I Spawned a wither boss at bedrock height in the nether, so its head would be trapped in the bedrock. when the wither boss exploded it broke the bedrock in a 3x3 around its spawn location.
The wither was spawned in horizontal.
FML Log. http://pastebin.com/L0j6vHYb
Image. http://postimg.org/image/ok9u2v5t1/