TERRAIN-SHAPING PROCESSES
In the last two decades two revolutions have occurred in our thinking about planets, particu larly the surfaces of the terrestrial planets: the recognition that the lithosphere of the earth is divided into large plates moving relative to each other (plate tectonics) and the results of planetary exploration from the United States and Soviet space programs.
Almost everywhere across the surfaces of the moon, Mercury, and Mars, we see evidence for impact cratering and volcanic plains, with the relative percentage of the two terrain-shaping mechanisms varying from one body to the next. And from radar studies of Venus it appears that evidence is there for both processes. For the earth, however, scientists have a very different picture: It is a planet dominated by tectontc and thermal activity.
Occurring some 3.2 to 4 billion years ago, a period of heavy bombardment produced much of the impact cratering seen on the terrestrial planets. A by-product of that period of heavy impact cratering was lava flooding of immense impact basins to form maria, impact basins filled by lava flooding. This is essentially why the surfaces of the moon and Mercury look as they do. For the larger of the terrestrial planets-earth, Venus, and Mars-the second major terrain-shaping mechanism has remade almost all of the surface in the case of the earth but only a part of the surfaces of Venus and Mars. This mechanism is the tectonic-thermal activity of lithospheric convection. The surface is fractured, deformed, and in the case of the earth replaced by erosion from water, wind, and life, over billions of years. Heavily cratered terrain is generally believed to be the oldest type of surface .. Its preservation implies a relatively stable history for a crust. Volcanic material can reach a planet's surface to develop plains or volcanoes only if the crust can be broken, allowing lava to flow onto the surface. Of the terrestrial planets the earth shows far and away the greatest volcanic activity, with presumably Venus next (we really don't know at this point), followed by Mars, Mercury, and the moon.
