Which term describes the stress regime where one principal stress is normal to the Earth's surface?

• A. Tensile regime
• B. Andersonian State of Stress
• C. Brittle-ductile transition
• D. Isostatic pressure

Answer: (B)

In this concept, the alignment of the rock’s principal stresses with respect to gravity is key. Gravity creates a vertical, lithostatic stress, while tectonic forces set up horizontal stresses. In Anderson’s framework, one principal stress naturally points vertical, normal to the surface, with the other two lying in the horizontal plane. So describing a stress regime where one principal stress is normal to the surface directly points to the Andersonian State of Stress, which specifically accounts for that vertical, surface-normal orientation. The other terms don’t capture this orientation aspect: a tensile regime describes the mode of deformation, the brittle-ductile transition refers to how rocks deform with temperature and pressure, and isostatic pressure describes buoyancy balance, not the principal-stress orientation.