Creep Measurements in Igneous Rocks

The Benioff torsion apparatus for creep testing of slender cylindrical specimens is described. Continuous creep and creep recovery curves for granodiorite and gabbro at room temperature and atmospheric pressure are given. For constant-torque tests of up to 1 week's duration, the results are closely represented by the equation {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}ε (t) = σ/μ [1 + q\ ln\ (1 + at)],{document}, which was found valid for low stresses, up to 0.05 per cent of the rigidity modulus. The results are compared with early torsion creep experiments by A. A. Michelson. The strain behavior of rocks at low stresses is discussed.