What would happen if a tiny black hole the size of a marble were placed at the center of the Earth? Rest assured, the Earth won’t completely be swallowed up by the black hole but that’s really the only good news to offer.
First of all, not all of the Earth would simply be sucked into the black hole. When the matter near the black hole begins to fall into the black hole, it will be compressed to a very high density that will cause it to be heated to very high temperatures. These high temperatures will cause gamma rays, X-rays, and other radiation to heat up the other matter falling in to the black hole. The net effect will be that there will be a strong outward pressure on the outer layers of the Earth that will first slow down their fall and will eventually ionize and push the outer layers away from the black hole. So some inner portion of the core will fall into the black hole, but the outer layers, including the crust and all of us, would be vaporized to a high temperature plasma and blown into space.
This would be a gigantic explosion — a significant fraction of the rest of the mass of the Earth matter that actually fell into the black hole will be converted into energy.
FYI, that marble-sized black hole would have about the same mass as the Earth. Not that they exist, mind you. Maybe, maybe not. Blackish holes? Dark grey holes? Anyway, really heavy.