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Joints, parts, varieties and clssification

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Joints, parts, varieties and clssification

  2. 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. 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. 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. 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. 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. 7. Tectonic joints
  8. 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. 9. Unloading Joints
  10. 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. 11. Cooling Joints