The absence of lineated marine magnetic anomalies over sedimented, recent spreading centers has been observed in several areas. We propose that magnetic anomalies can be suppressed as a result of pervasive hydrothermal reactions underneath thick blankets of sediment. The relatively impermeable sediment cover produces comparatively closed hydrothermal systems, which increase the residence time of the hot fluids in basaltic layer 2, causing more thorough leaching of the remanence-carrying iron-titanium oxides and diminishing the marine magnetic anomalies. This contrasts with lesser alteration at sparsely or unsedimented zones of extension with more open circulation. Conversely, spreading ridges and oceanic crust characterized by absent or subdued lineated anomalies may signal sites of extensive hydrothermal mineralization. This process might also provide an explanation for some magnetic “quiet zones,” such as in the Gulf of California, the Gulf of Aden, and the northern Red Sea.