The ocean floor forms at MOR (Mid Ocean Ridge) sites at very high (magmatic) temperature. The new plate cools as the result of convective cooling by sea water (leading to the hot vent systems) and by conductive cooling. As a result of the cooling, the plate shrinks, gets denser and will ride slightly deeper in the underlying mantle. Both effects lead to a deepening of the ocean. When the ocean floor is deeper, more water will press on it, leading to more subsidence. These 3 effects determine the depth of the oceans, which varies from 2500 m on the MOR tops to > 5 km in the abyssal plains. It turns out that the depth of the sea floor is proportional to the square root of the age of the plate, which is confirmed by theory of heat loss. In the plains, the plates have cooled largely to an equilibrium temperature and only cool very slow from then on. With further aging, the cool and dense plate ultimately will descend back into the mantle (subduction). The movement of the plates is driven by convection currents in the underlying mantle, and the whole process of ocean floor spreading is a result of the convective heat loss of the warm earth to the cold universe.

While the ocean plate forms, the hot seawater "alters" the volcanic rock and creates water- and Cl-bearing new minerals. The volcanic floor also received a steady rain of sediment from above, including debris of animal houses (foraminifera, diatoms, shells) and clastic sediment (debris of continental rocks). When the age of the plate increases, so does the load of sediment. The oldest marine sediment layers are about 200 Ma, the same age as the maximum age of the underlying floor. During the subduction process, both the plate and the overlying sediment are absorbed back by the mantle. At about 100-120 km depth, the water and Cl bearing minerals devolatilize and a flux of these compounds rises into the overlying mantle. This induces melting and these melts rise up into the crust, leading to volcanism (arc volcanism). These magmas have mainly a mantle source with added ingredients from the altered plate and the subducted sediment. The latter is particular important for the mineral calcite (beastie houses), which dissociates again into CO₂ and Ca-bearing minerals. The CO, which long time ago was fixed as CaCO₃ in the ocean is released again as CO₂ into the atmosphere, where it is available again for photosynthetic use, modulate climate or will become a new beastie house. This is part of the carbon cycle, where one nutrient essential to life is circulated through many different reservoirs.