that can support both abiotic and biological synthesis of organic molecules. Environment of Formation Serpentinization occurs at mid-ocean ridges, in the forearc mantle of subduction zones, in ophiolite packages, and in ultramafic intrusions. Mid-ocean Ridges Conditions are highly favorable for serpentinization at slow to ultraslow spreading mid-ocean ridges. Here the rate of crustal extension is high compared with the volume of magmatism, bringing ultramafic mantle rock very close to the surface where fracturing allows seawater to infiltrate the rock. Serpentinization at slow spreading mid-ocean ridges can cause the seismic Moho discontinuity to be placed at the serpentinization front, rather than the base of the crust as defined by normal petrological criteria. The Lanzo Massif of the Italian Alps shows a sharp serpentinization front that may be a relict seismic Moho. Subduction Zones Forearc mantle Serpentinization is an important phenomenon in subduction zones that has a strong control on the water cycle and geodynamics of a subduction zone. Here mantle rock is cooled by the subducting slab to temperatures at which serpentinite is stable, and fluids are released from the subducting slab in great quantities into the ultramafic mantle rock. Direct evidence that serpentinization is taking place in the Mariana Islands island arc is provided by the activity of serpentinite mud volcanoes. Xenoliths of harzburgite and (less commonly) dunite are occasionally erupted by the mud volcanoes, giving clues to the nature of the protolith. Because serpentinization increases the volume and lowers the density of the original rock, serpentinization may lead to uplift that creates coastal ranges above mantle forearcs. Further uplift can bring serpentinite to the surface when subduction ceases, as has taken place with the serpentinite exposed at the Presidio of San Francisco. Serpentinized ultramafic rock is found in many ophiolites. Ophiolites are fragments of oceanic lithosphere that has been thrust onto continents, a process called obduction. They typically consist of a layer of serpentinized harzburgite (sometimes called alpine peridotite in older writings), a layer of hydrothermally altered diabases and pillow basalts, and a layer of deep water sediments containing radiolarian ribbon chert. Because serpentinization increases the volume and lowers the density of the original rock, serpentinization may lead to uplift that creates coastal ranges above mantle forearcs. Implications Limitation on earthquake depth Seismic wave studies can detect the presence of large bodies of serpentinite in the crust and upper mantle, since serpentinization has a huge impact on shear wave