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The earth's crust is solid, but as new portions are constantly generated and others are destroyed, huge forces occur in its inner zone that eventually deform it.
These forces, acting for millions of years, cause the crust to undulate and form folds, in one place the ground rises, in another it sinks. Sometimes, these forces are so powerful that the elasticity of the materials cannot withstand them and the crease is broken.
The forces that bend the Earth
The rocky materials that form the earth's crust have a certain degree of elasticity, which is maximum in soft sedimentary rocks and minimal in metamorphic rocks. When intense forces act, such as those produced in the collision between continents, the rock yields elastically and bends in a way that depends on its elasticity and the intensity of the force.
These folding processes can occur at a shallow depth and are responsible for the formation of the great mountain ranges of the Earth. If the force exceeds the elasticity, the rock breaks and a fault is formed.
Most of the stratified rocks visible in rivers, quarries or coasts were, in their origin, sediments deposited in layers or horizontal beds. Today they are usually inclined in one direction or another. Sometimes, when the strata surface, you can see how they rise to an arc or descend into a breast.
Folds, anticline and syncline
Each folding unit is called a fold. The upper folds with a domed shape are called anticlinear and have a crest and two slanted branches that descend into adjacent breasts, where inverse bowl-shaped, or syncline folds can be formed.
The monoclines have an inclined branch and a horizontal one, while those of the isoclinals sink in the same direction and the same angle. The periclinales are folds like basins (internal inclination) or domes (external inclination). The folds are measured in terms of wavelength (from crest to crest or from sine to sine) and height (from crest to sine). These folds can be microscopic or have lengths of kilometers.
The surface rocks are so hard and brittle that it seems impossible for them to bend in a plastic manner during a deformation, and unless they flow between the cracks while folding. Heat is an important factor in the depths of the earth's mantle and can convert rocks from rigid to ductile, softening them.
The amount of time the rocks are under tension is also important. The difference in behavior can be explained by considering the example of tar: when hit with a hammer it breaks, but with the effect of gravity it spreads. Similarly, the rocks that undergo rapid deformation processes fracture and produce an earthquake, while the rocks themselves fold if they are subjected to long and continuous tensions.
Sometimes the ground undergoes a slight deformation that does not form a fold. The phenomenon is called "bending" of the terrain. On the other hand, some folds have areas of smaller slope in the middle of a uniformly inclined surface, called "terraces".
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