Relative rates of movement of the ocean floor∗

Much new information exists on the long‐term geologic motions of the ocean floor. Horizontal motions, when averaged over a few million years, are approximately 10 to 100 km/m.y. Relative motions across transform faults are twice these values. Uplift of the ocean crust in the rift mountains may be at a rate of 40 km/m.y. for short periods of time. Broader thermal cooling drops the ridge at a rate of about .175 km/m.y. for 1 m.y. old crust, and at a rate of about .02 km/m.y. for 80 m.y. old crust. Near subduction zones the ocean crust subsides at a rate of 2–4 km/m.y. near the oceanic trench axis and 40–60 km/m.y. on a Wadati‐Benioff zone which dips at 45°. Seamounts subside at a rate of .02‐.06 km/m.y. which is comparable to the rate of subsidence due to cooling of the oceanic crustal plates, but two or three orders of magnitude slower than seafloor spreading rates or the subsidence in some Wadati‐Benioff zones.