Granite, which usually forms from magmas that cooled and partially re-melted over and over—the minerals like quartz in granite melt easily, so they liquefy and separate from the leftover solid as rocks start to melt.
Now that we've got everything prepped, let's head to the stovetop where we're going to heat up a large pot that has a tight fitting lid, and melt some more butter in there.
This is freezer burn, and it happens because ice in the frozen food sublimates — that is, the water changes directly from a solid to a gas rather than first melting and then evaporating.