Rock Deformation and Geological Structures

by

John Cole, University of St. Andrews

This interactive module deals with the key areas of a standard structural geology course. It is aimed at second year students, and assumes a basic knowledge of structural geology at first year level (fault and fold classification, use of stereonets etc.). It may be of use to more advanced students who wish to revise various aspects of structural geology. The module concentrates on the concepts of stress and strain, the structures that may result, and how they relate to deformed rocks in the field.

The module is designed to be equivalent to about 4 hours of conventional teaching. The module occupies just under 7Mb and consists of six sections which are accessed via a main menu. One section is a tutorial that demonstrates the layout of the menu bars, and how to navigate through the module. The other five sections are sub-divided into topics that can be accessed at any time from sub-menus. Information is presented in a variety of ways, including multiple-choice questions. With these, if an incorrect answer is selected, the reasons why it is incorrect are explained, clues about the correct answer given, and the user asked to try again. When the correct answer is given, a confirming explanation is given and the user allowed to proceed. The sections are accessed from this menu.

1. Introduction

This section introduces the rationale behind structural studies in geology and hence why it is important to understand forces, stresses and strains. These concepts are outlined and differences between them explained. The other sections deal in more detail with the topics introduced here.

A page from the Introduction defining the difference between body and surface forces

2. Stress

This section defines stress and how it arises. The distinction is made between force and stress and the different types of stress are described. Normal and shear stresses, stress components, principal stresses and stress axes are covered, as well as hydrostatic and deviatoric stresses. The student is introduced to stress ellipses and stress ellipsoids and the uses of Mohr Circle Diagrams in solving structural problems.

3. Strain

This section discusses the different types of strain, units of strain, and how strain is measured. Homogeneous and heterogeneous strain are covered as well as strain ellipses and ellipsoids, and how these differ from stress ellipsoids. Also dealt with are pure shear and simple shear, Flinn diagrams and k values, deformation fields, deformation paths, and geological strain markers.

Various types of interaction are included. In this example the student is asked to calculate strain in a boudinaged quartzite

4. Brittle Deformation

Structures formed through tensile fracture and shear failure are discussed. The relationships between fault type (thrust, reverse, normal and strike-slip) and the state of stress are dealt with. The section describes the factors affecting failure using Mohr Circle diagrams and considers ductile-brittle shear zones.

5. Ductile Deformation

Block This section covers the rock fabrics that form during ductile deformation. This includes folding and boudinage in layered rocks, folding and boudinage in 3D, ductile shear zones and kinematic indicators. Fold mechanisms, the factors controlling fold wavelength, dip isogons and fold classification are dealt with. Flexural slip and neutral surface folding are considered, together with slaty, crenulation and spaced cleavage.

This page from Ductile Deformation explains the differences between buckling and bending

Here, the student is shown how bed thickness controls the wavelength of folds

Running the software

The different parts of the module are accessed via a main menu and navigation around the module is by means of buttons along the base of the screen. A glossary is accessed by clicking on a button along the base of the screen.