How does rock melt?


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

Yes, (almost) all things melt under the right conditions. When melting, the rock loses its crystalline structure and becomes a liquid. The atoms constituting the rock rearrange themselves as kinda free-moving molecules.

Not chemistry, melting is *physics*. Heat is basically the amount of random physical motion of the particles of a substance, the atoms and molecules all jiggling around. When something melts it is because that jiggling has become great enough to prevent the molecules of the substance staying linked to specific neighbors, and instead can all move around and trade neighbors.

Melted rock is still rock, the same substances as it was before. All substances can melt under the right circumstances, a combination of heat and pressure.

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]( floating in a melted lava of [silica]( (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]( 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).

So thermal energy (heat) is just a measure of atoms shaking about. If things only jiggle a little bit, then small electric forces (and some other stuff) can hold them in place on relation to one another which makes things solid. If they jiggle a bit more, you get a liquid. If they jiggle so much that other forces can’t restrain them much at all, you get a gas. If they jiggle so much they start falling apart, you get plasma (though things get a bit odd here). Making something really stable and big shake about requires a lot of energy, which is why some things can stay solid at higher temperatures while small and unstable things are already gases.

Things cooling down just goes in the opposite direction.

In this sense, yes. All things can melt. “Rock”, as everyday people think about it, have no singular composition. Some are uniformly one mineral, while others are made up of many. When these multi-mineral rocks melt, even if the minerals themselves don’t chemically change, the layers can separate much like how a milkshake left out on the counter will separate into different layers. In fact, some methods of purifying ores involve heating it to a specific temperature so that one mineral melts while the other doesn’t.

At other times, the melting helps the minerals rearrange into a nice pattern. Diamonds are just carbon in a very organized lattice. Rubies and sapphires, among other gemstones, are just aluminum oxide in a lattice, with their color depending on what other traces of minerals are caught in the lattice when it forms.

Melting rocks is very similar to melting anything else. Rocks just have a higher melting point than most other materials. Nearly everything can melt. There are a few exceptions, which include large and complex molecules that will simply fall apart into smaller molecules before they melt. Then there’s helium, which can’t melt because melting requires a solid, and helium can never freeze into a solid in the first place (at least not in conditions we can reproduce on earth).

All objects on earth are made of atoms. Most atoms are bound to other atoms to make molecules. Molecules can interact with each other in a large variety of ways, but some of the more common ways are hydrogen bonds, ionic bonds, dipole interactions, and van der Waals forces. I can go into more detail on each of these if you want but for now let’s just say that these are all ways that molecules can stick to each other.

In a solid, the molecules are stuck together so strongly that they stay in the same place relative to each other. Imagine a crowd of people where everyone is holding hands with everyone else. These people might have three, four or even more arms, so they can grab onto multiple neighbors. They can still move around a little bit, shuffling their feet one way or another, but these motions are small.

While in a liquid state molecules can flow past each other. Here, imagine that the crowd of people stops holding hands. They can now walk past each other and mingle. They still stay as a crowd, but it’s much more mobile than before.

If you look at these atoms and molecules with a strong enough microscope, you’ll see that they are constantly vibrating in place, even if the object as a whole is sitting still. The warmer it is, the faster these vibrations. If it vibrates fast enough, these random vibrations will become stronger than the forces causing molecules to stick together, and the object will melt.