In some science fiction, orbital bombardment is portrayed as being able to vitrify the soil, or even the underlying bedrock, of a targeted area. While this is certainly not out of the question for high-energy weapons (vitrifying a 40m × 40m × 1m area of packed, dry soil requires an amount of energy equivalent to roughly 800 tons of TNT going off, and glass formation has been documented as a result of terrestrial nuclear testing), my question is just how much energy is needed to glass an area, assuming that you are:
- Operating from orbit - either using some form of directed-energy weapon or the ability to detonate a high-yield warhead of some type at a set distance above the planet's surface.
- Seeking to destroy the surface of a given area (i.e. not concerning yourself with a Cheyenne Mountain-type underground fortification) and anything on it (buildings, aboveground fortifications, and any living thing that can't or won't get out of the way). More specifically, I'm working with areas the size of a fortress and its immediate surroundings ("who accidentally Sauron's tower?") or perhaps a small kingdom/country (who needs Lichtenstein or Luxembourg, anyway?), and thinking of a depth sufficient to render temperatures inhospitable no more than 50' underground (enough to deal with basements, shallow bunkers, and such, but a mere nuisance to Cheyenne Mountain-type fortresses)
- Targeting a temperate world (think Earth-clone).
Finally, what other consequences would the use of such high-energy weapons on a planetary target have? (Assume that said high-energy warheads/weapons are "clean" with regards to producing heavy radioactive products themselves - nuclear fallout isn't interesting for the purposes of this question.)