Joints, parts, varieties and clssification
- 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.
- 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
- 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