A number of features occur as a result of
plate tectonics for example, earthquakes and volcanism. The movement of the
plates exert tremendous forces of tension, compression and shear and produces some
impressive features over very long periods of time. Some of these features such
as folding and faulting occur in sedimentary rocks. Folding is the process by which the strata of rocks bend to form arches (anticlines) or troughs (synclines) as a result of compression in the earth's crust. Years of sediments or remains
form horizontal layers. This layering process is called stratification, sedimentary rocks are therefore called stratified rocks. The face of each layer
is called the bedding plane.
Figure A: Layers of sedimentary rocks built up from years of sediments.
The earth’s movement cause the sedimentary
rocks to be displaced. The rocks are pushed out of the horizontal plane and
become tilted or inclined. The inclination of the rocks is called the dip.
When earth movement occur folding and faulting of the sedimentary rocks result. Folding
results from lateral forces of
compression. In a simple fold, the
layers of rock which bend up form an upfold
or anticline. Those which bend down form a downfold or syncline. The sides of the folds are called the limbs. If compression continues then
simple folds are change first into asymmetrical
folds, then into overfolds, and
finally into overthrust folds.
Simple fold: A series of gently wave-like
folds formed by compression.
Figure 1: Simple fold (gentle sloping limbs).
Asymmetrical Fold: One of the limb in this
type of fold is steeper than the other.
Figure 2: Asymmetrical fold (one limb is steeper than the other).
Overfold: One limb is pushed over the other
limb.
Figure 3: Overfold (the anticline is pushed over one of the limbs)
Recumbent: With prolonged pressure the folds become increasingly complex and fold over themselves.
Figure 4: Recumbent or
Overthrust: If the folding is intensive the
recumbent fold moves forward over a thrust plane, until it lies almost
horizontal over the lower limb. Fractures occur eventually due to such
intensive folding. These types of folds are called recumbent or nappe fold.
Figure 5: Overthrust or Nappe fold (the anticline is pushed over the limbs and becomes almost parallel to the bedding plane)
Summary of folds
Fold Mountains
Fold mountains formed as a result of folding, includes the Alps, Andes, Himalayas and Rockies.
