Go read my bestie Lynne’s excellent essay on her work in volcanology, which includes this gorgeous description of what glass is, exactly. (Probably more accurate than the volcanology running through Anne Carson’s poetry, I guess.)
The difference between glass and liquid is, in a way, really a question of timescales. Glasses deform and flow, though so slowly we can’t see them do it in our lifetimes. They are internally amorphous, like liquids. Glasses exhibit brittle behavior when you exert forces on them that are relatively rapid, like smashing them with a hammer, and they do not relax in response to stress quite the way that liquids do. But a glass is considered thermodynamically “metastable”: it occupies a temporarily stable state, but it probably won’t last forever. In old volcanic rocks, glass breaks down to grow other, more stable substances long before everything else in the rock does so.