Ah, thanks for creating the topic for me, Mizal. I'll be adding some interesting passages soon, and I invite others to do so at their leisure when they have pertinent information to share. We can talk about such topics as the placement of water sources and how it affects civilization, exactly what happens during a volcanic eruption, and much more entertaining perils to throw at unfortunate victims in stories.
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.
This is a good book on this subject.
Favorite fun fact: Billions of years ago, an Earth day was only a few hours long and the newly-formed moon was very close above. However, days have been growing gradually longer as the moon drifts farther away from Earth.
Having covered the formation of Earth, discussing some volcanism is a logical next step. Convection currents lead to ascending wells of magma, which may settle below the surface as molten lakes called batholiths. Sections of this lake may rise into layers of rock above, spreading out along weak points where the rock has a lower melting point due to its composition. Magma can rise all the way to the surface in vertical shafts known as dykes. When this happens, the magma that issues forth is called lava. The spilling lava hardens into rock and builds up a mound called a cone. This is a volcano.
It's not all magma reaching the surface, however. As soon as possible, fluids will separate from the melt (on account of weighing significantly less) and course upwards through cracks in the existing (country) rock. They pour out as steam vents (fumaroles), geysers, mud pots and other things you might find at Yellowstone National Park. Hydrothermal vents are responsible for many precious crystal deposits, as well as a few major sulphur deposits. Lava can spill out into air or water; in the latter case, you get an underwater volcano.
Quick fun facts:
Aren't there different types of lava / magma that contribute to the formation of the oceanic and continental crusts (the latter of which tends to resurface up from the mantle)?
The process you're talking about is subduction. It occurs because of tectonic activity, and it's the density of the oceanic plate that causes it to sink while the continental crust "rides" on top.
The oceanic plate is mostly formed of basalts (originating from mid-ocean ridges), while the continents are mostly granitic. The difference is in many ways measurable by the concentration of silica. Felsic melts are siliceous (full of silica). This makes them acidic, causes them to flow slowly, and makes them lower temperature. They tend to have a higher quantity of dissolved fluids. The opposite is mafic. These are basic (on pH scale), less viscous, maintain a higher temperature, and have fewer dissolved fluids. They don't tend to create so many crystals as a result. (There are also intermediate and ultramafic melts.)
I managed to disclude from my response that the oceanic plate is mostly mafic and continental plates are mostly felsic. In case anyone was wondering why I went off talking about that.