• 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
• 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.
• 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
• Joints form in solid, hard rock that is stretched
such that its strength is exceeded (the point at
which it breaks).
• Joint set: A series of parallel joints is called joint
• 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.
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
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"
• Tectonic joints are formed during deformation
whenever the differential stress is high enough to
induce tensile failure of the rock, irrespective of the
• 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.
• 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
• Care needs to be taken when attempting to
understand past tectonic stresses to discriminate, if
possible, between tectonic and unloading 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