One language usage question, and one content question:

"Melted sand"?? Isn't it "molten sand"? Is my hunch completely wrong, or is the author not a native speaker? Neither am I, but melted sand sounds so weird to my ears.

    This all happened in a matter of seconds, so atoms didn’t have time to arrange into stable structures,[...]

To my ears “Molten” would imply that the sand is currently in liquid form. The sense ”melted” is used here, as having been melted, seems right. You melt sand to get molten sand, right?

"Molten" to me implies it is still liquid. Molten salt reactors, molten magma from a volcano, molten sand, molten steel, dipping something into molten cheese. All fluid.

If I was to nitpick, "melted" is kind of inaccurate and not entirely natural in this context. Technically, molten sand is also melted sand, because that's how you get it to that state? Usually, you'd hear about solidified magma, crystalized sand, cast iron, air-cast steel, unevenly settled corium... to make a better point on how it turned back into a solid and what to expect from it - something like "The molten sand crystalized into an unusual structure" would be clearer.

I'd usually rather hear "melted" if it is important to note that this had a phase change and back. Plastic on an electrical device may look melted, indicating heat. A hardened steel part may look melted, which may damage the hardening. Rubber on a hydraulic line may look melted, also indicating heat. A plastic container looking melted in the context of chemicals may indicate some compromise.

Now the words sound weird in my head. Thank you.

It’s being used as a participial adjective here. I don’t think there’s much semantic nuance in the distinction between “melted” and “molten.” It comes down to common usage. “Molten sand” sounds more suitable to my ears, but “melted butter” also sounds better than “molten butter.” Odd, I think it’s part of a trend to replace the use of the past participle in some adjectival contexts. Maybe melted/molten is just an incomplete transition.

Think of dropping a water balloon onto a pin. It pops instantly, but the water (like the temperature and overpressure) takes a while to dissipate into a puddle.

I think it's "molten" while it is still liquid but not once it crystalizes; at that point it's "melted"

The sand melted, yielding a type of glass. Melt emphasizes a change.