What are the non-uniformities and rheological details of the mantle? What is the structure of the 660 km discontinuity and its relation to…

What are the non-uniformities and rheological details of the mantle? What is the structure of the 660 km discontinuity and its relation to the correct model of the polar drift?

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Source: List of unsolved problems in geoscience (CC BY-SA 4.0)