To nuke a location or area so overwhelmingly that it is not only destroyed, but its entire surface is melted into glass.
When the first nuclear bomb,
Trinity, was tested in the New
Mexico desert, the explosion was so
hot that some of the sand near the epicenter was fused into a glassy material later called Trinitite. Modern nuclear weapons can be hundreds of times more powerful, and with so many in existence you could glass a large area indeed.
While most things won’t
fuse into glass (pretty much just sand), the idea is a powerful image of total destruction.