1. Joints and Veins 1 Lecture 14 – Spring 2016
Structural Geology
Image:
“Large joint in the Stoner Limestone near Eudora, Kansas. The original joint (fracture) was enlarged by groundwater solution and later infilled with glacial sediment and soil. Scale pole marked in feet. Photo date 1/77, © J.S. Aber.”
Photo: J.S. Aber
Joints and Veins 1
Lecture 14 – Spring 2016

2. Definition of Joint
Joints are naturally occurring, unfilled planar to curviplanar fractures that form as the result of tension, without displacement on the fracture plane
The term joint carries no genetic implications about its origin
Erosive forces often preferentially attack joint surfaces. Chemical weathering requires water, and water cannot penetrate to the interior of most rocks, but easily works its way along joint surfaces. Some physical weathering processes, such as frost wedging, also work on joints. Joints affect the morphology of the rock, the strength of rock, and permeability. They also provide a detailed history of stress and strain in a region, although reading the history isn’t easy in many cases.

3. Common Assumption
Nevertheless, most structural geologists believe they are the result of Mode I loading
This means they are tensile fractures, formed perpendicular to the σ3 trajectory
They parallel the principal plane of stress that contains σ1 and σ2
Some disagreement arises because the term joint is used by some to refer to shear fractures.
Since shear fractures involve displacement, they are faults, and the term joint should not be used to refer to shear fractures.

4. Diagram of Plumose Structure
This roughness is called a plumose structure
The size of the plumose structure depends on the grain size of the rock
Figure 7.3a in text
Joint surfaces are typically not smooth. Exposed joint surfaces often have a roughness that resembles the imprint of a feather.

5. Plumose in Siltstone A plumose pattern in thin-bedded siltstone
Joint origin at tip of pencil
Siltstones produce more prominent plumose structures than coal or shale
Figure 7.2b in text

6. Plumose in Sandstone Wavy plumose structure on a joint in sandstone
Plumose pattern still visible in the coaser grained sandstone
In coarse-grained rocks like granites, the size of the grains often obscures the plumose structure entirely
Figure 7.2a in text

7. Dimple The center of the plumose structure is called the joint origin
Typically there is a dimple in the fracture plane at the joint origin
Figure 7.3b in text

8. Plumose Inner Zones
Surrounding the joint origin is the mirror zone, so called because it is very smooth
Beyond the mirror zone we encounter the mist zone, slightly rougher
Figure 7.3a in text

9. Glass Fracture Photo
This photomicrograph shows the mirror, mist, and hackle zones in a broken glass rod
Even the graininess of fine-grained rocks may obscure these two zones
Note scale
Image source:

10. Plumose Outer Zone
Beyond the mist zone, the hackle zone is encountered
Within the hackle zone, lines, called barbs, curve away from the plume axis
The plume axis and barbs together make up the feathery structure of the plumose pattern
Figure 7.3a in text

