Joints-Structural Geology presentation.pptx

This presentation offers a detailed overview of joints within the field of Structural Geology. It begins by establishing a fundamental definition of joints as fractures in rock without relative displacement and contrasts them with other fracture types by explaining the different modes of rock failure (opening, sliding, and tearing) . The core of the presentation focuses on a comprehensive classification of joints, examining them from multiple perspectives. This includes classification by genesis, such as tectonic , cooling (columnar) , and unloading (exfoliation) joints , as well as by their geometry, such as systematic, non-systematic, conjugate, and orthogonal sets. The presentation also covers how joints are classified relative to rock bedding (dip, strike, and oblique joints) . Finally, it explores related geological structures, like veins and dykes, which are often associated with jointing. The concepts are supported throughout by clear diagrams and numerous photographic examples from notable geological sites

1. By: Harisankar SVE Department of Applied Geology National Institute of Technology, Raipur JOINTS 1 JOINTS Structural Geology

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3. GENERAL CHARACTERISTICS OF JOINTS JOINTS 3

4. What are Joints? Joints are fractures that divide the rock into different parts or blocks and there is no relative displacement. It occurs in all types of rocks. Rock body subjected to stress undergoes failure upon reaching the ultimate strength of the rock, this results in formation of marked discontinuities called Joints. JOINTS 4

5. What are Fractures? Fracture is any planar discontinuity that is very narrow in one dimensional form as a result of tectonic stress. Displacement field of fracture can be shown in these following modes, MODE 1: Opening (Extension) -Displacement perpendicular to walls of fracture. -This is mainly responsible for Joints, but relative displacement will be low. -Hence it is termed as Extension fracture. MODE 2: These fractures form by Sliding MODE 3: These fractures form by Tearing. JOINTS 5

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7. Orientation of various fracture types w.r.t to principal stress. Compressive stress: Joints form perpendicular to direction of least stress because they cannot resist the opening of the rock unlike maximum stress. JOINTS 7

8. JT1 :- Open Joints, JT2 - Closed Joints JT3- Smooth Joints, JT4 :- Rough Joints JT5 - Small Joints, JT6 :- Master Joints JOINTS 8

9. Systematic Joints: Sub-Parallel Orientation, Regular spacing. Non-Systematic Joints: Curved and Irregular fracture surfaces. JOINTS 9

10. CONJUGATE SETS: Joints always appear as Sets (Joints that share similar orientation in same area). Two sets intersecting each other forms a Joint System. Conjugate Joints are formed when two sets form and intersect at the same time. Conjugate Joints can be formed by both Tensional and Shear stress. Most Conjugate sets intersect at acute angle formed as a result of shear stress. Many intersecting Joint sets are generally taken to be Conjugates irrespective of when they formed, but detailed analysis of their stress history shows they form at different times. JOINTS 10

11. Dihedral angle = 3 0° - 60 ° (Acute angles) The Joint is termed as Conjugate Joints JOINTS 11

12. The Vertical Joints in granite of the Almo pluton at the Breadloaves formation at City of Rocks. JOINTS 12

13. Tensile stress - Joints form perpendicular to Maximum stress. JOINTS 13

14. STRUCTURES AS A RESULT OF JOINTS: VEINS: Extension fractures formed due to tensile stress creates open joints where minerals like quartz, feldspar can get filled forming a vein. JOINTS 14

15. A Dyke can exploit a Joint, since it shows weakness in the overlying rock. It can also results in formation of joints due to cooling of magma that expands the surrounding rocks. So a Joint can indirectly influence formation of dykes. DYKE: JOINTS 15

16. GEOMETRY & GENE T I C S : JOINTS JOINTS 16

17. GEOMETRY OF JOINTS: A single joint is a continuous fracture that has a planar or curviplanar geometery, planar geometry is referred to as regular joints (Systematic) and non-planar joints are termed as irregular joints (Non-systematic). Joints with similar orientation is termed as a Joint set , while two or more sets together forms a Joint system. JOINTS 17

18. Joint system formed by overlapping of 2 Joint sets. JOINTS 18

19. Formation of Joints: Joints results from brittle fracture of a rock body. Mainly formed as a result of tensile and shear stresses. Joints plane always forms perpendicular to σ3 (Minimum stress direction). JOINTS 19

