Comment on “if Not Brittle: Ductile, Plastic, or Viscous?” by Kelin Wang

In continuum mechanics, viscous materials are those that lack rigidity and elastic response under shear stress. We argue that using the term viscous to refer to the aseismic lithosphere is thus a misnomer because it denies the propagation of S waves through the lithosphere in total contradiction to decades of seismic surveys. Similarly, viscous materials lack yield stress, which is another feature expected in most situations within the aseismic lithosphere but is more difficult to assess. Aiming to reconcile the definitions of rheological terms between material and Earth and mineral sciences, we propose a decision tree chart for the use of the terms viscous, viscoelastic, plastic, and viscoplastic, all widely used terms in the materials and Earth sciences communities for describing fundamental macroscopic behavior of rocks under shear stresses.