A compilation of heat flow versus age of oceanic crust about all the midocean ridges demonstrates that low heat-flow anomalies associated with the crest and flanks appear to occur only on slow-spreading ridges. Such crestal anomalies are probably caused by large-scale hydrothermal circulation in fractures throughout the newly formed oceanic crust, which results in rapid cooling of the crestal zone and hydration of the crust. Away from the crest, sediments seal the fractures and circulation stops. The crust then begins to heat until it reaches equilibrium with the conductively cooling mantle, after which it cools with the rest of the lithospheric plate.
If, however, the crust heats to above the equilibrium stability temperature for its hydrous assemblage, dehydration occurs, absorbing enough heat to produce the observed low heat flow on the flanks of these slow-spreading ridges.