Abstract of Pollard's Talk

At the President's Day Ceremony, David Pollard delivered a talk entitled A New Curriculum for a First Course in Structural Geology.

Textbooks for a first course in Structural Geology typically provide encyclopedic descriptions of structures, but offer ad hoc approaches to the physical processes that give rise to structures. In contrast, the conservation laws of mass and momentum, leading to the relevant equations of motion, provide a unified and self-consistent approach to deformation and the evolution of geologic structures.

Publication of Newton’s Principia Mathematica in 1687 preceded Hutton’s foundational field trip for the science of Geology to Siccar Point, Scotland, in 1788 by 101 years. However, Newton’s Second Law applies to isolated point masses, not to a vast collection of rock particles. Thus, Newton’s mechanics was of no help to quantify, in Playfair’s words, “the immeasurable force which has burst asunder the solid pavement of the globe” at Siccar Point. Hutton was left with qualitative conjectures and analogies to observed active processes. Today, students of structural geology face the same dilemma.

However, the century from 1750 to 1850 that saw the founding of geology also witnessed the founding of continuum mechanics. Cauchy used the conservation laws to adapt Newton’s Second Law to the material continuum. The new curriculum resolves Hutton’s dilemma for students of structural geology by embracing Cauchy’s Laws of Motion. These equations apply at length scales from nanometers to thousands of kilometers, and at time scales from microseconds to millions of years. They apply to brittle, ductile, and viscous deformation.

Hutton’s Principle of Uniformitarianism and Cauchy’s Laws of Motion are fundamental concepts for the discipline of Structural Geology. The new curriculum, supported by the new textbook, makes it clear: 1) why one must choose a spatial or referential description of motion and a constitutive law to study geologic structures; 2) how the kinematic and dynamic variables are intrinsically linked by the rock properties; and 3) how deformation necessarily varies in space and time.