I have seen pictures of the cracks in the Earth's crust in Iceland and the San Andreas fault. To my eyes, the crack is relatively wide at the surface than a few meters down seems to be very narrow or just not there. I know this might seem stupid, but how come the crack does no go all the way down to the mantle? Is it that the crack opens and then the soft rock down below slides over to fill it in again?
How deep are the cracks at fault zones, I am thinking of the Silfra crack in Iceland between Europe and the US.