[quote=LeeRoy]
Yeah, just repeat the process with a single 20,000lb slab of stone. That'd probably be more than enough to wipe a planet clean.
[/quote]

Meteors weighing far more than that have hit Earth, and the planet was relatively unscathed aside from localized destruction and an effect on regional climate. Hell, the meteor that wiped out the dinosaurs was estimated to be 10km across. And that would weigh THOUSANDS of tons, if not millions.