Lesson on folding, types of folds and the resultant features of folding
Folding -Process in which crustal rocks are distorted by compressional forces by being caused to bend upwards and downwards. It occurs on fairly young sedimentary rocks. Parts of a Fold(a) Anticlines (upfolds)-parts of the earth’s surface which bend upwards when folding occurs. (b) Synclines (down folds)-Parts of the earth’s surface which bend downwards when folding occurs. (c) Crest-upper most part of Anticline. (d) Trough-lowest part of a syncline (e) Limp-rock layers sloping on both sides of a fold(f) Axis-imaginary line drawn vertically through the centre of the anticline. Types of Folds 1. Simple Symmetrical Folds - Which are symmetrical about the anticline. - Formed by 2 compressional forces of equal magnitude.2. Asymmetrical Folds - Which are asymmetrical about the anticlines axis or in which one limp is steeper than the other. - Formed by two compressional forces of unequal magnitude in which one is stronger than the other.3. Over Folds- In which anticline of one fold is pushed over the limp of the other. 4. Isoclinal Folds- Which are packed closely together and with limps almost parallel to each other. - Vertical Isoclinal folds are formed by compressional forces of equal magnitude while inclined Isoclinal folds are formed by forces of unequal magnitude. 5. Recumbent Folds- Which lie in a horizontal manner. - Formed by two compressional forces one of which is very strong. 6. Nappe/Overthrust Fold - In which one limp is pushed over the other limp. - The forces are very strong and they cause a fracture/fault to develop.7. Anticlinorium and Synclinorium Complex - Folds characterized by minor upfolds and minor downfolds. • Land is first subjected to weak compressional forces resulting into minor folds. • Later the land is subjected to much greater compressional forces resulting into new upfolds with minor folds (Anticlinorium) and new down folds with minor folds (Synclinorium).