During the first organization or differentiation, as the molten earth cooled gradually, the outer surface of mantle solidified into a crust which we now call lithosphere. This primitive crust of earth was constantly punctured (or cut) by meteorites falling from the outer space. Now, since the atmosphere was very thin at that time, it could not slow down the falling meteorites by offering sufficient air resistance. Due to this, the meteorites fell on the earth with great force and made many holes in the newly formed crust. These holes in the earth's surface were called vents. Through these vents, the molten rocks from the mantle of earth below, came above the surface of earth.
This molten rock called magma cooled down, solidified and became a part of the crust. This process of the molten rocks or magma rising to the surface of earth through vents, released more gases to the atmosphere. These gases were mainly nitrogen, carbon dioxide and water vapour which had been released by the heating of hydrated minerals present inside the earth. In due course of time, a lot of water vapour was concentrated in the atmosphere. When this water vapour present in earth's atmosphere rose up, it got cooled. The cooled water vapour turned into clouds and then poured on earth in the form of great.