How Earth formed (explains some planet-wide processes)
Earth formed from the cold accretion of debris from the solar nebula. About 4.6 billion years ago, it was a ball of magma due to gravity, radioactivity, and meteoric impact. Lighter compounds buoyed to the surface and created distinct zones: an inner core, outer core, mantle, crust, and early atmosphere. This planet-wide phenomenon of buoyancy is responsible for the ongoing convection currents of the mantle. These heat gradients drive seismic and volcanic activity. The movement of metals in the mantle also account for the magnetic field that bends charged particles away from the atmosphere, creating the Aurora Borealis and other solar storm phenomena.
A Mars-sized meteor struck Earth ~4.5 billion years ago, resulting in the formation of the moon (and raising the surface temperature to 2000°C). The existing atmosphere was replaced with carbon dioxide, nitrogen, and water. As Earth again cooled, the water rained down over a thousand years to form the oceans.
About 4.3 billion year ago, Earth was cool enough for life to form. The atmosphere was carbon dioxide and nitrogen; the ocean was acidic with iron. Anaerobic life developed and broke down carbon dioxide. Oxygen was the waste product; it bonded to the iron ions in the ocean to form oxides that fell to the bottom. Organic debris also fell to the bottom, creating layers of chert interleaved with the iron in what's known as a banded iron formation. These account for enormous quantities of iron mined today. After saturating the ocean with oxygen, the atmosphere and surface rocks were subjected to it. This was known as the Great Oxidation Event and gave us our "modern atmosphere". The convenient abundance of oxygen lead to life as we know it.