The properties of the lithosphere are different when measured at different scales. At a macroscale the detailed structures of the lithosphere are smeared, and it behaves as a continuum. At an intermediate scale called the mesoscopic scale, the material is composed of many small solid phase blocks whose interfaces play a significant role in the mechanical behavior of the macro phase. The smallest scale is the microscopic scale that associates with the atomic structure of the material. We introduce two state variables to describe the mesoscopic structure; entropy is a measure of the randomness of the organization of lithosphere blocks, and β describes the random strain energy stored in the system. Here the macroscopic stress and strain cannot uniquely determine one another, the relationship also depends on the internal structure of mesoscopic arrangement of the blocks. We can in principle derive an equation of state for the material that is composed of solid blocks of known geometry and mechanical properties, but calculations requiring realistic models are extensive. A one-dimensional model is useful as well as simple, however. ¿American Geophysical Union 1991