| The Earth's surface is young in geological terms, mostly less than one thousand million years old. This surface age indicates a geologically active planet, with evidence of the intense bombardment period of the early solar system having been largely obscured by crustal deformation, volcanic activity, erosion and deposition. The large-scale features of the Earth's surface, such as mountain ranges, rift valleys and ocean trenches, are controlled by the continuing motion of sections of the crust. This process, termed plate tectonics, seems to be unique to Earth among the terrestrial planets in the present day. Another striking feature of Earth's surface is the abundance of water - evidenced not just by the extensive oceans, but by the water erosion features which have shaped the land. Much of the continental landmasses are covered by a regolith of weathered debris - soils, muds, sands and loose rock - which provides a foothold for most of the planet's lifeforms. |