Serpentine minerals form where peridotite, Dante, and other ultramafic rocks undergo hydrothermal metamorphism. Ultramafic rocks are rare at Earth’s surface, but are abundant at the oceanic moho, the boundary between the base of the oceanic crust and the upper mantle. They are metamorphosed at convergent plate boundaries where an oceanic plate is pushed down into the mantle. This is where they are subjected to hydrothermal metamorphism. The source of water for this process is seawater entrained in the rocks and sediments of the oceanic slab.