If I read around most of comments and articles on the Internet give two main reasons for the heat found in the depths of our planet :
- Super high pressure at the center of it
- Radioactive decay
I need clarification on both points.
Pressure: while it's pretty clear why pressure and temperature are bound in gas (*) this becomes unclear when matter is in solid or liquid phase. Why pushing on a solid mass would cause its thermal agitation to raise? It looks to me like most of the people takes for granted such point without proper analysis which of course involves more physics than geo-science.
Radioactivity: if I put together radioactive matter it will produce heat by radioactive decay. That's fine. Nevertheless my assumption is that if I take 100kg of any matter here at Earth surface it should contain the same percentage of radioactive isotopes of 100kg of same matter taken from the center of Earth. If this is not the case, then I ask my self why we have higher percentage of radioactive isotopes in the center of Earth rather than in its surface? I've read somewhere that unstable isotopes are heavier and that explains why they accumulated at the center of Earth, just by gravity; but usually radioactive isotopes have just few neutrons more (or actually less!), and that make the element heavier than just 2 or 3 element next to them in the periodic table. For example Carbon-14 is lighter than Oxygen.
Can someone clarify if these two points are valid or not, and why?
(*): even if with volume and mass constant I would say you can augment pressure by increasing temperature and not the other way round as you can't augment pressure under the constraint of having mass and volume constant