Author : Researcher of Geodynamics, email In the last article of myths of mantle convection, we were put into a confusion that what the state of mantle is. Convection in a medium implied the medium should be fluid, on the other hand propagation of S-wave indicates that the material must be solid. How can a same material possess two states simultaneously? Let us discuss over this matter. If we observe racks of books shelves for a longer time, will see that the straight racks, made up of wood or metal, get little bit bent. This means that their initial length has increased due to some flow inside the material (this is not elasticity, as the bent woods never get back to original shape). However, their end points remained same. Important to mention, that such bending occurs in a time scale of several years (5-10 years, depending on wight of books and material of shelf). Let us take a simpler example. Everybody must be familiar with flour dough that is used to make bread (Rotti). If you take a piece of dough and keep it on a horizontal table, after some time, the height of the dough will be reduced, as it spreads and get flattened. This happens because, the dough material starts to flow from the top and accumulates at the bottom (following the simple gravity law). It is to notice that, this shape change of dough takes places within few hours, whether, the bending of book shelf, which is also caused by the flow of wooden material, takes a long period of time. The difference of time scale is manifested by strength of the material. It is easily understood that flour dough is much weaker (in common word, softer) than the wood or metal. So, it changes its shape quickly. The change in shape has a scientific name, called deformation. Practically, every material has a duel behaviour. If a material is solid or fluid, that depends on which time scale we are looking at. Depending on what is the strength of the material and how much force is applied to it, the material behaves as a fluid in different time scale. At a single instant, the wooden book shelf appears to be solid, however, in ten year time scale, same wooden shelf behaves as a fluid. But, as the strength of dough is low compared to wood, the flow behaviour in the dough is recognizable in less time (in the order of few hours). Similar things happen in case of mantle. Propagation of earthquake wave through the mantle is a matter of few seconds. In that time scale, mantle behaves as a solid material and permits S-wave to propagate through it. However, the mantle is much stronger that a wooden block or metal strip. So, it flows very slowly and the convection cycle takes place at a time scale in the order of million years.
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