Eighteen heat flow measurements on the Mid-Atlantic Ridge, in the detailed study area between 45 and 46 °N, have a pattern of low values up to 20 km from the median valley, high heat flow 30 to 40 km away, low values again 50 to 100 km away, finally increasing to normal heat flow at great distances. The smoothed heat flow profile is everywhere lower than that predicted by theoretical cooling plate models.It is concluded that convective water flow in the fractured, porous crustal rocks of the ridge is responsible for the low heat flow near the crest. Higher values (at 30 to 40 km from the crest) occur when the sediment cover is sufficient to cut off communication between the crust and seawater. The low heat flow zone at 50 to 100 km from the crest can be explained by heat required to warm the convectively cooled crust when the rocks are sealed and circulation stops, and by the heat absorbed in lower crustal metamorphic reactions.

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