Editing Seafloor spreading (section)

===Cooling plate model===

The depth predicted by the square root of seafloor age derived above is too deep for seafloor older than 80 million years.<ref name=":4" /> Depth is better explained by a cooling lithosphere plate model rather than the cooling mantle half-space.<ref name=":4" /> The plate has a constant temperature at its base and spreading edge. Analysis of depth versus age and depth versus square root of age data allowed Parsons and Sclater<ref name=":4" /> to estimate model parameters (for the North Pacific):

:~125 km for lithosphere thickness
:<math>T_1\thicksim1350\ {}^{\circ}\mathrm{C}</math> at base and young edge of plate
:<math>\alpha\thicksim3.2\cdot 10^{-5} \ {}^{\circ}\mathrm{C}^{-1}</math>

Assuming isostatic equilibrium everywhere beneath the cooling plate yields a revised age depth relationship for older sea floor that is approximately correct for ages as young as 20 million years:

:<math>d(t)=6400-3200\exp\bigl(-t/62.8\bigr)</math> meters

Thus older seafloor deepens more slowly than younger and in fact can be assumed almost constant at ~6400 m depth. Parsons and Sclater concluded that some style of mantle convection must apply heat to the base of the plate everywhere to prevent cooling down below 125&nbsp;km and lithosphere contraction (seafloor deepening) at older ages.<ref name=":4" /> Their plate model also allowed an expression for conductive heat flow, ''q(t)'' from the ocean floor, which is approximately constant at <math>1\cdot 10^{-6}\mathrm{cal}\, \mathrm{cm}^{-2} \mathrm{sec}^{-1}</math> beyond 120 million years:

:<math>q(t)=11.3/\sqrt{t}</math>