How did the earth form in the condensing nebula?
The earth may have formed through the accretion of many planetesimals and as the mass increased through gravitational attraction and compression (overhead). The earth was probably initially a homogeneous ball that heated from three sources: 1) energy of planetesimal impacts, 2) gravitational compression lowered potential energy releasing heat, and 3) heat from radioactive disintegration (20 cals is released for 1 cm3 of granite over 500 million years). As the earth heated it began to differentiate into various zones of matter with different properties (overhead). Differentiation was possible because molten material could rise or sink depending on density, be moved by convective currents, and localize due to chemical zonation (overhead). As the earth cooled outgassing of the mantle released compounds (water vapor, carbon dioxide, hydrogen, nitrogen) into a primitive atmosphere. Early geologists tried to determine how old the earth was from observations about the features of the earth. A
