QUESTION: When Earth formed from the accretion disk, I would imagine that the material from which it was formed was pretty well mixed, pretty homogeneous. So my question is, if Earth was formed from a well mixed medium of all the elements and compounds we find here, why do we find chunks of one element, like veins of gold? How did the gold clump up like that as the material of the planet coalesced?
ANSWER: Hi Cole,
This is a great question, one that researchers are working on today. I do not have a definitive answer as we are not sure of it, but I'll try to shed some light on the question.
Firstly, the molecular cloud of gas that collapsed down due to gravity to form an accretion disk was probably not that well mixed. That is because it would be consisted of different supernova explosions and other sources that would have created different densities of material/gas (elements).
Next, in the accretion disk that the gas cloud was pulled into by gravity, there would be slight variations in temperature. We know this is true as this is partially explains how we get different types of planets in areas (small rocky planets on the inside, large gas planets on the outside). We believe that in addition to large scale temperature variations, there would be small scale temperature variations. Since temperature and pressure are proportional, by changing the temperature in an area, you can change the pressure. This would mean that you can change in small areas, how well you can compress materials (i.e. make denser, more abundant pockets of an element).
Lastly, the volcanic and geothermal process over the 4 billions years of the Earth would have affected this too. These processes would have melted, changed, and moved some of the various elements, creating denser areas in one place, and less denser areas (of an element) in another.
I hope this helps with your question.
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QUESTION: I don`t understand how small pockets of temperature differential (and the coinciding ability to compress elements etc to varying degrees) would bring together clumps of one material. Any idea?
The key is actually nuclear physics. In order to get higher order elements (i.e. elements with more protons and neutrons than hydrogen), you need something called neutron capture. Since most of the gas in the early Solar System would have been hydrogen, you can fuse hydrogen together to get helium, and continue this process.
The second way (and more productive way) is if you have a lot of free particles (protons, neutrons, electrons) and spare energy. This would be the case with most planetary/solar systems, as they would have been from supernova explosions, which would have created a lot of energy but also split apart all of the atoms that existed in that previous star (therefore creating free protons, neutrons, etc.). With the extra energy, via nuclear fusion, you can start to pull these particles into elements. However, since the energy would not have been fully, evenly distributed (as is always the case with supernova explosions), different amounts of higher order (heavier nuclei) elements would be formed in areas where there was more energy
All of this energy would be thermal energy, which would alter the pressure to create these things. The direct implication is from the free energy, which is manifested as thermal energy (temperature) and then therefore pressure.