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. The basalts include both typical ocean island basalts and basalts resembling mid-ocean ridge basalts, with the former found deeper in drill cores. Hyaloclastites which outcrop at the margin of the summit platform indicate the occurrence of submarine eruptions.