At the very beginning of its existence, the Earth was an incandescent sphere that, while orbiting the Sun, collided with smaller or larger clumps of material. After each collision, its mass and the amount of thermal energy it possessed grew. Under the influence of gravitational forces, the “heavier” atoms moved toward its center, pushing the “lighter” ones toward the surface. Over time, Earth’s collisions with other celestial bodies became less frequent, and the temperature of its material steadily decreased.
The existence of layers in Earth’s physical structure is the result of physical and chemical processes that, from its formation to the present, have altered the spatial distribution and structure of the material from which it is built. The aggregate state of any given layer was determined by the structure of its constituent material and by the pressure and temperature to which that material was exposed.
The material at its surface formed the Earth’s crust, the lithosphere (a layer of solid material about 60 kilometers thick). The mushy, semi-fluid state of the next layer, the asthenosphere (magmatic zones about 140 kilometers thick), is the result of a temperature increase caused by the pressure of the lithosphere, the presence of radioactive material, and heat radiated by the material beneath it. Due to the extremely high pressure prevailing at greater depths, the mantle (a layer of solid, incandescent material about 2,900 kilometers thick) formed beneath the asthenosphere. In a similar way, the remaining two layers of Earth’s physical structure formed: the solid and very hot core (a sphere about 3,400 kilometers in diameter) and a layer of liquid metal (the layer between the mantle and the core through which electric currents flow, creating Earth’s magnetic field). The change in the aggregate state of the material at those “depths”, from solid to liquid, was caused by a rise in temperature brought about by the extremely high pressures to which it was exposed.
Simultaneously with the formation of layers in Earth’s interior, its gaseous envelope also took shape. The hyperactive Sun blew away the primordial atmosphere, which consisted of gases and dust produced during Earth’s frequent collisions with other celestial bodies. The new atmosphere consisted of gases released from the planet’s rocky surface and of complex inorganic molecules, gases, and water vapor, components of the material that, at the time, highly active volcanoes were ejecting from the asthenosphere into the space above the lithosphere. The composition of the atmosphere, which determines the physical and chemical changes of Earth’s surface layer and the rate of its cooling during the night, was also greatly influenced by living beings.