I was thinking the same thing. Perhaps originally they weren't buried. Are they designed in such as way as they could have stood upright by themselves?

I would guess that the people doing this work would be able to determine the dirt they were digging into was newer or older than the rock itself.

It's been ages since I read it, but I think that Jared Diamond's book 'Collapse' explains a bit about the island and the statues construction (his main focus is on the self-destructive cycles of behavior of the population however).

Also, I thought this was old news? I'm sure I have seen that photo quite a while ago.