AbstractIn the present paper, making the crusial assumption that the rheological state of the lower mantle silicate materials controlled by creep mechanism, we study: (i) the upper and lower limits in the depth (i.e. temperature and pressure) variation of the activation volume for creep and (ii) the lower permissible isoviscous temperature profile.The calculation indicates that: (i) the activation volume decreases by almost 50% from the top to the bottom of the lower mantle, (ii) it is impossible to maintain a rheological state in which both isoviscosity and adiabaticity can precisely exist, throughout the lower mantle.