Actually, I suspect that hitting atmosphere at that speed would be like hitting a brick wall.
At any significant percentage of C, the entire object would flash to energy, primarily heat, virtually instantaneously. It would be like a nuke blast, but many orders of magnitude more intense. Anything in visual range of the impact point would burned off instantly, and the shockwave would cave in that side of the planet.
Look at what happened in the Tunguska Event. The object never made it to earth - it probably exploded when it hit the lower atmosphere - but it devastated a huge region of Siberia.