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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.

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Cracks (actually the right word is fault or in some situations rift) can only occur in brittle material and only the Earths upper crust (top 5-25 km) is brittle. The lower crust and mantle are ductile, i.e., they have a relatively lower viscosity which means that they can indeed crack on short time scales but will heal quickly due to high temperatures and pressure.

Edit: In some ways you can think the lower crust and mantle to be like pitch (tar), which is solid at room temperature but flows over longer periods. E.g., see this link which also happens to be the worlds longest running experiment.

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