To Lilt:
Actually it was both Hashirama's and Tobirama's chakra that caused the room to crack like that. I looked at the previous and this chapter, and the floor was perfectly fine.
However, in the panel where Tobirama's fingers are shown, the floorboards are already breaking. This is before he was addressed by Hashirama. Then after Hashirama tells Tobi to stop there are also words that describe the floor as breaking. Whether it was from before or after Hashirama said something, I am not sure.
Although i also thought the same as you before i saw the panels posted by Ninja Hidden in The Leafs. In fact, i wanted to ask where is that version from, since all the other versions that i have read do not have the panels where it shows the floor breaking or Hashirama telling Tobirama there is no need to raise his chakra to such high levels.