Eruption of volcanic rocks at the midocean ridges is the major mechanism by which heat is lost from the interior of the Earth. Approximately one-third of the heat is removed from the spreading centers by convective circulation of seawater (1). The magnitude of this heat loss requires that the entire volume of the oceans circulates through the midocean ridges in approximately 10 million years. Seawater interaction with volcanic rocks at near 400°C results in substantial chemical flux and makes an important contribution to buffering the composition of some elements in seawater. Cations from seawater (Mg+2, Ca+2, and Na+) form hydroxyl-bearing alteration minerals in the volcanic rocks, releasing hydrogen ion to solution. The hot, acidic altered-seawater releases metals (Fe, Mn, Zn, and Cu) and reduced sulfur (H2S) from the volcanic rock; these are transported by hydrothermal solutions to the seafloor and form metallic mineral deposits.