11. Plume Axis
The plume axis may be straight
Figure 7.4a in text

12. Wavy Plume Axis
Curved plume axis
Figure 7.4b in text

13. Arrest Lines
The acute angle between the barbs and the plume axis points like an arrow to the joint origin
Concentric rings, called arrest lines, on the joint surface suggest that the joint opened in stages
Figure 7.4c in text
The presence of plumose patterns on many joint surfaces indicates there is an underlying control forming them. Yet a perfectly isotropic, homogeneous material undergoing Mode I loading should produce an perfectly smooth, planar fracture. The plumose pattern suggests that rocks are not perfectly isotropic, and often not homogeneous. Inhomogeneities may indicate differences in composition, and may result from grains not being in perfect contact. The local stress field is distorted by the presence of inhomogeneities, which means the principal stresses at the tips of growing joints are not always parallel to the remote-field σ3 axis. Joint propagation paths twist and tilt slightly as a result. Stress always changes as the crack propagates. We saw previously that the crack length, and particularly the elliptical axis ratio, controls the concentration of stress at the tips of the crack. As the crack lengthens, stress intensity increases, approaching a limiting value. The velocity of crack propagation has been found experimentally to depend on the stress intensity. Velocity is small near the joint origin because the new crack is short. As the crack moves away from the origin, it is longer, and propagates faster. Once the crack is long enough, the stress magnitude at the tip is greater than that necessary to create a single surface. The excess stress creates microscopic cracks away from the joint plane.
The dimple on the fracture service is the actual flaw which first concentrated the stress. Flaws includes pores, preexisting microcracks, inclusions, irregularities on a bedding plane, or primary sedimentary structures, such as ripple marks. Dimples form because the flaw was either not perpendicular to the remote-field σ3 or caused local changes in the stress trajectories. Propagation of the crack quickly returns to the σ1-σ2 principal plane. In the mirror zone, the crack is short, and stress concentration is minimal. Only atomic bonds exactly perpendicular to the local σ3 can break. As the crack lengthens, joint propagation is accelerating, and off-plane bonds begin to break, roughen the surface a bit. In the hackle zone, joint propagation has reached a terminal velocity. Energy is available to create larger off-plane cracks, and these propagate. The tip bifurcates forming microscopic splays. These penetrate the joint wall. The rapidly propagating joint tip may also twist and turn around inclusions, breaking into microscopic steps. The arrest lines represent places where propagation pauses. The visible ridge that forms along the arrest line represents a new mirror/mist zone forming just beyond the roughness of the preceding hackle.

14. Free Surface Definition
A free surface is one in which there is no cohesion between layers
No atomic bonds exist across the surface
This makes it impossible for shear stresses to be transmitted
By definition, a free surface is a principal plane of stress, although it may not parallel any principal plane of the far-stress field
Another factor which influences crack propagation is the presence of a free surface
Thus, the stress field must curve from the far-stress field to become oriented with the free surface as a principal plane of stress. Bedding planes often lack strong cohesion, acting as partially free surfaces, and twisting local stress-fields.

15. Twist Hackles
The twisting stress results in twist hackles, and the edge of the fracture plane where the twist hackle occurs is the hackle fringe
Figure 7.3a in text

16. En Echelon Surface
When the hackle fringe intercepts an outcrop surface, the trace of a large planar joint within the outcrop may resemble a series of small joints arranged en enchelon
En echelon cracks, resembling small offset normal faults, has recently been discovered off Virginia and North Carolina
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Slightly overlapping, approx. parallel faults with the same sense of displacement are called "en-echelon" faults
Joint may be either systematic or nonsystematic. Systematic joints are parallel, or nearly parallel, to a common plane, and maintain roughly equal spacing within a given field of view. While there is no limitation on the spacing, several of them must be visible within the field of view. Systematic joints can be limited to a single stratum, or may cut through many layers.
Nonsystematic joints violate the spacing or parallelism requirements, or both. Nonsystematic joints may terminate at other joints.

17. Nonsystematic Joints
Nonsystematic joints violate the spacing or parallelism requirements, or both
Nonsystematic joints may terminate at other joints
Figure 7.5b in text

18. Systematic Joint Photo
Three sets of systematic joint controlling erosion on Kangaroo Island, Australia
Note rock hammer for scale
Figure 7.5a in text

19. Vertical Systematic Joints
Systematic joints in a vertical outcrop
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20. Joint Sets and Systems A joint set is a group of systematic joints
If two or more joint sets intersect at constant, or nearly constant, angles, they form a joint system
The angle between joint sets within a system is the dihedral angle

