Seafloor spreading rate can be calculated from the age and distance of oceanic crust from a mid-ocean ridge axis; the full (total) spreading rate is twice the half-rate measured on one flank, since both plates move apart symmetrically.
The half-spreading rate (one flank) = distance / age = 1300 km / 65 million years = 1300x10^5 cm / 65x10^6 yr = 2 cm/yr....
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