Convection rolls within the mantle have an adiabatic heat gradient, starting at the border between tectonic plate bottom and the layer below. Logically, the mantle must therefore as a whole be on the verge of being ductile and stagnant. A tectonic plate is 120 km thick, according to the defination that its lowermost border is where convection, or constant flow of mantle material, is found.
It has been found in laboratories, that if mantle material at this point (convecting but very close to becoming stagnant) does form convection rolls. As reality and experimental results are to be compared, especially if no other factors affecting real circumstances than used during experiment can be pointed out, inserting the outcome of experiment into known and measured circumstances is indeed a piece of work any scientist should undertake.
In this case it is easy, because the thickness of inner layers of Earth are known. Putting togherher the outcome of the experiments in laboratories, the logic of adiabadic thermal gradient, and knowledge about the depth of each layer, this is the outcome:
This is the basic picture of a section of convection rolls within the Earth. Inserting the results of experiments, fits exactly into measured environment.
Most people recognize the core, mantle and crust, and some might notice the Gutenberg layer, also known as the core-mantle boundary or CMB. This is a beginning of a study described in the book found here on this webpage. Reading that book is of course more difficult than reading this short post, and most people do certainly not have time enough to read it. It can be said here, though, that all the implications have been worked out, and how the convection rolls form a 3D system within the Earth is thereby fully understood. In turn, it enhances our overall understanding of geology.
Magic Magma 