The only book I have at the moment is Mortal Dictata, it's reference to glassing is on pg129-130. Unfortunately it doesn't give a numerical area of effect (the AOE is the horizon), or a precise numerical depth (it is deep enough to make hills and cliffs disappear in the sea of molten glass).
"A Faint, tickling vibration climbed through the soles of Staffan’s boots. He didn’t dare take his eyes off the screens, but the back of his tongue started to itch, an irritation that made him want to scratch deep inside his ears. In the hull cam view, a pin-point of white-hot fringed with violet expanded into a swirling ball of energy in seconds.
The projection dimmed slightly. The deck shivered and the sensation in Staffan’s ears became so intense that he jammed his fingers into them to try to relieve it. Then all he could see for a painful second was blinding, blue-white light.
When he blinked again, he was looking down a long, harrow beam of fire apparently connected to the surface of Shaps 3, and a heartbeat later a white-hot ball spread like a bursting dam and turned red-hot in seconds. The beam vanished. The red area roiled like the surface of the sun, broken up by an undulating black mesh.
Molten rock. Smoke. Oh God.
One moment the horizon was low hills and cliffs with a cluster of ancient, gigantic, thoroughly alien ruins, and the next it was just pure white light, followed seconds later by what looked like an instant red ocean that lit the sky like a sunset behind dense black clouds. There was nothing left but heat and smoke. Even the geological features seemed to have vanished. The terrain was now almost completely flat.”
Anyone have another quotation that gives us a bit more to work off of?