Alteration of ultramafics

Serpentinization is the process by which deep mantle material (nominally Harzburgite) alters to serpentinite.  The mantle material, composed dominantly of peridotite with minor orthopyroxene and clinopyroxene, is stable at high temperatures and pressures but destabilizes in near surface conditions.  In the presence of water this instability allows the alteration of Harzburgite to Serpentine.  This process is exothermic and volume-increasing, which may contribute to the domed form of Oceanic Core Complexes seen around the world.