Blocking of the base question

[dnorthen]

I see this many times on pick-off attempts and want to make sure I'm thinking right about this.

I have been told, perhaps incorrectly, that the runner must be allowed access to the base when diving back into it. Many times the fielder will plant his foot along the inside of the base and the diving fielder has little base area to touch, many times ending up touching the fielder's foot, not the base on the throw. Should this be treated as obstruction? If so, would an extra base be given as a penalty or only allow the safety back to base he dives back to?