THE topographic roughness of mid-ocean-ridge flanks is known to increase with decreasing spreading rate 1-4, but the exact form of this relationship has not been established. As the topographic features that make up ridge flank roughness (abyssal hills) are created near the axes of mid-ocean ridges by faulting and volcanism 5-7, the relationship between roughness and spreading rate may shed light on the process of crustal accretion at spreading centres. Here I present measurements of ridge flank roughness on profiles crossing the world mid-ocean-ridge system, which show that roughness is proportional to the inverse square-root of the spreading rate. This result is consistent with some very simple inferences of how the topographic roughness of mid-ocean-ridge flanks scales with the lithospheric thickness near ridge axes, as determined by thermal models.
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