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Sec 3 IHE GE 2014/Chan ML. Fold is a bend in the rock strata caused by compression. Fold. Folds are formed by compression resulting in shortening and thickening of the crust. Parts of a fold: 1) limbs - the two sides of a fold.

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Folds are formed by compression resulting in shortening and thickening of the crust.

  • Parts of a fold:

1) limbs - the two sides of a fold.

2) axial plane - an imaginary surface that divides a fold as symmetrically as possible, with one limb on either side of the plane.

3) fold axis - a line drawn along the points of maximum curvature of each layer.

Also defined as the intersection of the axial plane with the beds. In some folds, the axis is completely horizontal.

In more complex folding, the fold axis is often

inclined at an angle known as the plunge

- a plunging fold.


types of fold

anticline - formed by upfolding, or arching, of rock layers. It is a structure in which the oldest strata are found in the center.

2) syncline - a downfold, or trough, with youngest strata in the center.

Symmetrical when the limbs on either side of the axialplane diverge at the same angle and asymmetrical when they do not.

Asymmetrical fold is said to be overturned if one limb is tilted beyond the vertical.

Some folds are said to be plunging since the axis of the fold penetrates into the ground.

Types of fold


fold mountains


  • two plates converge
    • two continental plates or
    • a continental and an oceanic plate. Usually found along continents’ edges
  • When the plates converge, the accumulated layers of rock crumple as sedimentary rocks are forced upwards into a series of folds

young fold mountains (10 to 25 million years), (Rockies and Himalayas)

Fold mountains

old fold mountains (over 200 million years), (Urals and Appalachians)


The French Alps




Overturned Folding


Faults are fractures in the crust along which displacement has taken place.

Faults can occur as single discrete breaks, or as part of a large fault system (the San Andreas fault has displacement of hundreds of kilometers and consist of many interconnecting fault surfaces). These so called fault zones can be several kilometers wide.

Sudden movements along faults are the cause of most earthquakes.



Formation of Block Mountains by:


  • Reverse faults
  • Rock layers are compressed
  • Central block is pushed upwards
  • Overhanging escarpment
  • Normal Faults
  • Rock layers are pulled apart
  • Side blocks are lowered relative to the central block

Examples: Sierra Nevada (USA), Flinders Range (Australia), The Tetons (USA)

Examples: Death Valley, California (USA), Black Forest (Germany)


Formation of Rift Valleys by:


  • Normal Faults
  • Rock layers are pulled apart
  • Central block is lowered
  • Example: Great Rift Valley, East Africa
  • Reverse Faults
  • Rock layers are compressed
  • Side blocks are pushed up
  • Overhanging escarpment
  • Example: Great Glenn Fault, Scotland
strike slip faults

Dominant displacement horizontal and parallel to the strike of the fault plane.

  • Rather than a single fracture along which movement takes place, large strike-slip faults consist of a zone of roughly parallel fractures up to several kilometers in width.
  • They DO NOT produce a fault scarp, like the normal and reverse faults
  • Many major strike slip faults cut through the lithosphere and accommodatemotion between two large crustal plates - transform fault.
  • Examples: San Andreas Fault, Anatolian Fault (Turkey)
Strike-Slip Faults

New Madrid



Strike Slip fault

the great rift valley system africa

+9656 km crack (fissure) in the earth's crust,

  • stretching from Lebanon to Mozambique.
  • It divides Kenya into two segments.
  • formed by violent subterranean forces that tore apart the earth's crust.
  • forces caused huge chunks of the crust to sink between parallel fault lines and force up molten rock in volcanic eruptions.
  • this process, called rifting, is still in progress
  • many active and semi-active volcanoes; and hotsprings are along the Rift.
The Great Rift Valley System(Africa)

stages of formation

2 continental plates moving apart (African Plate: Nubian) and Somalian Subplate

  • Due to the rising convection currents
  • Producing Normal Faults
  • The central block is lowered relative to the adjacent blocks
  • The process is accompanied by earthquakes and volcanic eruptions
  • This process is widening the valley
  • The Somalian plate may be pulled apart from Africa, just like what happened to Madagascar
Stages of Formation


The continent of Africa is thought to have been split by a series of rift valleys in various states of development. Those in East Africa are still in thick crust. Those in West Africa are associated with thick oil-bearing sediments. In the Red Sea area the rifting has gone so far as to form a narrow ocean. In the south-east Madagascar has been completely separated from Africa by rifting.

r e f e r e n c e s

  • Chong, Marianne. Aspects of Physical Geography
  • Waugh, David. Integrated Approach
R e f e r e n c e s