Website Earth Science, Geography, Geology, Engineering Geology, Oceanography, Meteorology, Astrononomy, Human Geography, Environmental Sciences, mapping, map usa Joints and their classification - Geo Studies Organization
Menu

Joints and their classification

Joints:

Regular and irregular fractures develop in the rock masses when they are subjected to tensional or compressional forces. Such fractures, along which there has been no relative displacement are called ”Joints”. Joints occur in almost every type of rock. They may be vertical, inclined or even horizontal.

Rocks commonly contain a large number of joints, which lie parallel to one another. These parallel joints together form a ”joint set”. Very often two or more sets of joints occur in rocks, which break them into large nearly rectangular blocks. Two or more sets together constitute a ”joint system”.

Since joints have well defined fracture surfaces, their position in space is recorded in terms of dip and strike. The dip and strike of joints are measured in the same way as that of sedimentary strata.

Classification of Joints:

Joints can be classified on two different bases: (i) Mode of Joint formation, (ii) Geometry Base

Classification of joints on the basis of their mode of formation

Depending on their mode of formation joints can be classified into two groups: (i) tension joints, and (ii) shear joints.

  1. Tension Joints: The tension joints are those, which are formed as a result of tensional forces. These joints are relatively open and have rough and irregular surfaces. The columnar joints in lava flows and the longitudinal joints in anticline that run parallel to the fold axis are examples of tension joints. Tension joints can further be classified into following types;
    • Tectonic Joint: 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 nonsystematic and systematic joints, including orthogonal and conjugate joint sets.
    • Hydraulic Joints: 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 preexisting 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 nonsystematic and systematic joints, including orthogonal and conjugate joint sets. In some cases, joint sets can be a tectonic-hydraulic hybrid.
    • Exfoliation Joints: Exfoliation joints are sets of flat-lying, curved, and large joints that are restricted to massively exposed rock faces in a 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.
    • Unloading Joints: 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 buildup 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 preexisting structural elements (such as cleavage) or perpendicular to the former direction of tectonic compression.
    • Cooling Joints: Cooling joints are also called Primary joints. Columnar joints are good example of this type of joint. They 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.
      • Mural Joints: Granites commonly show three sets of joints mutually at right angles, which divide the rock mass into more or less cubical blocks. Such joints are known as the ”Mural Joints”.
      • Sheet Joints: Sheet Joints are often seen in the exposures of granites. These joints run in the horizontal direction and are formed as tension cracks. They are believed to have developed by unloading through the removal of underlying rocks by erosion. The sheet joints are somewhat curved and are essentially parallel to the topographic surface. They are more conspicuous and close together near the ground surface. 
      • Longitudinal Joints: Joints, which are roughly parallel to fold axes and often fan around the fold.
      • Columnar Joints: Columnar joints are formed in tabular igneous masses such as dykes, sills, and lava flows. These joints divide the rock masses into hexagonal, square rhombic, or triangular columns. Such columns develop at right angles to the chief cooling surface. In lavas and sills, the column is vertical while in dykes they are nearly horizontal.
  2. Shear Joints: The shear joints are those, which are formed due to compressional forces involved in the folding and faulting of rocks. These joints are rather clean-cut and tightly closed. Shear joints occur in two sets, which intersect at a high angle to form a ”conjugate point system”.

Classification of Joints on the basis of their attitude and geometry

Joints can be classified into three types depending on their attitude and geometry; (i) Systematic joints, (ii) Non-systematic joints.

(1). Nonsystematic joints:

Joints that are irregular in form, spacing, and orientation, and can not be easily grouped into any distinctive type of joints are known as the Non-systematic joints.

(2). Systematic joints:

Systematic joints are planar, parallel, joints that can be traced for some distance, and occur at regularly, evenly spaced distances on the order centimeters, 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 contain 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.

Based upon the angle at which joint sets of systematic joints intersect to form a joint system, systematic joints can be subdivided into conjugate and orthogonal joint sets. The angles at which joint sets within a joint system commonly intersect are called by structural geologists as the dihedral angles. When the dihedral angles are nearly 90° within a joint system, the joint sets are known as orthogonal joint sets. When the dihedral angles are from 30 to 60° within a joint system, the joint sets are known as conjugate joint sets.

Within regions that have experienced tectonic deformation, systematic joints are typically associated with either layered or bedded strata that have been folded into anticlines and synclines. Such joints can be classified according to their orientation in respect to the axial planes of the folds as they often commonly form in a predictable pattern with respect to the hinge trends of folded strata. Based upon their orientation to the axial planes and axes of folds, the types of systematic joints are:

  1. Cross Joints: Joints, which are approximately perpendicular to fold axes.
  2. Diagonal Joints: Joints, which typically occur as conjugate joints sets that trend oblique to the fold axes.
  3. Cross-strike Joints: Joints, which cut across the axial plane of a fold.
  4. Strike Joints: The joints which run parallel to the strike of country rocks, are called the ”strike joints”.
  5. Dip Joints: The joints, which run parallel to the direction of dip of the country rocks are known as the “dip joints”.
  6. Oblique Joints: The joints, which run oblique to the dip and strike directions of the country rocks, are known as the ”oblique joints”.
  7. Master Joints: In sedimentary rocks, the joints usually run in two directions at nearly right angles. One set of joints run parallel to the dip direction and the other parallel to the strike direction. Of these one set of joints is commonly more strongly developed and extends for a long distance. Such well-developed and vast joints are known as ”Master Joints”.
  8. Mural Joints: Granites commonly show three sets of joints mutually at right angles, which divide the rock mass into more or less cubical blocks. Such joints are known as the ”Mural Joints”.
  9. Sheet Joints: Sheet Joints are often seen in the exposures of granites. These joints run in the horizontal direction and are formed as tension cracks. They are believed to have developed by unloading through the removal of underlying rocks by erosion. The sheet joints are somewhat curved and are essentially parallel to the topographic surface. They are more conspicuous and close together near the ground surface. 
  10. Longitudinal Joints: Joints, which are roughly parallel to fold axes and often fan around the fold.
  11. Columnar Joints: Columnar joints are formed in tabular igneous masses such as dykes, sills, and lava flows. These joints divide the rock masses into hexagonal, square rhombic, or triangular columns. Such columns develop at right angles to the chief cooling surface. In lavas and sills, the column is vertical while in dykes they are nearly horizontal.

Related posts:

  1. Dip and Strike (geological structure)
  2. Folds (elements of folds, types of folds,)
  3. Mechanics of Folds
  4. Recognition of a Fold
  5. Introduction to faults
  6. Classification/types of the Faults
  7. Pieces of evidence of Faults
  8. Unconformities
  9. Overlap

Leave a Reply

Your email address will not be published.