1. Joints and Shear Fractures (D & R; p. 205-226)
Figures not from D & R are from: Earth Structure: An Introduction to Structural Geology and Tectonics, by Ben van der Pluijm and Stephen Marshak. Copy available in TA office, Rm. 314.

2. Joint: A natural fracture that forms by tensile loading- walls of fracture move apart slightly as joint develops

3. Joints/Fractures: Geometry
Planar and often smooth; no appreciable displacement. Most abundant structural element in crust. What do the surfaces look like?
Moscow Kremlin - Bell Tower of Ivan the Great. Fractured in 1737 due to uneven cooling

4. Plumose structure: A subtle roughness on surface of some joints; resembles imprint of a feather. Due to inhomogeneity of rock.

5. Joints: commonly elliptical

6. Close-up views of hackles in plumose structure
Close-up views of hackles in plumose structure. Plumose structure is more prominent away from origin due to stress concentrations at crack tips

7. Joints/Fractures: Kinematics
ribs are arrest lines- opening is not instantaneous, but rhythmic, like splitting wood

8. Griffith cracks: preexisting microcracks and flaws in a rock
The largest properly oriented Griffith crack propagates to form a through-going crack

9. Joint arrays

10. Three competing mechanisms that contribute to joint formation during uplift and erosion:
(1) Contraction during cooling
(2) Poisson effect- e.g., rock expands in vertical direction and contracts in horizontal direction during unloading
(3) Membrane effect- expansion due to increase in curvature of layer

11. Cooling joints: form by thermal contraction

12. Exfoliation joints: Form by unloading of bedrock through erosion.
They form parallel to topography

13. Exfoliation joints: Form by unloading of bedrock through erosion.
They form parallel to topography

14. Tectonic joints: Form by tectonic stresses as opposed to
stresses induced by topography.

15. Joint analysis
Significance: determine orientation of tectonic stresses

16. Significance for Engineering
Planes of weakness!

17. Significance: Geologic Hazards

18. Joints and Geomorphology

19. Shear fracture: A fracture that grows in association with a component of shear

20. Shear fractures
en echelon tension gashes
-form ~45 degrees from plane of max. shear stress
-preexisting vein material rotates while new vein material grows

22. What is it?
What are these structures? What is the sense-of-shear? Describe how the veins grew.
en echelon tension gashes right lateral or top-to-the-right from center to tips during rotation

23. What is it?

24. Determining the sense of shear

25. Vein filling during crack opening

26. Significance: Economic Geology
Alteration/Mineralization along fractures; Veins preserve dilational separation

27. Joints/Fractures: “no appreciable displacement” Next: Geometry and Kinematics: Faults (Read D&R, p ; )

28. Important terminology/concepts
Joints- what are they?
Joint ornamentation- plumose structure
Joint kinematics: opening, sliding, scissoring
Griffith cracks and tensile crack formation
Tectonic joints
Exfoliation/unloading joints
Cooling joints
Joint arrays and joint analysis
Shear fracture formation
- en echelon tension gashes
- sense-of-shear indicators
Significance
- tectonics
- engineering
- economic geology
- hazards