Joints are fractures in rocks where the rock has broken, creating two free surfaces. Joints form due to contraction from cooling, consolidation, or tectonic stresses. Joints are classified based on their formation process or geometry. Tectonic joints form from differential stress and may indicate past stress orientations. Unloading joints form from uplift and erosion reducing compressive loads. Cooling joints commonly form vertically in cooling lava.

1. JOINTS, PARTS, VARIETIES AND
CLASSIFICATION

2. Joints
• Fractures are surfaces along which rocks or
minerals have broken, thus generating two free
surfaces where none existed before.
• Typically, there is little to no lateral movement
across joints.
• Joints occur in all types of rocks. These may be of
small sizes extending only for a few centimeters
in length or may be extremely extensive.

3. • Joints are formed as a result of contraction due to
cooling or consolidation of rocks and also by
tectonic movements (compressional and
tensional or shearing forces).
• When rock masses are subjected to these forces ,
joints may be developed in more or less regular
pattern.
• Joints form in solid, hard rock that is stretched
such that its strength is exceeded (the point at
which it breaks).

4. Joints Terminology
• Joint set: A series of parallel joints is called joint
set.
• Joint system: combination of two or more joint
sets intersecting each other.
• Conjugate: two sets of joints crosses nearly at
right angle to each other.

5. Varieties & Classification of Joints
• Joints are classified by the processes responsible for their
formation, or their geometry.
• Depending upon the formation joints are classified as
1. Tectonic joints
2. Unloading joints
3. Cooling joints
• Joints can be classified into three groups depending on their
geometry
1. Strike joints – Joints which run parallel to the direction of
strike of adjacent rocks are called "strike joints"
2. Dip joints – Joints which run parallel to the direction of dip of
adjacent rocks are called "dip joints"
3. Oblique joints – Joints which run oblique to the dip and strike
directions of the adjacent rocks are called "oblique joints"

6. Tectonic joints
• Tectonic joints are formed during deformation
whenever the differential stress is high enough to
induce tensile failure of the rock, irrespective of the
tectonic regime.
• They will often form at the same time as faults.
• Measurement of tectonic joint patterns can be useful
in analyzing the tectonic history of an area because
they give information on stress orientations at the
time of formation.

7. Tectonic joints

8. Unloading joints
• Joints are most commonly formed when uplift and
erosion removes the overlying rocks thereby
reducing the compressive load and allowing the rock
to expand laterally.
• Joints related to uplift and erosional unloading have
orientations reflecting the principal stresses during
the uplift.
• Care needs to be taken when attempting to
understand past tectonic stresses to discriminate, if
possible, between tectonic and unloading joints

9. Unloading Joints

10. Cooling joints
• Joints can also form via cooling of hot rock masses,
particularly lava, forming cooling joints, most
commonly expressed as vertical columnar jointing.
• The joint systems associated with cooling typically
are polygonal because the cooling introducing
stresses that are isotropic in the plane of the layer

11. Cooling Joints