<p>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.</p>

<p>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 <a href="https://en.wikipedia.org/wiki/Pitch_drop_experiment" rel="noreferrer">this</a> link which also happens to be the worlds longest running experiment.</p>

CC BY-SA 3.0
2015-07-17T15:19:32.000Z
2015-07-17T15:27:41.553Z
2015-07-17T15:27:41.553Z
stali
<p>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?</p>

<p>How deep are the cracks at fault zones, I am thinking of the Silfra crack in Iceland between Europe and the US.</p>

<geology><plate-tectonics>
How deep are the cracks in the earths crust at fault zones?
Nat
# How deep are the cracks in the earths crust at fault zones?

(Asked by: username_0 on 2015-07-17)

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.

**Tags:** geology, plate-tectonics


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## 1 answers

### Answer by username_1 on 2015-07-17. Score: 6

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](https://en.wikipedia.org/wiki/Pitch_drop_experiment) link which also happens to be the worlds longest running experiment.


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