Never understood why people say glassing about bombing a planet. Dropping 50 MT bombs on a planet will that not cause something to explode? Not turn it into glass? Vaylin in Old Republic say it once too and I was just scratching my head wondering why say it like that.
Ever hear of trinitite? The intense heat of a nuclear blast tends to melt and fuse the various minerals in the blast area into a glassy mineral that is often generically referred to as trinitite, named after the Trinity nuclear test and the glassy residue it produced (technically, only nuclear glass from the Trinity test is trinitite, but the term is often used as a generic for all nuclear blast formed glass).
Short version: nuke hot, melt minerals into glass
Similarly, other high-energy events such as meteor impacts (or kinetic weapon bombardment), lightning strikes (or arc emitter bombardment), or exposure to high-powered lasers and plasma bombardment also tend to produce extreme heat and melt minerals into lava and/or glassy slag.