1. What are folds and folding? 2. What the different types of folding? 3. How does folding occur?
Over a long period of time, stress and strain (pressure) causes rocks to buckle and fracture or crumple into folds. When rocks deformed in such a way, it bends instead of breaking. A fold can be defined as a bend in rock that is the response to compressional forces. This is when the rock is being pushed inwards from both sides.
Folding is the process by which rocks bend in response to the compressional force.
When large-scale folding occurs and massive layers of the earth’s crust are uplifted due to compressional force, this pushes towards each other in opposite directions. The uplifted crust form fold mountains, example, The Himalayas.
There are different types of folds created by compressional stress, depending on which way the rock bends.
An anticline is a fold that arches up as both sides of the rock are pushed inwards. The arch looks like an ‘A’. It convex up and has its oldest beds at its core.
A syncline is a fold that sinks down as both sides of the rock are pushed inwards. The fold ‘sinks’ downwards, with the younger layers closer to the center of the structure.
A monocline is a fold where the rock layers form an Sshape, as the sides of the rock are compressed from vertical movement. All the layers of rock are horizontal, going in one direction. Monoclines are layers in only ‘one direction.’ Rock beds lying at two levels are separated by steep inclined limbs.
We also have domes, which are like anticlines but instead of an arch, the fold is in a dome shape, like an inverted bowl.
Similarly, there are also basins, which are like synclines but again, instead of a sinking arch, the fold is in the shape of a bowl sinking down into the ground.
TYPES OF FOLDS
Symmetric fold: Both limbs equally bent, imclined at almost equal angles to the horizontal.
Asymetrical fold: One limb pushed too far, one side gradual slop, other side steeper. Monoclinal fold: One fold is vertical. Isoclinal fold: Both sides are inclined in the same direction. Recumbent fold: One limb is puhed over in the ame direction.
Overthrust fold: Fracture occurs, one limb slides forward over the other limb.
Symmetrical folds are folds with the same angle. This type of fold has two limbs of equal steepness. It results when the two opposing forces of equal strength move towards each other.
Asymmetrical folds are folds with different angles. This type has one limb steeper than the other. It results when the opposing force is greater than the other.
Isoclinal folds are similar to symmetrical folds. These folds both have the same angle and are parallel to each other.
Overturned folds occur when the folding is so intense that the fold appears to have turned over on itself. One limb pushes over the other limb, until the axis is almost horizontal and almost parallel. This occurs when opposing force is much greater than the other.
Recumbent folds are folds that are nearly horizontal. These folds are noted to be lying down sideways.
Chevron folds are folds in a zigzag pattern. The angles of the folds are shaped like chevrons, and instead of slowly curving, the edges are sharp and straight.
Folds are most visible in rocks that contain layering.
Let’s look the difference between folding and faulting
FOLDING AND FAULTING
Forces in earth’s surface caused by the movement of eath’s plates squeezes and bends the surface of earth. Folding occurs when the crust is bent upwards or downwards. Faulting occurs when the crust is actually broken. Judith Henry teaches at Ardenne High School. Send questions and comments to firstname.lastname@example.org
Folds forms at different angles.
The part of a fold