Hydrothermal activity and the volume of the oceans

The volume of the world ocean can be regulated by exchange processes between the crust and the mantle. Hydrothermal systems of fast-spreading mid-ocean ridges operate at the critical pressure of seawater, which ensures optimal heat transport and maximal penetration of hydrothermal fluids. If the oceans were shallower, convective heat transport would be reduced and the depth of hydrothermal penetration and crustal hydration would decrease; outgassing would exceed ingassing, and the ocean volume would increase. If the oceans were, instead, deeper than today, inefficient cooling would lead to ultrafast subduction and ingassing of water into the lower mantle. These mechanisms could explain why the continental freeboard has remained about the same since the Archean Eon despite increases in continental area.