Two major families of faults dominate the tectonics of slow spreading ridges: the transform faults paralleling the spreading direction and the 'GLORIA' faults, observed by large-scale side-scan sonar, normal to it. The observation of microearthquakes to 8 km depth beneath the Mid-Atlantic Ridge axis implies that not only the transform faults but the GLORIA faults as well penetrate into the mantle. Since large horizontal temperature gradients are often associated with them, both types of fault provide the means of circulating large volumes of sea water through the oceanic mantle. Under these conditions, serpentinization of upper mantle rocks will occur in a predictable way. Its influence on the tectonic development of slow spreading ridges is discussed.