|MadSci Network: Environment|
The inner Earth looks like this; the center is thought to be solid iron, the mantle is molten rock (mostly silicon and oxygen), and the outer few percent - just an eggshell, really - is the crust. All of the processes that created oil, coal, and gas, which resulted from the burial and compression of dinosaur-era biomass, happened on the very surface of the crust. That is to say, there's no oil anywhere near the Earth's core, and oil has nothing to do with the Earth's geological heat - indeed, oil has almost nothing to do with the Earth's overall geology.
Oil has a lot to do with your home furnace's heat (or maybe this is a bad analogy for a Floridian!) ... but remember that getting heat from oil requires burning it. Burning is the process of fuel combining with oxygen (atmospheric oxygen, in this case) to make carbon dioxide and water. Oil underground does not and cannot burn, since there is no oxygen for it to combine with. (The oxygen in rocks is generally tightly bound up with the silicon and other stuff.)
Yeah, so oil has nothing whatsoever to do with lava and the Earth's heat. The heat is thought to come mostly from radioactive elements decaying naturally.
Regardless, in this day and age, do we really care whether the Earth's core is molten or not? With the exception of geothermal power and hot springs (which are nice, but not indispensable) the Earth's ongoing geological activity is more often a nuisance than a blessing - volcanos, earthquakes, etc. (I've seen joke political protest signs saying "Stop plate tectonics!") The Earth will be habitable for a very long time after the continents stop drifting and mountain ranges stop uplifting. More subtly, though, the fluid mantle is probably responsible for the Earth's magnetic field, which, aside from guiding orienteers and birds, protects the atmosphere from solar winds. That's actually quite important; I'm not sure what the immediate effect would be if we lost the magnetosphere. But as I said, humans are not having any influence on these processes at all.
The other side of the coin is that we may be "causing our early demise" by pumping so much oil and burning it - the carbon dioxide produced by six billion humans and their automobiles is almost certainly causing a greenhouse effect. We will see what comes of this, environmentally and socio-politically, in the next few decades.
Hope this clears things up,
Try the links in the MadSci Library for more information on Environment .