Glassing

Summary
Glassing is a technique used by the Covenant in which they map the latitude and longitude of a planet's surface, divide it into grids, and bomb each grid, therefore obliterating the planet and leaving it as a desolate wasteland. After the glassing is done, all vegetation is vapourized and the planet is a red smoldering sphere. After a few days, the atmosphere of the planet corrodes and dissipates, leaving it truly barren. This technique was used to obliterate the UNSC outer colonies and Reach. For some particular reason (probably the presence of Forerunner artifacts), this was not used against Earth as of yet.

Images
Covenant forces preparing to glass Reach