--- title: "Genetic classification of fault and joint" subject: "Engineering Geology I" chapter: 5 key: "semester_3/engineering_geology_i:genetic-classification-of-fault-and" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/genetic-classification-of-fault-and site: BCT Engineering --- # Genetic classification of fault and joint Engineering Geology I · Chapter 5: Structural Geology - Asked in: 2073 Shrawan, 4 marks - Video: [CE8392-ENGINEERING GEOLOGY- UNIT4 - MODULE 02- FAULTS , JOINT AND CONFORMITY](https://www.youtube.com/watch?v=wdcQdw9SXxs) — Nirmal Ponnambalam - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/genetic-classification-of-fault-and - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5 ## Question How do you classify fault and joint genetically? Describe. ## Short answer Faults are genetically classified by relative movement: normal (tension), reverse/thrust (compression), strike-slip (shear), oblique-slip, rotational, and horst/graben. Joints are classified by origin: tension joints (open, irregular), shear joints (smooth, conjugate), release joints (unloading), cooling joints (columnar), mural joints, and tectonic joints related to folding. ## Answer Genetic classification groups a structure by the way it was formed (its origin and the nature of the movement or stress). #### Genetic classification of faults **By relative movement** 1. **Normal fault** - the hanging wall moves down relative to the foot wall; caused by tension. 2. **Reverse fault** - the hanging wall moves up relative to the foot wall; caused by compression. A reverse fault with a dip of less than 45 degrees is a **thrust fault**. 3. **Strike-slip (lateral) fault** - the blocks move horizontally along the strike of the fault; caused by shearing. It may be right-lateral (dextral) or left-lateral (sinistral). 4. **Oblique-slip fault** - the movement has both dip-slip and strike-slip components. 5. **Rotational (hinge/pivotal) fault** - one block rotates about a point on the fault. 6. **Horst and graben** - a horst is a block raised between two normal faults; a graben is a block dropped between them. #### Genetic classification of joints 1. **Tension joints** - formed by tensile stress (stretching); usually open and irregular. 2. **Shear joints** - formed by shear stress (usually under compression), in conjugate sets; smooth and closed. 3. **Release (unloading) joints** - formed when pressure is released after the overburden is removed; for example, sheeting joints. 4. **Cooling (contraction) joints** - formed in igneous rocks during cooling, such as columnar joints in basalt. 5. **Mural joints** - cubic joints in granite produced by cooling and unloading. 6. **Tectonic joints** - related to folding and faulting; longitudinal, transverse or oblique joints. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which fault has the hanging wall moving down, caused by tension? A. Reverse fault B. Strike-slip fault C. Thrust fault D. Normal fault Answer: D. Normal faults form by tension with the hanging wall down. (Key point: Normal fault) 2. When is a reverse fault called a thrust? A. When its dip is less than 45 degrees B. When its dip is more than 45 degrees C. When it moves horizontally D. When it is caused by tension Answer: A. A low-angle reverse fault is a thrust. (Key point: Thrust) 3. What is a graben? A. A block raised between two normal faults B. A block dropped between two normal faults C. A block rotating about a point D. A block sliding horizontally Answer: B. Graben is dropped; horst is raised. (Key point: Horst and graben) 4. Which joints form in igneous rocks during cooling, such as columnar joints in basalt? A. Shear joints B. Tension joints in sediments C. Cooling (contraction) joints D. Tectonic joints Answer: C. Contraction on cooling gives columnar joints. 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