This is a cheesy tourist shot. Iceland sits on the junction of the two tectonic plates that make up this part of the earth. On one side you have the plate that carries North America, and on the other side you have the plate that carries Europe. It's the banging together of these landmasses that makes Iceland so geologically active.

Anyway, the tourist office would have you believe that this little rift is the junction of the plates. Hmmm, I'm not convinced it's not just a convenient crack in the bedrock. See next photo...