"Melted" vs "molten"

Is there any difference (e.g. regionality) between the two forms of the past participle of melt (melted and molten)?

Solution 1:

They're not really alternatives. Molten specifically refers to liquids which are extremely hot, and whose usual form is as a solid. It is used as an adjective.

By contrast, something may be melted (whether used as a transitive verb, or an adjective), but need not be hot, and it need not be liquid (something that has begun to lose solidity can be considered melted), nor does it even need to still be subject to the melting process (i.e. it may have fully resolidified).

Solution 2:

Marcin's answer is, I believe, very good, from the perspective of every day usage. However both terms relate to concepts in physics, so let me give some more perspective.

If you look in wikipedia entry for molten/melting you'll find this distincion:

An object that has melted completely is molten.

This is further enforced by dictionary definitions, but also expanded with what used to be colloquial use,

mol·ten (mōlt′'n)

transitive verb, intransitive verb

• (Archaic) melt

adjective

• melted or liquefied by heat
• (Now Rare) made by being melted and cast in a mold

and for melt

melt (melt)

transitive verb, intransitive verb

• to change from a solid to a liquid state, generally by heat
• to dissolve; disintegrate
• to disappear or cause to disappear gradually: often with away
• to merge gradually; blend: the sea melting into the sky
• to soften; make or become gentle and tender: a story to melt our hearts

noun

• a melting or being melted
• something melted
• the quantity melted at one operation or during one period
• a dish, esp. a grilled sandwich, containing or covered with a layer of melted cheese: a tuna melt

From these definition you can conclude that

• melting describes the process from the moment the object starts changing its shape (due to the fact that it is not solid anymore)
• melted is used for object that changed its shape due to melting, even though it might not be melted (in stricter sense) anymore and it completely re-solidified
• molten is typically used with glass, lava, rock, all of which all require to be very hot to be melted, so high temperature is usually implied (though strictly speaking it is not necessary)

So, you should make a distinction between a strict physical meaning and colloquial use. I believe that discrepancies due to loose vs strict meaning are much more pronounced than any distinction due to regional meanings.