21. Orthogonal System
If the dihedral angle is ninety degrees, the system is orthogonal

22. Conjugate System
Although many geologists use the terms orthogonal or conjugate to imply that the different joint sets formed simultaneously, the definitions do not require any time relationship, and many system have sets formed at different times.
Many possible relationships exist between joint sets in a system. Figure 7.6 shows the outcrop patterns of some possibilities as seen on bedding surfaces. Joint patterns are distinguished by the angle between the joint sets, and on the relative lengths of joints in different sets. If one set has relatively long joints, and the other set is relatively short, the long joints are called master joints, and the short set is called cross joints. Where flat-lying sedimentary beds are present, joints sets are usually perpendicular to the ground surface and to bedding. Orthogonal systems are common.
If the angle is considerably less than ninety degrees, usually in the 30-60º range, the system is conjugate

23. Joints in Gently Folded Strata
Where sedimentary strata are gently folded, such as near mountain ranges, strata often contain both vertical joints and joints at a high-angle to the bedding
Both orthogonal and conjugate systems are likely to be found in gently folded regions
Figure 7.6b in text
Joints may have a relationship to the folds. Some joints may parallel the strike, and are called strike-parallel joints. Cross-strike joints are formed at a high angle, generally between 60 and 90̊, to the regional bedding strike. If conjugate systems are present, two cross-strike joint sets with their acute bisectrix at a high angle to the hinge of the fold.
In mountainous regions, intense deformation is likely. Metamorphism has generally occurred. Many nonsystematic joints may be present, and recognizing joint systems may not be possible. Joints formed prior to deformation, and especially before metamorphism, may have been erased. New joints form during deformation, during the uplift that follows the initial deformation, and in response to more recent stress. Rocks are likely to be extremely heterogeneous with widely varying stress fields. Joints then occur with a wide variety of orientations. Younger joints, formed during uplift or from recent stress fields, may stand out.
Crystalline rocks lacking strong schistoscity, such as granite or gneiss, often contain a set of joints roughly parallel to ground-surface topography. The spacing decreases progressively toward the surface.

24. Exfoliation Domes
With a nonhorizontal ground surface, sheeting joints curve, following the face of a mountain
Rock-sheets peel off the surface creating smooth, dome-shaped structures called exfoliation domes
Crystalline rocks lacking strong schistoscity, such as granite or gneiss, often contain a set of joints roughly parallel to ground-surface topography. The spacing decreases progressively toward the surface. These joints are called sheeting or exfoliation joints
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25. Sentinel Dome, Yosemite N.P.
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There are a number of such domes in Yosemite National Park in California

26. North Dome, Yosemite N.P. Photographed from Stoneman Meadow
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Photographed from Stoneman Meadow

27. Half Dome, Yosemite N.P. Photo from Glacier Point
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Photo from Glacier Point
Half of the dome was removed by a glacier moving down Yosemite Valley

28. Stone Mountain, Georgia
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Hypabyssal intrusions, dikes or sills, sometimes display columnar jointing.
Stone Mountain is a large pluton that shows exfoliation

29. Columnar Joints
When viewed edge on, these columns usually have a hexagonal cross-section, although 5-7 sides are possible
Photo of top of Devil’s Postpile N.M.
Figure 7.06a in text
Image source:

30. Devil’s Postpile, California
The long axis of the columns are perpendicular to the boundaries of the sheet. In some bodies, the columns curve.
Left image source:
Right image source:

31. Devil’s Tower N.M., Wyoming
Left image source:
Right image source: past/dtower/dt.htm

32. Giants Causeway, Ireland
Left image source:
Nonparallel joints can be studied at their intersections to determine the relative age relationships. A joint surface is a free surface, and another joint cannot propagate across it. Thus, if one joint terminates at another joint surface, the one which terminates is younger. Older joints may influence younger joints in another way. As it approaches an older joint, the younger joints orientation may change. If the older surface is acting as a free surface, the younger joint must be either parallel or perpendicular to the joint surface, so that it is perpendicular to σ3. Near a joint surface, the local stress field usually differs from the remote stress field, unless the free surface parallels a principal plane of stress. So young joint surfaces are likely to curve as they approach older joints.

