1. Compressional Forces
Geography grade 8 and 9

2. Folding Occurs along Convergent plate Boundaries. Himalayas
Deep layers of sedimentary
Rock laid down between the plates are squeezed and bent.

3. Himalaya Mountains

4. Fold Mountain Ranges

5. Young Fold Mountains Found along plate boundaries.
Formed 35 million years ago.
Very rugged, steep slope, deep valley and very high ( 6000 m )

6. Old Fold Mountains Not on plate boundaries. 250 million years old .
Formed when the continents were joined.
Eroded for longer.
Lower in altitude and less steep than young fold mountains.

7. Areas between the Fold Mountains
Shields : stable areas, where there has been very little earth movement.
They are flat areas.
Plateaus : Flat high lying areas. ( African Plateau )
Basins : between the shields and the fold mountain .
Called lowlands.

9. The nature of Fold Mountains
Symmetrical Fold
Asymmetrical Fold
Overfold
Overthrust Fold
Parts of a fold limb, anticline and syncline

11. FOLDS

12. Isostasy
Long periods when mountains are eroded and weathered and material is deposited on the
sea-bed.
This causes a redistribution of weight on the surface of the earth.
The balance between the continents and the sea-bed must be maintained.

13. Maintain the balance
Erosion of the continent leads to less pressure on the mantle beneath the continent.
Weathered material deposited on the sea-bed cause more pressure on the mantle beneath the sea-bed.

14. Slow movement of balance
Movement of material from beneath the sea-bed to below the continents.
Mantle : plastic material that can move.
The continent rises and then sea-bed sink slowly.

16. Faults
In unstable parts of the earth the crust is subjected to TENSION and COMPRESSION forces.
When they crack or break it is called a FAULT.
Fault line : the line along the surface where the break has occurred.

17. San Andreas Fault

18. Types of faults
Normal Fault : tension forces (apart)/ rocks are thrown down.
Reverse fault : compression forces (together)/ rocks are pushed up.
Tear fault : tearing forces (past).

19. Normal Fault

20. Reverse fault

21. Tear Fault

22. Block mountains and rift valley
Faults usually occur in groups .

23. Rift valleys