GeographyISC Class 11 · Plate Tectonics and Endogenous Processes

Endogenetic Forces, Isostasy, Folds and Faults (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Endogenetic forces act from inside the earth: slow diastrophic movements (vertical epeirogenic, horizontal orogenic) and sudden earthquakes and eruptions. Compression bends rocks into folds and thrust faults, tension makes normal faults, rift valleys and block mountains, and isostasy explains how light crust floats in balance on denser mantle.

Understand Endogenetic Forces, Isostasy, Folds and Faults

Endogenetic forces. Driven by internal heat; they build relief while weathering and erosion wear it down. Slow (diastrophic) forces: epeirogenic (vertical uplift or subsidence of continents) and orogenic (horizontal; compression gives folds and thrust faults, tension gives normal faults). Sudden forces: earthquakes and volcanic eruptions.

Isostasy. A state of balance in which lighter crust floats on denser plastic material, like wooden blocks of different heights in water.

Pratt: columns of different heights have different densities; higher mountains are lighter; all columns reach the same level of compensation.
Airy: density is similar, but high mountains have deep roots.
Holmes: the sub-crustal material is solid but plastic over long periods and convects, so crust rises when load is removed (melting ice, erosion) and sinks when load is added (ice, sediment). Scandinavia and Canada are still rising after Ice Age ice melted.

Folds. Anticline = up-fold, syncline = down-fold, with limbs and an axial plane. Types: symmetrical (equal limbs), asymmetrical (one limb steeper), monocline (one steep limb), isoclinal (parallel limbs), overturned, recumbent (almost horizontal) and nappe (recumbent fold broken and pushed over other rocks). Fold mountains: Himalayas, Alps, Rockies, Andes. Anticlines can trap petroleum and gas.

Faults. Normal (tension, one block slides down, crust extended); reverse (compression, one block pushed over the other); thrust (low-angle reverse fault moving long distances, common in the Himalayas); tear or strike-slip (horizontal sliding, San Andreas).

Rift valley (graben): block sunk between parallel normal faults (East African Rift, Rhine, Narmada and Tapi valleys).
Block mountain (horst): raised block (Vosges and Black Forest).
Fault scarps are steep slopes along fault lines.

Endogenetic Forces, Isostasy, Folds and Faults: key facts

Endogenetic forces: $E p e i r o g e ni c : v er t i c a l ∣ O r o g e ni c : h or i z o n t a l ∣ S u dd e n : e a r t h q u ak es, v o l c an oes$
Fold types: $S y mm e t r i c a l ∣ A sy mm e t r i c a l ∣ M o n oc l in e ∣ I soc l ina l ∣ O v er t u r n e d ∣ R ec u mb e n t ∣ N a pp e$
Fault types: Normal tension
Reverse compression
Thrust low-angle reverse
Tear horizontal
Isostasy: Pratt density varies by column
Airy roots
Holmes plastic mantle

Normal	tension
Reverse	compression
Thrust	low-angle reverse
Tear	horizontal

Pratt	density varies by column
Airy	roots
Holmes	plastic mantle

How to answer Endogenetic Forces, Isostasy, Folds and Faults questions

Typical 4-mark questions ask you to compare, describe types or explain isostatic adjustment.

1State the force and its direction (tension or compression).
2Describe the structure and name its parts.
3Give a landform and a place.
4For isostasy, add loading or unloading and the direction of adjustment.

Endogenetic Forces, Isostasy, Folds and Faults in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Wooden blocks

Suppose a light block and a heavy block float in a tub of water, and then a stone is placed on the light block.

What does this model and what would happen if the stone were removed?

Try it first, then show the solutionSolution

The blocks model crustal columns floating in balance; the load makes the block sink.
Removing the load lets it rise: isostatic adjustment, as in Scandinavia.

Answer: Isostasy; the block rises when unloaded.

Memory hook: Float, load, sink; unload, rise.

Squeezed carpet

Suppose a carpet is pushed from both ends and rises into a ridge.

Which structure does this resemble and which force acts?

Try it first, then show the solutionSolution

Pushing from both ends is compression.
The ridge is an anticline fold.

Answer: An anticline from compression.

Memory hook: Push the carpet, make a fold.

Tricks to remember Endogenetic Forces, Isostasy, Folds and Faults

Short memory aids. Say each one aloud once, then use it on a question the same day.

Tension drops, compression lifts
Tension: normal fault, rift valley; compression: reverse fault, fold mountain.
Use it when: Force and result.
Graben sinks, horst stands
Graben = ground down; horst = high.
Use it when: Rift and block.

Endogenetic Forces, Isostasy, Folds and Faults: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Force type · 1 mark · MCQBoard level

Forces acting horizontally to build mountains are called:
(a) epeirogenic
(b) orogenic
(c) exogenetic
(d) sudden

Try it first, then show the solutionSolution

Horizontal.

