The earth is probably at least 4.5 billion years old. The oldest rocks ever discovered are about 4.3 billion years old. Scientists learn the age of rocks by measuring the amount of radioactive isotopes in them.
The nebular theory assumes that the earth was first a gas and then a liquid, and finally cooled enough to have a solid crust.The planetesimal theory assumes that the earth was made of solid particles from the beginning.
The Earth's Early Development
Scientists theorize that the earth began as a waterless mass of rock surrounded by a cloud of gas. Radioactive materials in the rock and increasing pressure in the earth's interior gradually produced enough heat to melt the interior of the earth. The heavy materials such as iron, then sank. The light silicates (rocks made of silicon and oxygen) rose to the earth's surface and formed the earliest crust.
The heating of the earth's interior also caused other chemicals inside the earth to rise to the surface. Some of these chemicals formed water, and others formed the gases of the atmosphere. Over millions of years, the water slowly collected in low lying areas of crust and formed the oceans. As land developed on the earth, rainwater and rivers dissolved salts and other substances from rocks and carried them to the oceans, making the oceans salty.
The earth's earliest atmosphere may have contained hydrogen, helium, methane and ammonia, similar to the present atmosphere of Jupiter. Or it may have contained a large amount of carbon dioxide, as does the atmosphere of Venus. The earth's earliest atmosphere probably did not contain much free oxygen. The oxygen in the atmosphere comes mainly from plants that use carbon dioxide and give off oxygen through the process of photosynthesis. The amount of oxygen gradually increased in the atmosphere of the early earth as plants developed and increased in numbers..The organisation of molten earth into various layers having different densities is called differentiation. As a result of differentiation, the earth was divided into three major layers: crust, mantle and core - the crust being lightest and the core being densest. It was during the differentiation process that the water vapor and gases present in the molecules of the minerals were released. This water formed the oceans on the earth where as gases formed the atmosphere.
The planet we live on is not just a ball of inert material. During the past ages, dramatic changes have taken place inside the earth. Without these changes, life could never have originated on the earth. And changes are still going on today, like the occurrence of earthquakes, outburst of volcanoes, weathering, erosion, mass movement etc.