[Humabout]

I would generally avoid any calculations of energy or energy-pressure generated by the falling object, just as I would avoid checking the shear modulus for the wooden beams or measuring the length of time of contact between the boulder.

The number one rule of RPM is that it is a Rule Zero system. If the intent of casting the spell is to inflict a bunch of damage, the vehicle by which said damage is delivered is irrelevant, especially if that vehicle will blink out of existence right after dealing that damage. As such, this is simply adding the Damage modifier to the spell with sufficient quantity and modifiers to represent a massive falling boulder. That it creates a massive falling boulder simply gives the option to cast this as a Create Matter effect instead of a Destroy Matter effect. I'd allow either for this sort of thing, personally, but others might be more stringent.

If the boulder is to stick around afterward for a while, I'd treat this as two effects: Create Matter (to make a semi-permanent boulder) and Destroy Matter (to trash the bridge/squash the troll/etc.).

Personally, I'd be curious to hear how PK would adjudicate this situation.