Heat Flow - The heat energy coming from the earth is about the same order of magnitude of the heat energy of the Sun hitting the surface. The uncertainties are so huge that you might as well consider them to be about the same.
I just put up that interesting tidbit. If you want to be a geophys, you must think in huge amounts of energy. The geothermal heat energy blows up volcanoes, and moves continental plates around like a toy train set. The Sun also puts in a huge amount of energy. But the uncertainties in the day to day flux makes humanity look like a bump on a frog, on a log in the middle of the swamp.
The Earth has to get rid of this heat, or else it will be a boiled frog. The amazing thing is that for the 2 billion years or so of life, the Earth has been shirt-sleeve temperature somewhere. We know that because we are here to ask the question.
In all that time with the tectonic plates bouncing off each other, huge continental volcanism, giant meteorite impacts, there has been no runaway thermal reaction that kills everything, like on Venus. A geophys must be in awe that for billions of years, the Earth has been dancing on the point of a pin. Solar power may go up and down, with the Earth's heat blanketed by super-continents, and the temperatures laugh at all that.
So, if you want to ask the question "Why are we here?", you are going to have to go seriously at the Earth's heat balance with all the physics experiments you can rustle up. (Hint -- not done)
-- to be continued (now up to 2 fans, plus the old guy)
ps. I absolutely have run out of ideas for this. End of series.