33. J Joints
If the local σ3 is parallel to the older joint walls, the younger joint will curve so it is orthogonal to the older joint at the point of intersection
This behavior is called hooking, and forms “J” joints

34. Sigmoidal Joints
If the local σ3 is perpendicular to the older joint walls, the younger joint becomes parallel to the older joints, creating a sigmoidal appearance
Photo shows calcite infiliing of sigmoidal joints to form veins
Figure 7.06a in text
In some cases, joint sets may intersect without apparent interaction. There are three possible reasons:
The older joint did not act as a free surface
The intersection of two younger joints at the same point on an older joint is a coincidence
Within the body of the outcrop, the older joint had terminated. The younger joint grew around it, giving the illusion of cross-cutting
Image: RWA/GS_326/326_Gallery.html

35. “x” and “+” Joints
Within stratified sedimentary rocks sets, joints are often evenly spaced. We can measure the distance perpendicular to the joint surface and report it as the joint spacing. Joint spacing measurements are typically an average of as many spacings as can be conveniently measured.
If the mutual crosscutting joints are orthogonal, they a called “+” joints
If they are not orthogonal, they are called “x” joints
Figure 7.06a in text

36. Joint Development The development of even spacing
At time seven, we see five evenly spaced joints
How did they develop?
They could have formed simultaneously, or in sequence
Figure 7.08 in text

37. Sequential Formation Experimental work suggests they form in sequence
When a new joint forms, it is at some distance from a preexisting joint
This may be greater than the minimum distance, dm, over which the joint formation relieves tensile stress
The zone near the joint in which stress is reduced is called the joint stress shadow. To create a new joint, stress must build up, and this can only happen outside the stress shadow. Traction between beds above and below the bed in question, as well as far-field stress that may have passed around other joints, accumulates and starts a new joint.
Figure 7.09a in text

38. Closely-Spaced Joints
Thin-bedded contain joints with narrow stress shadows
Joint spacing is thus closely spaced
Figure 7.09b in text

39. Widely-Spaced Joints In thicker strata, stress shadows are larger
This results in joints that are widely spaced
Figure 7.9c in text

40. Joint Spacing Parameters
The width of the stress joint shadow determines the spacing of joints
Joint spacing depends on four parameters:
Bed thickness
Strain
Stiffness
Tensile strength

41. Bed Thickness and Strain
As bed thickness increases, so does the width of the stress shadow
Because the wider bed joints have larger stress shadows, the joints are spaced further apart
As extensional strain increases, so does the number of joints
Figure 7.10 in text
Cutting one spring affects only
a few springs – as more are cut,
the effect increases

42. Stiffness
The stiffness, or Young’s modulus, depends strongly on the type of rock
Dolomite has three times the stiffness of sandstone (600 MPa vs. 200 MPa)
A three layer sequence of dolomite, sandstone, and dolomite
Figure 7.11 in text

43. Effect of Stretching
Hooke’s Law tells us what happens when the layers are stretched:
σ = E•e
If each layer is stretched by the same amount (e = constant) than the stress depends on the stiffness
Rocks with higher E will have more stress, and will fracture earlier
Before the sandstone develops its first joint, the dolomite might fracture several times
Stiffer beds show greater jointing than beds with lower stiffness. However, we need to consider tensile strength as well. If the rock with lower E also has a lower tensile strength, it may crack more than the stiffer bed. If other factors are equal, rocks with lower tensile strength fracture more than rocks with higher tensile strength.

44. Grand Canyon N.P. Reddish slopes of the Hermit Shale
The canyon country of the southwestern U.S. allows us to see these principles in action.
Shales are often thinly bedded, and are weak. They contain many joints, and break into fragments. The fragments slip, forming slopes at the angle of repose.
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Reddish slopes of the Hermit Shale

45. Walnut Canyon N.M., Arizona
Cliffs of Permian Kaibab Limestone
Sandstones and limestones, usually more thickly bedded, and stronger, develop fewer joints.
The joints control erosion, and maintain the high cliffs we see exposed in canyons.
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