Endogenetic Forces — Diastrophism, Folding & Faulting
Learn the Concept
AllEndogenetic (internal) forces draw their energy from within the Earth (radioactivity, primordial heat, gravity, rotation) and act to build up the relief of the surface. They operate as slow (diastrophic) movements — which warp, fold, uplift and fracture the crust over long spans — and as sudden movements (earthquakes and vulcanism, covered in later leaves). Fix the logic: diastrophism is the umbrella term for crustal deformation; where rocks are compressed and bend plastically we get folds and fold mountains; where they are stressed beyond their strength and break we get faults, rift valleys and block mountains. These structures are the raw framework on which exogenetic forces later carve the detailed landscape.
- Diastrophism: slow crustal deformation — epeirogenesis (continent-building, vertical) & orogenesis (mountain-building, compression).
- Folding: compression bends rock — anticline (up-arch), syncline (down-fold) → fold mountains.
- Faulting: stress fractures rock — normal (tension), reverse/thrust (compression), strike-slip/transform (horizontal).
- Products: horst & block mountains (up-thrown), graben & rift valleys (down-dropped).
- Diastrophism: all movements that deform the crust (warping, folding, faulting, uplift/subsidence).
- Epeirogenesis: broad vertical uplift/subsidence of continents (building continents).
- Orogenesis: mountain-building by intense horizontal compression.
- Anticline / Syncline: an up-arched fold / a down-sagging fold.
- Hanging wall / Footwall: the fault block above / below an inclined fault plane.
- Horst / Graben: an up-thrown block / a down-dropped block between faults.
1. Diastrophism: epeirogenesis & orogenesis
- Diastrophism covers all slow deformations. Epeirogenesis (“continent-making”) is broad, radial/vertical movement of large areas with little folding — producing crustal warping (gentle up- or down-bends), continental uplift (raised beaches, wave-cut platforms above sea level) and subsidence (submerged forests, drowned valleys). Orogenesis (“mountain-making”) is intense, tangential/horizontal compression concentrated in narrow mobile belts, crumpling rocks into fold mountains (Himalayas, Alps, Andes, Rockies).
2. Folding — anatomy and types
- Under horizontal compression, plastic layered rocks buckle into folds. Parts of a fold: the two sloping sides are limbs; the line of maximum curvature is the axis/hinge; the imaginary plane dividing the fold symmetrically is the axial plane; the highest point is the crest, the lowest the trough; a fold that tips so its axis is not horizontal is a plunging fold.
- Types by form: the up-arch is an anticline; the down-sag a syncline; a one-limbed step-bend a monocline; a circular up-dome a dome and down-sag a basin. A series of anticlines is an anticlinorium, of synclines a synclinorium.
- Types by symmetry/intensity: symmetrical (upright, limbs dip equally), asymmetrical (one limb steeper), overturned (one limb pushed past vertical), recumbent (axial plane nearly horizontal), isoclinal (both limbs parallel), and the open/closed(tight)/box/fan folds. An extreme recumbent fold that fractures and slides forward as a sheet is a nappe (common in the Alps and Himalayas). Large-scale folding builds fold mountains.
3. Faulting — anatomy and types
- When stress exceeds rock strength, rocks fracture and the blocks slip along a fault plane. Parts: the block above an inclined plane is the hanging wall, below it the footwall; vertical displacement is the throw, horizontal displacement the heave; the angle of the fault from the vertical is the hade.
- Normal fault — caused by tension; the hanging wall slips down relative to the footwall (crust extends). Reverse fault — caused by compression; the hanging wall is pushed up (crust shortens). A low-angle reverse fault is a thrust fault (e.g. Himalayan MCT, MBT, HFT). Strike-slip (tear/transcurrent) fault — blocks slide horizontally past each other (e.g. San Andreas, Chaman fault). A transform fault is a special plate-boundary strike-slip fault that offsets mid-ocean ridges. Where many parallel normal faults occur, they form step faults.
4. Fault-produced landforms: horst, graben, rift
- An up-thrown block between two faults is a horst, forming a block/fault mountain — e.g. the Vosges & Black Forest (flanking the Rhine), and in India the Satpura range. A down-dropped block is a graben, forming a rift valley — e.g. the East African Rift (Great Rift Valley), the Rhine graben, the Dead Sea, and in India the Narmada and Tapi troughs. Faulting also yields fault scarps (and, after later erosion, fault-line scarps).
