incorrect. About 100 and 80 million years ago a pulse of volcanism occurred in the Pacific Ocean; the formation of Horizon Guyot may have coincided with this pulse. Basalt lava flows were emplaced on Horizon Guyot during the Cretaceous, before or during the Albian. A second volcanic phase occurred during the Turonian (93.9 – 89.8 ± 0.3 million years ago) and Cenomanian (100.5 – 93.9 million years ago); thus volcanic activity on Horizon Guyot was recurrent.