Serpentinites form when seawater reacts with peridotite - rocks that form the Earth's mantle and have been brought up to the seafloor by tectonic processes. This picture shows a hand sample of a serpentinite recovered from the Atlantis Massif. Thin fractures in the serpentinite are filled with calcium carbonate. Sample is 16cm wide. Image courtesy of G. Fruh-Green.