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22. Tectonic Joints: Tectonic joints are formed in response to regional tectonism happening on the Earth crust. As it undergoes subtle movement, it experiences stress, leading to development of Joint patterns.This is generally observed over regional scale Parallel joints in Navajo sandstone, Utah. Formed by tectonic forces. JOINTS 22

23. Cooling Contraction: When a Lava body cools and contracts it forms jointed structures in a systematic manner. Often termed as Columnar Joints. Columnar joints have 5-6 planar sides forming a most efficient geometric resolution of fractures with stress fields is set of hexagonal prisms. If stress, cooling rate, thermal gradient were perfect throughout the cooling magma body, A highly symmetrical cylindrical stress field will form as magma contracts. JOINTS 23

24. Columnar Joints in St. Mary Island, India JOINTS 24

25. Giants causeway, Ireland JOINTS 25

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27. Exfoliation joints is where Curved sheet like fractures are formed. Exfolaition joints forms a Dome like structure (Yosimite National park). Exfoliation forms by long term erosion, expansion in massive rocks, meanwhile unloading joints form by rapid pressure release from glacial erosion or excavation. Unloading joints generally forms when more than half the original overburden has been removed by sudden erosion. UNLOADING JOINTS: JOINTS 27

28. Exfoliation forming a half dome, Yosemite National park, California JOINTS 28

29. Exfoliation joints in Granite, Texas JOINTS 29

30. Fluids trapped in rock layers exerts stress that exceeds the rocks tensile strength. Hydraulic Joints: JOINTS 30

31. CLASSIFICATION & TYPES OF JOINTS JOINTS 31

32. Bedding based Joints: Dip Joints Strike Joints Oblique Joints Joints based on Geometry : Systematic and Non-Systematic Joint Conjugate Sets Persistent and Non-Persistent Joints Orthogonal Joints Mural Joints Based on Regional stress: Tensional Joints Compressional Joints Shear Joints Based on Genesis: Tectonic Joints Columnar Joints Exfoliation Joints Plumose Joints Classification of Joints JOINTS 32

33. BEDDING RELATED JOINTS -Strike Joints -Dip Joints -Conjugate Set JOINTS 33

34. BAS ED ON GEOMETRY Symmetric joints: Joints that are regularly arranged and evenly spaced are said to be symmetric. Non-Symmetric Joints: Joints that are irregularly spaced and unevenly spaced are said to be Non-Symmetric joints. JOINTS 34

35. Persistent Joints: Joints that extend for long distance without interruption. Non-Persistent Joints: Joints that do not extend continuously, they terminate within the rock mass . JOINTS 35

36. ORTHOGONAL JOINTS - Joints in rock that intersect at exactly right angle. (DIHEDRAL ANGLE - 90) ° JOINTS 36

37. MURAL JOINTS: In Granitic Rock masses three sets of joints may occur. In such a way one set is horizontal and the other sets are vertical. All three sets being mutually right angles to each other this type of joints dividing the rock mass into cubical blocks or murals is called mural joints. JOINTS 37

38. BASED ON GENESIS: TECTONIC JOINTS FORMED FROM STRESS ACTING ON EARTHS CRUST JOINTS 38

39. COLUMNAR JOINTS JOINTS 39

40. Weathering leads to Ex foliated Joints. JOINTS 40

41. Exfoliation in Granite, Texas, USA. JOINTS 41

42. Exfoliation Joints in Barela Hills, Madhya Pradesh. JOINTS 42

43. -Forms by tensile stress propagation -Common in fine grained rocks. -The fracture propagates outward from a initial point giving rise to Feather like appearance this is known as Plumose joints. PLUMOSE JOINTS PLUME AXIS JOINTS 43

44. Hinge of Parallel folds have Extension joints in the outer arc and compressional Joints in the Inner arcs. Rock expands in the direction of minimum stress ( σ 3) and rock compress in the direction of maximum stress ( σ 1) . BASED ON REGIONAL STRESS: JOINTS 44

45. (B and C) Shear Joints and Shear Joints with a finite normal displacement JOINTS 45

46. THANK YOU Reference: Structural Geology by Haakon Fossen Structural Geology by R.D Hatcher Jr. JOINTS 46