The planet Earth and the solar system
The composition of the planet Earth
The planet Earth is composed of four interacting components; namely, the rockosphere, atmosphere, hydrosphere, and biosphere. These component parts of the planet were formed one after the other.
Geologists believed that the young proto-planet was molten mainly because of the extreme heat generated by the accretion of materials as it grew, as well as the heat generated by radioactive materials.
Even today, the Earth collides with about 100 million "shooting stars" and unaccounted millions of micro-meteorites every day. Taken all together, these collisions add more that 2 million tons of matter to the earth each year. If this has been going on for about 4.6 billion years, the amount of matter added is equivalent equivalent to about a layer 10 feet deep over the planet's surface.
As the hot and molten proto-planet orbited through space, eventually surface layer cooled off and solidified to form a thin layer, the Earth's crust (or the primordial rockosphere); while the inner portion of the planet remained hot and molten. Since temperature increases with depth, thermal convection currents of molten materials (magma) persisted within (similar to convection currents in a cauldron of boilinfg water. In the weaker and fractured segments of the crust, the magma was extruded. And so the earth passed through an extensive period of volcanic activity.
For thousands of years, the volcanoes ejected lava and tons of gases such as superheated steam, hydrogen sulfide, carbon monoxide, and other volcanic gases. Because of the Earth's gravity, these gases formed the first Earthly primordial atmosphere.
Some geologists believe that the young planet was initially enveloped by gases which are left-over materials of the cosmic cloud; gases like hydrogen, helium, methane, ammonia, and miniscule amounts of the inert gases neon, argon, and xenon. When the young sun started radiating energy, it blew off most of these materials enveloping the planet. Thus, the young Earth was initially devoid of an enveloping atmosphere.
The Earth continued to cool off as it orbited around the sun. Occasionally, the water vapor in the atmosphere would condense to form rain drops which would evaporate as it fell on the hot Earth's surface. Eventually the Earth's surface cooled off sufficiently and the liquid water slowly accumulated to form pools, then lakes, and eventually oceans of water. Thus, from the Earth's interior, gases were ejected to form the atmosphere; and from the atmosphere, liquid water precipitated to form the hydrosphere.
The composition of the primordial atmosphere was unlie the present. Nitrogen was in the form of ammonia (NH3); carbon was in the form of methane (CH4), and there was hydrogen sulfide (H2S). There was no molecular oxygen (O2). Some geologists refer to the primordial atmosphere as a "reducing atmosphere."
For about 8 billion years, the rockosphere, the atmosphere, and the hydrosphere interacted chemically. From that interaction evolved the biosphere about 2 to 3 billion years ago in the primeval ocean of the young earth. Since then, the four Earth spheres have continuously interacted resulting in the recycling of matter.