Weathering, or rock decay processes are at the center of the Critical Zone. The weathering engine modifies the Earth's crust to adjust to surface atmospheric, hydrologic, and biotic conditions, relevant to the many fields that find interest in the Critical Zone. Regolith refers to the body of decayed rock or sediment, coincidental to (and parallel and synonymous with) the Critical Zone. Weathering processes are synergistic, involving combinations of mechanical and chemical decay, taking place mostly at a nano-scale boundary layer at the mineral surface. Numerous factors intrinsic to the rock itself and of the surrounding subsurface and subaerial environment influence the availability of weathering agents and the rates and efficacy of decay. When environmental conditions allow, the terrestrial Critical Zone, regolith, can attain significant thickness, tens of meters, in most environments. Rates of formation calculate the oldest regolith profiles to exceed one million years in age.