Part I: the scope of structural geology
Part 1 includes Chapter 1, which recounts the founding of structural geology by Hutton at Siccar Point and summarizes the scope of structural geology by addressing the following questions:
- What forces cause deformation in Earth’s lithosphere and asthenosphere?
- What are the three major mechanical styles for this deformation?
- What five broad categories of geologic structures result from this deformation?
- What is the methodology for analyzing and understanding these geologic structures?
In 1788 John Playfair and Sir James Hall accompanied James Hutton on a geological field trip to this outcrop at Siccar Point, Scotland. Hutton and his colleagues did not have the tools to quantify the timing of this angular unconformity, but they appreciated that an immense span of time must have passed, and that enormous forces were required to deform these rocks.
Church dogma of the day put the age of the Earth at 6,000 years, and described the only natural event of any consequence during Earth's history as the biblical story of the great flood. Here at Siccar Point, Hutton saw the geologic evidence for a much longer and more profound history. With a combination of field observations, relative ages, uniformitarianism, and kinematic reasoning Hutton produced a compelling story that was a key building block in the founding of structural geology.
Forces causing deformation in Earth's lithosphere and asthenosphere.
Schematic (not to scale) conceptual model of convecting asthenosphere and overlying lithosphere from one of the classic papers on crustal deformation. Closed curves parallel to velocity vectors (black) are flow lines for convection cells driven by body forces. Discrete arrows (red) are normal and shear tractions (surface forces per unit area) acting on the base of lithosphere and causing deformation within the lithosphere. Modified from Hafner, 1951.