Energy required to "glass" a given area of a temperate planet's surface 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.)
 A: The worlds land area is about 1.491×10^14 m^2  (square meters)
divide by 1600
So you'd need 93,187,500,000 of your 800 ton piles of tnt though much of the energy would be lost to the atmosphere. 
But let's try with some kind of directed energy weapon and using figures I'm finding elsehwere. 
I'm going to use the figures for creating 1 ton of glass: about 7,000,000,000 joules. 
I'm going to assume that about that much energy is applied to every square meter of the earths land surface. In some places that will melt deeply, in others the ground is soggy and it will only melt a few tens of cm deep. 
So that's going to take about 1.044×10^24 Joules of energy to glass the earths surface. 
lets assume we want to do it with a laser or with focused light from orbit. it's not going to be perfectly efficient, lets assume 50% losses to the atmosphere or inefficiencies so we'll need about 
2.088×10^24 Joules
Which is about 580 trillion MW h  (megawatt hours)
Now that's about 0.54 × total yearly solar energy available to the Earth (~~ 3.85×10^24 J )
so if the aliens wanted to glass the planet in the course of 1 year they'd need about 46900000000 copies of Isar nuclear power plant (one of the worlds largest nuclear reactors) or a set of mirrors the size of the planet to focus a planets worth of sunlight onto a small area and slowly burn the world. ("slowly" is relative, since the earth is turning the beam would be moving at about a 1600 km per hour along the ground)
with these levels of energy being thrown about I think the air would get pretty toxic from 1: the year long wild fires 2: the plasma from around the points that are being melted 3: everything that vaporises bellow the melting point of glass vaporising, there's going to be a lot of heavy metals in the atmosphere along with thick smoke as every forrest, oil field and coal seam near the surface burns.
Hell on earth comes close to describing it.
I wouldn't be surprised if there was a constant gigantic lightening storm around the focus point along with the mother of all huricanes due to the beams charging particles in the atmosphere and the huge energy involved along with the huge quantity of boiling water and smoke. I can't begin to calculate the effects on pressure but you're going to want your facility to be airtight if you want to live even if you're deep. 
Let me know if I've put any decimal points in the wrong place.
