Some important inferences arise from the observation that freeboard (i.e., sea level) on the continents has fluctuated by only around +/- 0.5 km from the middle Archean to the present. The magnitude and interplay of the various contributing factors have been examined by combining the plate model of the oceanic lithosphere with parameterizations of ocean crustal thickness based upon isentropic melting beneath spreading ridges. For a constant thickness continental crust and constant volume ocean, freeboard depends upon the (1) volume of continental crust, phi-c, (2) production rate of new ocean floor, C0 (km2 a-1), (3) thickness of the oceanic lithosphere, a (km), and (4) mantle potential temperature, T(P) (degrees-C). The thickness of the oceanic crust probably exerts the strongest influence through isostatic readjustment of the continents, and is a strong function of mantle potential temperature. Freeboard changes are illustrated graphically, and solutions are presented among these parameters explicitly for the case of constant freeboard. Criteria for ocean ridge crests to be raised above ambient sea level are also presented. The principal results are the following: (1) Freeboard is especially sensitive to aesthenospheric mantle potential temperature. A reasonable balance of the parameters maintains constant freeboard if T(P) never exceeded approximately 1430-degrees-C (i.e., approximately 150-degrees-C greater than today). (2) A middle Archean (approximately 3.5 Ga) continental crust of comparable size to today is possible only if T(P) was within approximately 50-degrees-C of its present value. (3) The crests of ocean ridges have probably always remained below sea level, though this is still not completely certain. (4) Somtime prior to approximately 3.8 Ga, potential temperatures must have been greater than 1600-degrees-C in the shallow mantle. In such circumstances, there can have been little hypsometric or tectonic distinction between "continental" and "oceanic" regimes at the Earth's surface. If any continental material existed before approximately 3.8 Ga it would have been subject to quite different tectonic and sedimentary processes than have typified continents from the middle Archean to the present.
