What is Glacial isostatic adjustment, mantle viscosity?

Glacial isostatic adjustment (GIA) is the slow deformation of the solid Earth, and the change of its gravity field and of sea level, as it returns towards isostatic equilibrium after an ice sheet grows or melts. The mantle viscosity is the effective viscosity $\eta$ that relates the slow, steady creep of the mantle to the stresses that drive it, over times of thousands of years and more.