In this lecture we start out with a lesson in mechanical engineering, then apply that knowledge to the study of the mechanical properties of the Earth.

Stress and Strain

  • Stress: A force acting on a surface. Generally, the stress is measured as force per unit area of the surface. E.G. the stress applied to the inside of a car tire might be 60 pounds per square inch. Strictly speaking, stress and pressure are the same thing, but in common usage, "stress" is applied to a wider variety.

    NOTE! Stiffness and strength are two different things. A steel bar and a biscuit are both stiff but they aren't both strong. Nylon and steel are both strong but they aren't both stiff. Materials that are both stiff and weak are said to be brittle.

    More on Young's modulus.
    Structural Geology: The study of stress and strain in rocks and the rock structures that result from them.

    Basic descriptive terminology: Structural geologists are preoccupied with the orientation of things in space. To describe the orientations of lines and planes, three terms are used.


    Fractures: Fractures are simply cracks, the traces of brittle deformation of rocks.


    In the field, faults almost never appear as in the nice clear block diagrams. Weathering and erosion immediately attack any topographic expression of faults, like in the image above. Once that happens, faults are inconspicuous and reveal their presence only indirectly. Some keys:


    Folds: Folds and associated structures are the record of ductile deformation of rocks.