Answer: (b) Orogenic.

Example 2: Fold parts · 2 marks · Short answerBoard level

Define anticline and syncline.

Try it first, then show the solutionSolution

Anticline: up-fold or arch.
Syncline: down-fold or trough.

Answer: Anticline is an up-fold; syncline is a down-fold.

Example 3: Normal versus reverse · 4 marks · Long answerHarder board level

Compare a normal fault with a reverse fault on force, block movement and effect on the crust.

Try it first, then show the solutionSolution

Normal fault is caused by tension; reverse by compression.
In a normal fault one block slides down; in a reverse one block is pushed up over the other.
Normal extends the crust; reverse shortens it.
Landforms: rift valley or block mountain from normal faults; thrust sheets from reverse faults.

Answer: Normal: tension, one block drops, crust extended; reverse: compression, block pushed up, crust shortened.

Example 4: Rift valley formation · 4 marks · Long answerHarder board level

Explain how a rift valley forms and give two examples.

Try it first, then show the solutionSolution

Tension produces two roughly parallel normal faults.
The block between them sinks, or the side blocks rise.
The result is a long, steep-sided valley.
Examples: East African Rift Valley, Rhine rift valley, Narmada and Tapi valleys.

Answer: A block sinks between parallel normal faults; e.g., East African Rift and Rhine.

Example 5: Isostatic reasoning · 5 marks · Case-basedHOTS / competency

Case: A region of thick ice melts, and the land later stands higher than before. (a) Name the process. (b) Explain using Holmes. (c) Name a region the chapter gives as still rising.

Try it first, then show the solutionSolution

(a) Isostatic adjustment.
(b) The plastic sub-crustal material flows slowly, so the unloaded crust rises.
(c) Scandinavia or Canada.

Answer: (a) Isostatic adjustment (b) plastic flow allows uplift (c) Scandinavia or Canada.

Where students lose marks in Endogenetic Forces, Isostasy, Folds and Faults

The slips examiners see most often, and what to write instead.

Mistake: Reversing tension and compression
Forces and faults mismatched.
Do this: Tension gives normal faults; compression gives reverse faults and folds.
Usually costs: 1 to 2 marks.
Mistake: Calling a fold an anticline in both directions
Up and down folds are confused.
Wrong working
Syncline is an arch
Right working
Anticline is an arch
Do this: Anticline up; syncline down.
Usually costs: 1 mark.
Mistake: Writing horst for a rift valley
Terms swapped.
Do this: Graben is the rift valley; horst is the block mountain.
Usually costs: 1 mark.

Exam tips

Name the direction of force for each fold or fault.
Give a named example for every landform.
Use "level of compensation" for Pratt.
Mention both slow and sudden forces.

Practice questions on Endogenetic Forces, Isostasy, Folds and Faults

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The San Andreas Fault is an example of a:
(a) normal fault
(b) thrust fault
(c) tear fault
(d) reverse fault
Hint
Horizontal sliding.
Answer
(c) Tear fault.
Q2 · 3 marks · Short answerHarder board level
What is a nappe and how does it differ from a recumbent fold?
Hint
Broken and pushed forward.
Answer
A recumbent fold lies nearly horizontal; a nappe forms when compression is so great that it breaks and its upper part is pushed forward over other rocks, sometimes for many kilometres.
Q3 · 4 marks · Long answerHarder board level
Compare Pratt's and Airy's views of isostasy.
Hint
Density versus roots.
Answer
Pratt: columns have different densities (higher mountains lighter) ending at one level of compensation. Airy: densities are roughly equal but mountains have deep roots in denser layer, like an iceberg.
Q4 · 4 marks · Long answerHarder board level
Explain how horizontal tension and compression produce different landforms.
Hint
Rift valley versus fold.
Answer
Compression causes folding and reverse or thrust faults, building fold mountains. Tension causes cracks and normal faults, forming rift valleys and block mountains.
Q5 · 4 marks · Case-basedHOTS / competency
Case: Drilling finds petroleum in the crest of an upfold of layered rock. (a) Name the fold. (b) Explain why it traps petroleum. (c) Name the force that formed it.
Hint
Anticline.
Answer
(a) An anticline. (b) The arch holds petroleum and gas in folded sedimentary rock. (c) Horizontal compression.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Up-fold name?Show answer
Anticline.
2.Block mountain term?Show answer
Horst.
3.Vertical movements are?Show answer
Epeirogenic.
4.Nappe is?Show answer
A recumbent fold broken and pushed forward.
5.Fault with tension?Show answer
Normal fault.

Endogenetic Forces, Isostasy, Folds and Faults: frequently asked questions

Is the Himalaya a fold mountain?

Yes, it has folds and thrust faults from continental collision.

What is a graben?

A sunken block between faults, forming a rift valley.

Is isostasy a process?

It is a state of balance, with adjustment when loads change.