Evolution of the Earth,artwork. The Earth's internal structure is shown in the insets,with the corresponding state of the Earth's crust across top. At left,from 4.6 to 2.3 billion years ago,the mantle only had weak convection currents (dotted lines) and a fluid core-mantle boundary. The surface was cratered and barren. At centre,from 2.3 billion years ago,soil and oceans form. Post-perovskite forms the core-mantle boundary,reinforcing convection cells. Volcanism begins. At right,1 billion years ago,convection within the iron core forms a magnetic field. Plate tectonics begins,and mountains,an atmosphere,and life form |