Geol342
2008
Outline
• Review of Brittle Failure
• Tensile Fractures
– Joints, cleat
– Geometry of Joints and joint sets
– Relationship to stress field
– Mechanisms for joint formation
Once a fracture exists deformation continues by frictional sliding of cataclastic flow
Two modes of Brittle Failure
σ
3
σ
1
Failure= rupture of atomic bonds
1

σ
1
σ
3
Role of Holes
Stress Concentration around holes
C=2(b/a)+1
10:1
Stress concentration= 21X
Stress field around a hole
σ
3
σ
1
Microscopic flaws control the macro strength
Role of Fluid Pressure
Formation of Shear Fractures
σ
3
σ
1
2

Otter Creek, WV
Joints
• The most common type of tensile fracture
• Form near the Earth’s surface
• Control the bulk strength of the rock
– Coal Cleat
• Important fluid conduits
– Groundwater
– Hydrocarbons
Jointed Rock Face, NH
Jointed limestone bed at Lilstock Beach on the southern coast of the Bristol Channel, England (Pollard and
Fletcher, 2005).
Joints in every rock outcrop
Monument Valley, AZ-
Landscapes controlled by Joints
3

Arches National Park-
Landscapes controlled by Joints
Joints are systematically oriented over huge areas
Related to regional stress fields
Surface Joint Morphology
Fracture surface in Plexiglas with fracture propagation textures (Pollard and Fletcher, 2005)
4

Joint Features record fracture propagation
Systematic Joint Sets
Join Spacing
•Spacing decreases with layer thickness
•More joints in stiffer layers
•Often joints are confined to specific beds
•Spacing increases with strain
Cross cutting relationships
Seco nd S et
C
What are the relative ages of the joint sets?
B
A
Appalachian Joint Sets
5

Appalachian Joints
• Set 1 Parallel to folds –strike joints
• Set 2 Perpendicular to fold – cross-fold joints
• What is going on?
Competition between local and remote stresses
Other Common Mechanisms for
Joint formation
• 1. Unroofing – exfoliation
– Rock is elastic!
– It springs back when the load is removed
• 2. Cooling Joints
Exfoliation Joints
Exfoliation Joints in Yosemite National park
Exfoliation Joints
6

Hexagonal cooling joints
Columnar Jointing
Basalt
Cooling Joints in Basalt
7