Mantel rocks exposed at the ocean floor react with seawater to form serpentine. This is accompanied with a volume expansion that causes the rock to burst and form cracks. While the rock is reacting with seawater, carbonates are formed in veins and magnetite is oxidized to red hematite.
Formation of oceanic crust along a slow spreading ridge:
Image: Brun and Beslier, 1996, EPSL
A) first stage of the opening with formation of faults B) slow uplift of the mantle (gray) C) exposure of the mantle to the seafloor
Petrology studies the formation of rocks. Howsoever a rock is formed, during mountain building or on the ocean floor, its composition reflects the conditions at which it was formed and the time it needed to develop.