I have looked up typical values of the viscosity in the asthenosphere, which are given as 1020 Pa s in seismically active regions (see e.g. Kaufmann and Lambeck, JGR 107, B11 (2002)  2280), and 1024 Pa s beneath Cratons. Compare this with 1017 Pa s which is quoted as a typical value for glass, and 1025 Pa s for Earth's crust. If you are not looking at very long time scales, and very big sandboxes (if I may take this expression literally), it will probably not matter much in everyday terms. What kind of experiment do you have in mind?

Dear Jan-Martin

I am convinced that the sandboxes are providing outcomes which are very good approaches of what is really happening in earth's crust.

I am also convinced that, these experiments don't take in mind some parameters, which I cannot express in public. I can develop my thinking in a personal message.

Thank you for the suggestions, they are helpful and they alredy solved some of my  questionings.

Best regards

Ioannis Alexandrides

It is difficult to comment on things which are still to be developed, but if you wish to investigate the influence of the properties of the underground on the behaviour of a sandpile as a small-scale laboratory model for tectonic processes, I assume that you have already thought of looking for materials with considerably lower viscosity to speed up the experiments. Instead of a sandbox to represent the crust, you may also wish to use a more cohesive material. Also, take into account the relative densities of the materials involved.