CSIR NET June 2016 Q117: Plate Tectonics
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. Since the ridge spreads symmetrically in both directions, the full rate at which the Atlantic Ocean widens (total separation rate between the two plates) is twice the half-rate: 2 x 2 = 4 cm/yr. Option 4 is correct.
1) and 2) 1 cm/yr and 2 cm/yr correspond to the half-spreading rate on one flank only, not the full ocean-widening rate. 3) 3 cm/yr does not correspond to either the half- or full-spreading rate calculation. 4) Correct -- 4 cm/yr is the full spreading rate, twice the one-flank rate computed from the given age and distance.