Structural geology – get acquainted with the basics. Structural geology is the branch of geology that studies the geometry, distribution, and formation of geological structures.

Highlights

• Main geologic structures are created during rock formation
• Studying structural geology means identifying, measuring, and explaining the information to a non-professional.
• There are various methods for structural geology.

What Is Structural Geology?

The term geologic structures refer to the geometric configuration of rocks. This includes studying this configuration when the rocks have undergone some deformation, and this implies that there are two types of geologic structures: primary and secondary.

Primary Geological Structures

Primary geologic structures are those that are created during rock formation. Among these are stratification, diagenetic foliation, flow banding, eutaxitic structure, etc.

• Stratification
• Diagenetic foliation
• Flow banding
• Eutaxitic structure

Secondary Geological Structures

Secondary geologic structures are created during a deformation process that the rock undergoes after it has been formed.

As an example of secondary geological structures, the following can be named: metamorphic foliation, folds, cleavage associated with folding, joints, and faults, as the main ones.

• Pleats
• Cleavage
• Joints
• Faults

We said earlier that structural geology is responsible for studying the geometry, distribution, and formation of geological structures. Therefore, we are going to analyze each of these aspects.

Studying the Geometry of Geological Structures Involves Identifying Their Shape

Studying a structure geometrically implies identifying, measuring, and describing its geometry in an easy way. This done so that both the person who takes the data and the person who reads the data can imagine practically the same thing.

This information will later be useful for elaborating geological models and the respective report that accompanies the models.

Studying the distribution of geologic structures means understanding how the geometry of these structures is spatially located within the geological context in which they are found.

For this purpose, the geologist in the field uses tools such as the topographic map and/or the GPS to know the location of the site. A geological hammer, and a magnifying glass is then used to identify the outcropping rocks. Also, a compass is needed to orient himself, and to orient the geological structures and tools like tape measure is used to know the dimensions of the outcrop, geological units, and geological structures.

Subsequently, the geologist will capture the information collected in his field report. Still, he will do so mainly on a map called a geological map (if it shows all the geological information: lithological units, geological structures, structural data, etc.) or a structural map (when it emphasizes geological structures).

Then, based on this map and, depending on the information collected, the geologist will expand his analysis by preparing geological or structural profiles to understand the geometry of geological units and structures in depth.

Studying the formation of geologic structures implies understanding from theory, in principle, the geological mechanisms and processes that can generate a particular geological structure.

The fundamental principle to start any job is to have a good understanding of the theoretical foundations. If you can identify the primary structures, you will know if an observed structure is secondary or not.

Basic Methods of Structural Geology

Work in structural geology, as well as in any branch of knowledge, requires a methodology that allows obtaining the desired results. In this item, four of the most critical steps of the work methodology in structural geology will be mentioned.

Understand How Rocks Are Deformed

The fundamental principle to start any job is to know and understand the theoretical foundations. If you can identify the primary structures, you will know if an observed structure is secondary or not. Likewise, suppose the type of secondary structure found is identified, its geometry, geometric configuration, spatial distribution, formation process can be analyzed, and, additionally, other geology structures that are possibly associated with the observed structure can be searched.

Describe and Collect Measurements of The Current Geometry of Rocks

Once the primary and secondary structures have been identified, a detailed description of each of them must be made. This description must include the type (or types) of rock present, the type of structure, the type of deformation observed in the secondary structures (continuous, discontinuous, homogeneous, heterogeneous). Also, the response of the rocks to stress (brittle behavior, ductile or brittle-ductile), the area occupied by the deformation. Lastly, the geometric description of each structure, including the structural data taken with the compass.

Interpret the Configuration and Distribution of Geological Structures

The information collected in the previous step must be captured on a geological map to understand the geometric configuration and spatial distribution of the geologic structures. Then, this distribution must be analyzed within the local and/or regional geological context, and, preferably, geological profiles (or three-dimensional models) must be made to infer and understand the distribution of the structures in depth.

Understand the Deformation History of Observed Rocks

Based on the above processes, it is possible to arrive at one (or several) hypotheses about the deformation history of the observed rocks, including kinematic factors, dynamic factors, time, and the evolution of the deformation.