Our planet Earth was formed some 4.5 billion years ago. At the beginning, the Earth was very different from what it is now. There was no atmosphere and no water bodies on the surface of the Earth. It is widely accepted by research scientists that the sun, the planets and their satellites in our solar system were all formed out of solar nebula. We can define solar nebula as a huge mass of spiral cloud made up of dust particles and various types of hot gases.
The key constituent elements of the solar nebula were hydrogen and helium gases and there were some other heavier elements too. Solar nebula began its contraction about 4.6 billion years ago probably because of shock wave caused by stellar explosion in the nearby region. During the contraction process, its temperature came down and it began to rotate very fast. As a result of cooling, shrinking and faster rotation, the outer part of the cloud got detached from the main body. The solar system was formed out of one of these detached rings of the gas cloud. The ring or disk shrank further and the center of the dense cloud heated up to form the sun, while the outer parts of the ring clustered together due to gravity to become protoplanets. One such cluster of gases created the planet Earth. This ball of hot gases was by now, cooled so much that all the gases in it were condensed into liquid or lava. Over a period of time, with further decrease in temperature, the lava solidified and led to the formation of the crust of the Earth. During the process of solidification, the liquid heavy metals like iron, descended to the center due to high density, forming the core of the Earth. The remaining part of the lava formed the Earth's mantle, which lies just below the crust.
Due to the cooling of Earth, large amount of steam escaped from its crust. As a result of the eruptions of volcanoes, substantial amounts of steam and various types of gases were also released. Geologists believe that a good amount of water was imported to the Earth during its collisions with several comets that contained ice. When the temperature on the Earth was very high, all the water remained in vaporous state and surrounded the surface of the Earth. Then, as the temperature on Earth decreased, all the steam condensed to form clouds. The resultant rainwater got accumulated in the craters formed on the Earth's surface by the impact of smashing of comets. This led to the formation of the oceans.
In the early stage, the Earth's atmosphere composed of hydrogen and helium gases. However, as the Earth's gravity was not that strong then, it could not hold on to these light gases and they were lost in the space. Later on, the impact of collisions with comets enriched the Earth with water as well as many other essential gases like carbon dioxide, methane, nitrogen, ammonia, etc. The gases discharged from volcanic eruptions and collisions with comets also contributed to the formation of the new atmosphere over the Earth. Free oxygen was not available in the new atmosphere; it was either bound by hydrogen or some other elements. The ozone layer was absent in the newly formed atmosphere. As a result, the Earth's surface was exposed to ultraviolet radiation. Significant amount of free oxygen was found in the atmosphere only after life forms came to the Earth when the photosynthesis process supplied oxygen to the atmosphere.
According to theories of geology, the present-day continents were formed due to fragmentation of a huge, single mass of land. The Earth's crust comprises a number of large plates of solid rock floating on the liquid mantle. The molten rock of the Earth's mantle is in constant motion due to convection of heat that occurs deep inside it. Due to this internal motion, some of the plates are constantly sliding at the edges (more commonly) in relation to others. This kind of continuous movement detached one plate from other. The broken pieces of land masses then drifted away from each other and caused the division of the continents. It is believed that the mountains were formed when one plate of the crust pushed itself against another and the resultant pressure thrust a part of the land upwards.
All the information about how was the Earth formed provided in this article is based upon various research studies conducted by geologists, cosmologists, etc. The changes that we have discussed here took place on the planet Earth gradually over a period of several millions of years. The geological features of the Earth then provided a suitable condition for evolution of life on the planet which appeared within one billion years of its formation. However, the human species came into existence much later.