The East Pacific Rise in the North Pacific drops from a crestal elevation of ∼2,800 m to a depth of 5,600 m for 75-m.y.-old crust. All other active midocean ridges except the North Atlantic close to Iceland show the same increase in depth with age but, within a 400-m spread, have generally shallower crestal elevations. Depths and basement ages from JOIDES Deep Sea Drilling Project site cores in the Pacific are in excellent agreement with the empirical depth versus age curve for the North Pacific determined from magnetic and topographic profiles. Sites in the northwestern Pacific are used to extend this empirical curve from a depth of 5,600 m at 75 m.y. B.P. to 5,900 m at 135 m.y. B.P. JOIDES data from the North Atlantic fall close to this extended curve but consistently show 200- to 300-m shallower crestal elevations in the oldest regions—evidence that certain geographic areas show small (±300 m) but significant departures from the uniform depth versus age relation for the North Pacific. The cause of these departures is, at present, unknown.

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