The Early Earth and Plate Tectonics

So, after watching Brian Cox's documentary Wonders of the Solar System, you now have a basic idea of how the solar system came together. But what about our own planet?

Things may normally look stable and sedentary to our eyes, but there are still great geological forces constantly at work (hot enough to liquify rocks!), reshaping the planet and literally moving the ground beneath us...

And for this next part, I will admit almost complete ignorance: I have no idea how they've managed to reconstruct this much geological history, so for all I know, this could be a combination of evidence-based reconstruction mixed with a bit of theoretical speculation.

Any geologists out there who can help clarify?