- Origin of rift valleys — two views: the tension theory (J.W. Gregory) holds that crustal stretching lets a central block subside between normal faults; the compression theory holds that squeezing thrusts the side-blocks up over a trapped central block. Modern plate tectonics favours tension at divergent/incipient-rift boundaries (East African Rift = a continent beginning to split).
| Feature | Stress | Movement / Result | Example |
|---|---|---|---|
| Anticline / Syncline | Compression | Up-arch / down-fold | Himalayan folds |
| Nappe | Extreme compression | Recumbent fold thrust forward | Alps, Himalaya |
| Normal fault | Tension | Hanging wall down | Rhine graben faults |
| Reverse / thrust | Compression | Hanging wall up | MCT, MBT (Himalaya) |
| Strike-slip / transform | Shear (horizontal) | Blocks slide sideways | San Andreas; ridge offsets |
| Horst / Graben | Tension (block) | Block mountain / rift valley | Satpura / Narmada, E. African Rift |
- Epeirogenesis = continent-building (vertical); orogenesis = mountain-building (compression).
- Fold parts: limb, axis, axial plane, crest, trough; anticline up, syncline down; extreme → nappe.
- Fault parts: hanging wall, footwall, throw, heave, hade.
- Normal = tension (down); reverse/thrust = compression (up); strike-slip/transform = horizontal (San Andreas).
- Horst → block mountains (Satpura, Black Forest); graben → rift valley (Narmada, Tapi, East African Rift).
- Plate tectonics: compression at convergent, tension at divergent boundaries (next leaves).
- Mountain building / orogenesis: fold mountains & nappes (later leaf).
- Earthquakes: sudden slip along faults; elastic rebound (later leaf).
- Drainage & Indian geography: trellis drainage; Narmada–Tapi rift valleys.
Prepare for the Exam — Prelims
PrelimsTested on epeirogenesis vs orogenesis, fold anatomy/types, fault anatomy/types with their stresses, and horst/graben examples (incl. Indian).
- Epeirogenesis = vertical (continents); orogenesis = compression (mountains).
- Fold parts: limb, axis, axial plane, crest, trough.
- Normal = tension (down); reverse/thrust = compression (up).
- Strike-slip/transform = horizontal (San Andreas; ridge offsets).
- Horst = block mountain (Satpura); graben = rift valley (Narmada, E. African Rift).
- Normal fault = tension (NOT compression).
- Thrust = low-angle reverse fault (Himalaya: MCT, MBT, HFT).
- Nappe = extreme recumbent fold thrust forward as a sheet.
- Narmada & Tapi occupy graben (rift) troughs.
Prepare for the Exam — Mains
MainsOptional Paper I / GS Paper I. The examiner rewards linking stress type to the resulting structure and tectonic setting, with labelled sketches.
- Classify folds and discuss their geomorphic significance. (15)
- Discuss the types of faults and the landforms they produce. (15)
- Distinguish between epeirogenesis and orogenesis. (10)
- Examine the origin of rift valleys. (10)
- Diastrophism: epeirogenesis/orogenesis.
- Folds: anatomy, types, fold mountains.
- Faults: anatomy, types, stresses.
- Landforms: horst, graben, rift (two theories).
- Cite MCT/MBT/HFT thrusts (Himalaya).
- San Andreas (strike-slip); Gregory tension theory.
- East African Rift, Narmada graben.
How to Write the Answer
Mains“Discuss the types of faults and the landforms associated with them.” (15)
Classify by stress and movement, then tie each to its landform, with labelled sketches.
- Swapping tension/compression.
- Confusing horst and graben.
- No labelled diagram.
Prelims Questions
Prelims30 interactive MCQs — previous-year questions are included and tagged with their year. Select an answer for instant feedback and an explanation; your score is tracked and you can review wrong answers at the end.
Mains Questions
Mains15 mains questions — previous-year questions are included and tagged. Write a full answer, then click Evaluate My Answer for paragraph-level feedback. Reveal the model structure after you attempt.
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Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
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Body 1: conceptual / historical basis with a named fact, date or example.
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