TYPES OF ROCKS..

1. JOINTSJOINTS
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3. Joints and veins
Joint: a fracture without measurable shear displacement (cracks or tensile
fractures)
Fault: a fracture with measurable displacement
Vein: a fracture filled with minerals precipitated from solution
Calcite veins fill joints
A fault offsets layers of sediment
SS and clay layers

4. TYPES OF JOINTS
Classification of joints by geometry
1) Non-systematic joints
2) Systematic joints

5. Non-systematic joints are joints that are so irregular in form, spacing, and
orientation that they cannot be readily grouped into distinctive, through-going joint
sets.
Systematic joints are planar, parallel, joints that can be traced for some distance,
and occur at regularly, evenly spaced distances on the order centimetres, meters,
tens of meters, or even hundreds of meters. As a result, they occur as families of
joints that form recognizable joint sets. Typically, exposures or outcrops within a
given area or region of study contains two or more sets of systematic joints, each
with its own distinctive properties such as orientation and spacing, that intersect to
form well-defined joint systems.

6. Systematic joints
Longitudinal joints – Joints which are roughly parallel to
fold axes and often fan around the fold.
Cross-joints – Joints which are approximately perpendicular
to fold axes.
Diagonal joints – Joints which typically occur as conjugate
joint sets that trend oblique to the fold axes.
Strike joints – Joints which trend parallel to the strike of
the axial plane of a fold.
Cross-strike joints – Joints which cut across the axial plane
of a fold.

7. Columnar jointing is a distinctive type of joints that join
together at triple junctions either at or about 120° angles.
These joints split a rock body into long, prisms or columns.
Typically, such columns are hexagonal, although 3-, 4-, 5-
and 7-sided columns are relatively common. The diameter
of these prismatic columns range from a few centimetres to
several metres. They are often oriented perpendicular to
either the upper surface and base of lava flows and the
contact of the tabular igneous bodies with the surrounding
rock. This type of jointing is typical of thick lava flows and
shallow dikes and sills.

8. Types of joints with respect to formation.
Joints can also be classified according to their origin. On the
basis of their origin, joints have been divided into a number of
different types that include tectonic, hydraulic, exfoliation,
unloading (release), and cooling joints depending on the
specific author and publication. Also, the origin of many joint
sets often can be unclear and quite ambiguous. Often, different
authors have proposed multiple and contradictory hypotheses
for specific joint sets and types. Finally, it should be kept in
mind that different joints in the same outcrop may have formed
at different times and for different reasons.

9. Tectonic joints are joints that formed when the relative
displacement of the joint walls is normal to its plane as the
result of brittle deformation of bedrock in response to regional
or local tectonic deformation of bedrock. Such joints form when
directed tectonic stress causes the tensile strength of bedrock
to be exceeded as the result of the stretching of rock layers
under conditions of elevated pore fluid pressure and directed
tectonic stress. Tectonic joints often reflect local tectonic
stresses associated with local folding and faulting. Tectonic
joints occur as both non-systematic and systematic joints,
including orthogonal and conjugate joint sets

10. Hydraulic joints are joints thought to have formed when pore
fluid pressure became elevated as a result of vertical
gravitational loading. In simple terms, the accumulation of either
sediments, volcanic, or other material causes an increase in the
pore pressure of groundwater and other fluids in the underlying
rock when they cannot move either laterally of vertically in
response to this pressure. This also causes an increase in pore
pressure in pre-existing cracks that increases the tensile stress
on them perpendicular to the minimum principal stress (the
direction in which the rock is being stretched). If the tensile
stress exceeds the magnitude of the least principal compressive
stress the rock will fail in a brittle manner and these cracks
propagate in a process called hydraulic fracturing. Hydraulic
joints occur as both non-systematic and systematic joints,
including orthogonal and conjugate joint sets.

11. Exfoliation joints are sets of flat-lying, curved, and large
joints that are restricted to massively exposed rock faces in
an deeply eroded landscape. Exfoliation jointing consists of
fan-shaped fractures varying from a few meters to tens of
meters in size that lie sub-parallel to the topography. The
vertical, gravitational load of the mass of a mountain-size
bedrock mass drives longitudinal splitting and causes
outward buckling toward the free air. In addition, paleostress
sealed in the granite before the granite was exhumed by
erosion and released by exhumation and canyon cutting is
also a driving force for the actual spalling.

12. Unloading joints or release joints are joints formed near
the surface during uplift and erosion. As bedded sedimentary
rocks are brought closer to the surface during uplift and
erosion, they cool, contract and become relaxed elastically.
This causes stress build-up that eventually exceeds the
tensile strength of the bedrock and results in the formation of
jointing. In the case of unloading joints, compressive stress
is released either along pre-existing structural elements
(such as cleavage) or perpendicular to the former direction of
tectonic compression.

