Board Thread:Theories and Speculation/@comment-26112212-20141110192310/@comment-26112212-20141111095137

You are right about that, I guess Black Zetsu is not a solid mass, because he was ripped off when he was absorbing chakra of Naruto and Sasuke. But that doesn't explain how he impaled Madara, So possible explanation would be that he can control when to be hard solid and when to be a gooey material.