When we talk about the structure and composition of the Earth's crust in geology, we inevitably mention folds. Folds are the most common structures affecting all geological materials. The best known are the anticline and synclineTherefore, we're going to dedicate this entire article to discussing the structures and importance of the different types of folds and their characteristics.
If you want to know more about anticlinal and syncline folds, this is your post.
What are folds
The folds are nothing more than structures that are formed as a result of the deformation of geological materials. It is important to add that these folds of materials do not produce a fracture. These geological structures are formed through a plastic deformation that is formed through the pressure of some tectonic stresses, both compression and expansion. You can read more about this topic in our article on structural geology y geomorphology.
If we remember everything related to Tectonic plates, we can see that the Earth's crust is composed of different tectonic plates and they are not fixed. There are the so-called convection currents The mantle's folds cause the plates to shift continuously, and this is why the continents are constantly moving. Therefore, folds are formed thanks to the existence of different materials with plastic or deformable behavior, such as sedimentary rocks.
Parts of a fold
Each fold has different important parts to analyze. The flanks are the lateral planes that form the fold. Two flanks are needed to form a fold. This component can be interpreted as following the surface where the materials are stratified. You can read more about this topic in our article on cleats.
Another part of a fold is the axis or hinge, which is the line that corresponds to the massive curvature of the fold and is formed by the intersection between the flanks and the different layers or stratification surface. The axial plane is another part of a fold and is formed by the junction between lines of the axes of each layer of the fold. Depending on the axial plane of each fold, it will have different dip angles.
Classification of folds
Now we are going to talk about the different types of folds according to their composition and shape. There are several ways to classify the folds and the most common are those that are related to the way the main elements are oriented. The axial plane, the axis and the angle between the flanks are what make the difference between one type of fold and another.
The first classification we have is according to its shape. This classification is divided into: anticline and synclinal fold. There is also the symmetrical fold. The second classification is based on the dip of the axial plane: here we have the inclined, inverted and lying fold. According to the axis of the fold we have the cylindrical and conical ones.
Another classification, not as widely used, uses the angle between the limbs. Here we have the following folds:
- Weakly bent, interlimbal angle greater than 120 °
- Open fold, interlimbal angle 70 ° to 120 °
- Closed fold, 30 ° to 70 ° interlimbal angle
- Narrow fold, 10 ° to 30 ° interlimbal angle
- Isoclinal fold, interlimbal angle = 0 °
Anticline and syncline
An anticlinal fold is characterized by a convex shape toward the top. This is because the youngest materials that make up the fold are at the top, while the oldest ones form the core. Sometimes we can't determine the age of the materials, and in these cases, it's better to name this structure as antiform. To delve deeper into the topic, you can consult our article on orogenesis y lithosphere.
On the other hand, we have the syncline fold. Its main characteristic is that it is concave toward the top. This is because the youngest materials are found in the core, while the oldest forms the lower part. As with the anticline fold, if we don't know the age of the materials, it is better to name this structure as shapeless.
When we classify a type of fold according to the dip of the axial plane, we must take into account the type of angle we have. In these cases we find symmetrical, inclined, inverted and lying folds. All of these folds have a range of between 0 degrees and 90 degrees.
Symmetrical folds are those where the angle formed by the axial plane is equal on both sides. In this case, the angle formed with the axial plane is vertical. The other type of fold is inclined so that one flank has a greater dip angle relative to the other.
Morphology of the anticline and synclinal fold
We are going to begin to describe the anticline fold. It has its center with a symmetrical axis. The two sides of the anticline show different tilt directions. The strata slope towards the flanks. From the center to the flanks, the dip gradually increases. However, the dip in the center is smaller or zero. To better understand the importance of the lithosphere, you can read our article on .
We proceed to describe the synclinal fold. The center is an axis of symmetry. The two sides of the syncline show different tilt directions. The layers of its interior always lean towards the nucleus. The manteo, in this case, is also zero. The youngest strata emerge in the center and the oldest remain on the flanks.
To see these folds on geological maps, it's as simple as identifying the symmetrical repetition of the materials with respect to a central axis. This is the intersection of the axial plane with the topographic surface. In this symmetrical repetition of materials, we don't have to take into account the outcrop of the materials. This is because the surface thickness and roughness of the materials It depends on the degree of dip that the materials have and the surface we are on.
As you can see, the whole topic of folds is quite complex. I hope I've helped you identify anticlinal and syncline folds on a geologic map.