Fold (geology)
A fold is rock that is bent, instead of being broken, by its deformation. The term is used in geology when one or a stack of surfaces that were originally flat and level, such as sedimentary strata, are bent or curved as a result of high pressures and temperatures. The basic cause is likely to be some aspect of plate tectonics.
When two forces act towards each other from opposite sides, rock layers are bent into folds. Folds are formed by compression, known as folding, which is an endogenetic process (it takes place within the Earth's crust).
Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes on a variety of scales. A set of folds distributed on a regional scale is a fold belt, a common feature of orogenic zones.
There are large-scale and small-scale folds. Large-scale folds are found mainly along a collision boundary between two tectonic plates.
Structure of a fold
[change | change source]The upfold is called an anticline. The downfold is called a syncline.
The imaginary line joining the highest points along the upfold is called the crest line.
The flanks of a fold are known as the limbs.
The central line from which the rock strata dip away in opposing directions is called the axis of fold.
According to the degree of folding of the layers, folds can be classified into five main types.
- Symmetrical fold: two limbs are of equal steepness
- Asymmetrical fold: one limb is steeper than the other
- Overfold: one limb is pushed over the other
- Recumbent fold: two limbs are nearly parallel
- Overthrust fold: the compressional force is so great that the rock strata are broken and slide over one another.
Formation of a fold mountain
[change | change source]Large depressions called geosynclines form between plates. Seas filled the geosynclines and rivers flowing into them carried sediments (sand and silt) which build up on the sea bed.
Over millions of years, the sediments were compressed by their own weight into sedimentary rocks: sandstone, limestone, etc.
Landforms formed by folding
[change | change source]Large-scale folding develops parallel ranges of round-top mountains along destructive plate boundaries, which are known as fold mountains.
Examples of fold mountain ranges: