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Thinning of the crust occurs at points on the Earth's surface where tectonic plates are separating. This initially causes normal faulting to occur which leads to the creation of down thrown blocks or valleys known as "grabens". These are bounded on either side by "horst" structures.

If the separation of the plates continues, then a rift valley will form, along with associated thinning of the lithosphere and the onset of volcanism. Continued riftying will ultimately form a new oceanic spreading centre.

This process is currently occurring in the East African Rift zone.

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