13. Cooling joints are columnar joints that result from the
cooling of either lava from the exposed surface of a lava
lake or flood basalt flow or the sides of a tabular igneous,
typically basaltic, intrusion. They exhibit a pattern of joints
that join together at triple junctions either at or about 120°
angles. They split a rock body into long, prisms or columns
that are typically hexagonal, although 3-, 4-, 5- and 7-sided
columns are relatively common. They form as a result of a
cooling front that moves from some surface, either the
exposed surface of a lava lake or flood basalt flow or the
sides of a tabular igneous intrusion into either lava of the
lake or lava flow or magma of a dike or sill.

14. Surface morphology
Plumose structure: wavy structure
on joint
Spreads outward from joint origin

15. Divergent Shear (Transform) Dip-slip & rotation
MODES

16. Surface morphology
Why does the plumose structure form?
Mode 1 loading: should yield smooth fractures perpendicular to σ.
BUT real joints are not perfectly smooth.
Rocks are not homogeneous.
these imperfections distort the local stress field
The stress field at the tip of the propagating crack changes

17. Joint Spacing in sedimentary rocks
Joints are mostly evenly spaced
Widely or closely spaced, partially
depending on length of time tensile
stress applied
Joint spacing and bed thickness:
Closely spaced in thin bedded rx
Wide spaced in thick bedded rx

18. Stress shadows
Cutting many strings in a
row causes a wide band of
strings to relax – larger
area affected
Rigid grid Cutting one string
causes only a few
remaining strings to
relax.
Greater length of joint has
a wider stress shadow

19. Dolomite stiffness >> Sandstone
1) Stretch a block
2) Stress in each bed controlled by
Hookes law (magnitude of stress
depends on E)
3) Beds with large E (stiffness)
develop a greater stress and fracture
first.
Joint spacing and Lithology:
1) Stiffness = Elastic value E,
Youngs’ modulus
2) Hookes law
where e is the elongation
strain
e⋅= Eσ
Stiff dolomite fractures a few times
before the sandstone fractured the
first time.

20. Rocks with low tensile
strength develop more
closely space joints
More tensile strain
(stretching) yields more
joints
AND

21. Joint arrays
Systematic vs nonsystematic joints
Systematic joints:
Planar joints
Joints are parallel or
subparallel.
Same average spacing
Nonsystematic joints:
Irregular spatial distribution
Not parallel to one another.
Different average spacing

22. Joints in the field
Methods
1) Inventory
a) Sample fracture density
b) Sample joint orientation (strike and dip of
joints)
2)Relate to tectonics

23. CATEGORIES OF BRITTLE DEFORMATION
• Frictional Sliding on preexisting fractures
• Cataclastic flow due grain scale fracturing
• Shear rupture at acute angle to max. principle stress
• Tensile cracking perpendicular to dir of min. stess

24. STRESS CONCENTRATION AND
GRIFFITH CRACKS
• A stress concentration is a location in an object where
stress is concentrated.An object is strongest when force is
evenly distributed over its area, so a reduction in area, e.g.
caused by a crack, results in a localized increase in stress.
• Griffith cracks are preexisting microfractures and flaws in
the rock, weakening it. Reason rock failure less than theory

25. Origin and Interpretation of joints
Sheeting joints – uplift and exhumation
Sheeting joints form in a location where σ1 is horizontal while σ3 is
vertical near the ground surface. Joints become more closely placed
near the ground surface

26. Origin and Interpretation of joints
1) Sheeting joints – uplift and exhumation
A cooling pluton contracts more
than country rock. Here, σt
(tensile stress) is oriented
perpendicular to the intrusive
contact.
After exhumation, joints form
parallel to intrusive contact and
creates an exfoliation dome.

27. MECHANICAL EXFOLIATION IN
GRANITEYOSEMITE NATIONAL PARK
Source: Phil Degginger/Earth Scenes

28. Origin and Interpretation of joints
2) Natural hydraulic fracturing
Stresses in the Earth’s crust are
mostly compressive.
How do joints form in such a tectonic
environment?
Effect of pore pressure on fracture.
Increase of pore pressure in a pre-
existing crack pushes outward
Increase of σt (tensile stress) that
allow crack tip to propagate.

29. Origin and Interpretation of joints
3) Regional divergence
High pore pressures in blocks subject to
divergence, weakens confining pressure
Formation of joints in hanging-
wall block with normal faults
Tensile stress σ3
weakest is horizontal,
joints form
perpendicular to σ3

30. http://www.ged.rwth-aachen.de/

31. Calcite-filled wing crack with tip splayCalcite-filled wing crack with tip splay

32. KEEP ATTENTION
Common questions you should ask when mapping
Should I pay attention to joints and veins?
It depends.It depends. What is the purpose of the map?
You should pay attention to them IF the map purpose is, for example:
1) to locate faults
2) to define variations in permeability
3) to define joint intensity for oil and gas exploration (maybe with drill
core data)
4) to define and explore orientation of veins for ore deposits
5) to predict groundwater transport
But IF the purpose is, for example,
(1) understanding stratigraphy, or
(2) the history of folding in high-grade metamorphic rocks
then joint analysis will not help.