If a black hole 12 inches in diameter were situated (say) 10 metres off the ground, what would happen?
And how long would it take to devour the earth?
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Assuming it had no initial momenta, it would quickly drop to the ground. It would instantly start devouring encountered the Earth's matter, as it would continue its travel towards the center. The matter that would be falling into the black hole would grind against itself, because of the friction converting most of its potential energy into heat, and then radiation. The efect would be something similar to a continuous nuclear explosion. I can't tell how long it would take, but eventually the Earth would have melted around the black hole, the melted matter dropping on the black hole and forming an accretion disk (because of the Earth rotation), though some portion of it would be blown away into the space. The disk would then be slowly devoured.