Effective teaching of structural geology and tectonics: What to teach?
David D. Pollard, Stanford University
Effective teaching of any subject depends upon both content and technique: what is presented and how is it presented. In this talk I focus on the content for an undergraduate course in structural geology and address the question: What concepts are fundamental to our discipline that every undergraduate should understand and utilize?
To set the stage I recall the field trip led by James Hutton to Siccar Point on the east coast of Scotland in 1788. Hutton utilized field observations, relative ages, uniformitarianism, and kinematic reasoning to produce a compelling story for the angular unconformity at Siccar Point. These four methodological ingredients are as vital to a first course in structural geology as they were to Hutton in 1788, but they do not provide answers to many questions students now ask about geologic structures. Therefore, we add a framework of continuum mechanics to understand rock deformation and address the questions: how, how rapidly, and why do geologic structures form? The answers require a quantification of the forces that cause deformation, the constitutive properties of rock that resist deformation, and the accelerations, velocities, and displacements that provide the kinematics of deformation.
Isaac Newton published The Principia in 1687, so the underlying principles of mechanics were available to Hutton, but they focus on the motions of discrete bodies. What was needed was a mechanics of continuous bodies, in which every particle is acted upon by its neighbors, and every particle can move relative to its neighbors. In the decades just before Hutton's visit to Siccar Point, Leonhard Euler and Joseph-Louis Lagrange developed the spatial and referential descriptions of motion that are central to the kinematics of the fluid and solid continuum. In the first few decades after Hutton's death in 1797, Augustine-Louis Cauchy introduced the general equations of motion for a continuous body, which account for the mechanical action of every particle on its neighbors.
Today, we have the opportunity to make these concepts an integral part of teaching an effective first course in structural geology. They apply to all rock types under all relevant conditions of temperature, pressure, and rate of deformation in Earth's lithosphere and asthenosphere. They apply to rock that is brittle, ductile, or fluid, and to geologic structures of all types, including fractures, faults, folds, fabrics, and intrusions. Thus, they are inclusive and extensible, two hallmarks of what constitutes the fundamental content of an effective course.