Forces Rocks deform in different ways depending on the type and strength of the tectonic force exerted on them. These forces lead to the formation of folded and faulted rock structures.
Folded structures
Fold mountains occur at the edge of continents, along convergent plate boundaries. The continental plate (in collision with either a continental or oceanic plate) is compressed by the forces exerted from both sides. The flat body of rock in the continental crust buckles or folds as it is squeezed. It doesn’t fracture as it is uplifted for two reasons:
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Chapter 1: Plate Tectonics
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the plates are more flexible, as the collision occurs at depths where the rocks are subjected to high temperatures;
the process happens over a long period of time, allowing the rocks to adjust to their new position slowly without breaking.
Elements of a fold
The extent to which the rock surface has been compressed will determine the type of fold. zz
The layers of rock that fold upwards form an anticline, which looks like an arch. zz zz
The layers of rock that fold downwards form a syncline, which looks like a trough.
The sides of a fold that join an upfold/anticline to a downfold/syncline are called the limbs. (Fig. 4.2)
Anticline Limb Compression
Fig. 4.2(a) Elements of a fold. Fig. 4.2(b) A folded rock structure.
Syncline Types of fold zz
Simple (symmetrical) fold: The limbs on either side of the fold have the same steepness and length. Equal pressure has been applied from both sides.
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Asymmetrical fold: The limbs on either side of the fold are different in steepness and length. One limb is steeper and shorter than the other, as more pressure has been applied from one side than the other.
