Density and porosity of sediments were measured on DSDP legs 69 and 70 samples from the Galapagos spreading center. Permeability and the hydraulic impedance of each sediment layer were estimated from measured values of porosity. The gradients of porosity and density with depth where sediment layers are thin (≲50 m thick) are anomalously high compared with those of other areas and with the upper part of thicker sediment layers in this area. A good correlation was found between the anomalous porosity and density gradients and the present-day heat flow. We interpret these observations to suggest that these high gradients may be due to active hydrothermal circulation through a thin sediment cover, which is inhibited by a thicker sediment layer, and that the pattern of hydrothermal circulation may be essentially fixed with the moving plate. Hydraulic impedance of the sediment layer was estimated from the observed depth variation of porosity and was shown to increase rapidly with its thickness. Our interpretation that a threshold thickness of about 50 m would inhibit direct diffuse discharge or recharge of hydrothermal flow through the undisturbed sediment layer yields an average permeability of the underlying basement layer of about 3-6¿10-14 m2 (30-60 mdarcies).