<P> Note that the assumption the v is relatively large is equivalently to the assumption that the thermal diffusivity κ (\ displaystyle \ kappa) is small compared to L 2 / T (\ displaystyle L ^ (2) / T), where L is the ocean width (from mid-ocean ridges to continental shelf) and T is its age . </P> <P> The effective thermal expansion coefficient α e f f (\ displaystyle \ alpha _ (\ mathrm (eff))) is different from the usual thermal expansion coefficient α (\ displaystyle \ alpha) due to isostasic effect of the change in water column height above the lithosphere as it expands or retracts . Both coefficients are related by: </P> <Dl> <Dd> α e f f = α ⋅ ρ ρ − ρ w (\ displaystyle \ alpha _ (\ mathrm (eff)) = \ alpha \ cdot (\ frac (\ rho) (\ rho - \ rho _ (w)))) </Dd> </Dl> <Dd> α e f f = α ⋅ ρ ρ − ρ w (\ displaystyle \ alpha _ (\ mathrm (eff)) = \ alpha \ cdot (\ frac (\ rho) (\ rho - \ rho _ (w)))) </Dd>

How does sea-floor spreading lead to the formation of ocean trenches