I feel like I’m missing a part of basic chemistry understanding. What happens to rock on a molecular level when it melts? Does its composition change? Do all things melt under the right conditions?
In: Chemistry
Rock melts the same way ice does. The molecules in the rock jiggle around more and more as they heat up, until eventually the attachments between them can’t hold them in place and they begin to slide around one another. The only difference is that because rocks are usually not a single chemical (“mineral” is the term for those that are), they often have a partial-melt phase where some (but not all) of the minerals in the rock are melted.
To see this in reverse in your kitchen, melt some butter and mix it with hot water, then let it cool: you’ll see the butter freeze out before the water does, producing globules of butter floating in liquid water. In rock you might have, say, crystals of [fosterite](https://en.wikipedia.org/wiki/Forsterite) floating in a melted lava of [silica](https://en.wikipedia.org/wiki/Silicon_dioxide) (I am picking two minerals at random, I don’t know if these commonly occur together in practice). This process, which can separate minerals of different melting points, is called [fractional crystallization](https://en.wikipedia.org/wiki/Fractional_crystallization_(geology)) and is important to understanding how minerals are deposited by volcanoes.
[EDIT: Actually, butter itself is an example, which is why melting a stick of butter in the microwave can produce a partial melt of some solid fats in melted oil.]
> Does its composition change?
Usually not on a molecule-by-molecule level, but as a bulk rock it can (because of the fractional crystallization just mentioned). If one mineral with a high melting point cools and hardens and deposits earlier on in the flow of melted rock, it can reduce the concentration of that mineral later in the flow (and thus increase the concentrations of things with lower melting points).
Imagine pouring your water/butter hot liquid down a very cold slope. The butter would freeze out immediately and you’d get solid butter near the top of the slope, while the water would flow further and you’d get mostly butter-free ice at the bottom.
> Do all things melt under the right conditions?
Yes – or they do unless they break down into some other chemical below their melting point (as e.g. wood does).
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