1. Introduction Introduction to Brittle Deformation

2. Outline Principal stresses Stress ellipse and stress ellipsoid Fundamental fracture modes Crack propagation Extensional joints Interpretation Problem

3. Principal Stresses  1 - greatest principal stress  2 - intermediate principal stress  3 - minimum principal stress Stress is a force/area Principal stress directions are mutually perpendicular

4. Principle Stresses Principal stresses are normal stresses only

5. Stress Ellipse In 2-D,  1 and  3 are mutually perpendicular and form the axes of a stress ellipse 11 33

6. Stress Ellipsoid In 3-D,  1,  2, and  3 are mutually perpendicular and form the axes of a stress ellipsoid The length of an axis is related to the magnitude of the stress The stress ellipsoid to the right might represent the stress at a point in the shallow subsurface

7. Stress Ellipsoid This stress ellipsoid could represent the stress at a point located in the suture of a collision zone between two continents

8. Fracture Modes Mode I Opening Mode II Sliding Mode III Scissoring Direction crack tip propagates relative to sense of slip defines Mode type Direction crack tip propagates relative to sense of slip defines Mode type

9. How to Stop a Propagating Crack… Extremely high stress concentration

10. How to Stop a Propagating Crack… Extremely high stress concentration Drill a hole and increase the area of the crack tip Lower stress

11. Extensional Joints Joint surface oriented perpendicular to  3 Mode I fracture - opening, no shear, no scissoring Opening direction 33

12. Growth of Joints Growth direction

13. Joints in Sedimentary Rocks

14. Joints in Igneous Rocks