GEOGRAPHY OF THE SURFACE
As far as surface geography is concerned, there appear to have been two major terrain-shaping mechanisms at work on the earth (and for that matter presumably on the moon, Mars, Venus, and Mercury, the terrestrial planets). These are impact cratering by meteoric bombardment and thermal-tectonic activity due to an outflow of thermal energy from the deep interior of the earth. Erosion by wind, water, and life and tectonic activity (deformations and motions of the crust), with its accompanying crustal strains and slippages, are the dominant mechanisms now (only on the earth). They have all but erased the results of the impact-cratering phase in the earth's history, except that remnants of the last of that phase remain in nearly a hundred ancient impact structures, some of which re as large as the largest visible ones on the moon. It is estimated that on the earth tectonic activity with its accompanying volcanic activity dominates better than 90 percent of the present terrain, with not more than 10 percent of the cratered terrain remaining. Present evidence suggests that surface evolution on the other terrestrial planets, as revealed by various space missions, has not been so heavily influenced by thermal-tectonic activity as that for the earth.
