AN OVERVIEW OF STRUCTURAL GEOLOGY
Earth Science / IS102 – Thomson
Louella Amor Ramos
John Matthew Castro
Fredz NecesitoRamil Lopez
Senior High School
Structural geology is the study of the three – dimensional disposition and composition of rock structures and using that knowledge to figure out the history of their deformation and how previous tectonic events and geological surroundings affected them that they obtained their current construction. Mapping and fieldwork are fundamental to structural geology. The concept of plate tectonics is also important in this field of study. Since the beginning, the interaction between the plates in the Earth’s crust has played a dominant role in both the formation and deformation of rock bodies. Nicolaus Steno stated that the structure of each layer of the earth told us something important about the history of the earth. So basically, the structure of each layer of the earth has a vital part with the history of the earth therefore, everything that we see now about the earth’s layers is the result of what happened in the past.
Structural geology is the study of the three – dimensional disposition and composition
Structural geology is the study of the three-dimensional disposition and composition of rock structures and using that knowledge to figure out the history of their deformation and how previous tectonic events and geological surroundings affected them that they obtained their current construction. These data could tell us about how and when the rock features formed, as well as about the dynamics and movement of the crust back then. (Russell, 1955)
Mapping and fieldwork are fundamental to structural geology. The primary sources of structural geologists are field relations. These enable them to critically observe rock formations and their composition.
The concept of plate tectonics is also important in this field of study. Since the beginning, the interaction between the plates in the Earth’s crust has played a dominant role in both the formation and deformation of rock bodies. This factor must be considered in observing and determining rock structures’ history.
Though laymen would think of rocks and land formations as hard, immovable solids, through the eyes of a structural geologist, this is not exactly true. In a geological time scale or “deep time”, rocks could be thought as “fluid”, forming and deforming continuously. This type of thinking is fundamental for a structural geologist as it leads to a brand-new perspective of how rock formations are constructed.
Another two significant concepts to be considered in structural geology are stress and strain, as well as the relationship between them.
Stress can be defined in extremely simplified terms as force that tends to deform a body or material, though unlike traction, stress is not a vector quantity but a tensor quantity, which means it acts on volume elements rather than surface elements (Babaie, 2010). It can be separated into two classifications; stress at a point and stress on a plane. Stress at a point can be illustrated using the stress ellipse and its 3D expansion—stress ellipsoid. Stress on a plane can be visualized using the Mohr’s circle or Mohr stress diagram.
As stress is the force exerted on rocks, strain is the reaction of their structures. A rock undergoing deformation, as well as translation and rotation, can change in size and shape, either through dilation (change in size), or distortion (change in shape). There are two types: homogeneous strain (straight lines remain straight after deformation); and heterogeneous strain (straight lines become curved after deformation).
In summary of these two concepts, their relationship can be described in simple terms as stress being the “cause” and strain being the “effect”. Observing this connection, scientists have discovered three basic models of rocks responding to stress, namely: viscous, plastic, and elastic behavior.
Development of the Subdiscipline
Nicolaus Steno, a 17th-century anatomist, could be considered as the first practitioner of structural geology. He recognized that the structure of each layer of the earth held useful information on the history of the earth. An eye-opening journey across the Alps and Apennines fueled Steno’s captivation with the earth, as well as his creation of the fundamental principles of structural geology. In that trip, he “had the chance to see with his own eyes layers of rock packed with fossilized shells, and strata raised and contorted into mountains. In fact, “Steno was the first to assert that history might be recoverable from the rocks and took it upon himself to unravel that history.” (Cutler, 2003).
Another pioneer of structural geology is Grove Karl Gilbert, also known as G. K. Gilbert in academic literature. From late June to the middle of August 1875, he studied an area from Salina, Utah to the west of the Henry Mountains in 1875, where he coined the term “laccolite” (now called laccolith) in describing the intrusions of diorite in the rocks of the mountains. The “peculiar features of the structure” Gilbert noticed divulged the two major processes that formed the Henry Mountains, the upward displacement of the strata and the progression of erosion that shaped the mountains as they are now (Gilbert, 1877).
Christian Otto Mohr, a German civil engineer and graphical tool enthusiast, developed a method for visually representing and analyzing stress known as the Mohr’s circle in 1882, which he improved upon from Carl Culmann’s proposal. Using this approach, he put forth an early theory of shear stress-based strength.
Alfred Wegener and all proponents of the theory of continental drift, which later developed into the theory of plate tectonics, also helped in the development of structural geology, as the concept of tectonic plates is a crucial factor to be considered in researching in this field of study.
Structural geologists are also indebted to John Ramsay, who wrote the classic text Folding and Fracturing of Rocks (1967), which emphasized the significance of strain analysis and provided a detailed and informative depiction of the basics and applications of structural geology. Strain analysis is primarily a challenge in geometry, even a form of art, although it is expressed mathematically (Ramsay, 1967).
Today, structural geology has developed into a diverse science that intertwines both historical-based and process-based methods to understand the evolution of geological structures.
III. Applications of the subdisciplines
Structural geology deals with the geometric relationships of rocks and geologic features in general. The scope of structural geology is vast, ranging in size from submicroscopic lattice defects in crystals to mountain belts and plate boundaries.
There are numerous methods used in structural geology. At the microscopic scale, transmission electron microscopes are employed to enlarge images of lattice defects and dislocations in crystals several thousand times in order to study them, as well as similar techniques applied in petrology, in which sections of rock mounted on glass slides are ground into fine pieces then examined by transmitted light with polarizing microscopes. Nevertheless, some structures can be studied at the macroscopic level using hand specimens, especially when collected in the field.
Techniques of field geology are employed on larger scales. These pertains to: the preparation of geologic maps that show the areal distribution of geologic units selected for representation on the map; and the plotting of the orientation of such structural features as faults, joints, cleavage, small folds, and the attitude of beds with respect to three-dimensional space. A common objective is to interpret the structure at some depth below the surface. It is possible to infer with some degree of accuracy the structure beneath the surface by using information available at the surface. If geologic information from drill holes or mine openings is available, however, the configuration of rocks in the subsurface commonly may be interpreted with much greater assurance as compared with interpretations involving projection to depth based largely on information obtained at the surface. Vertical graphic sections are widely used to show the configuration of rocks beneath the surface. Balancing cross sections is an important technique in thrust belts. The lengths of individual thrust slices are added up and the total restored length is compared with the present length of the section and thus the percentage of shortening across the thrust belt can be calculated. In addition, contour maps that portray the elevation of particular layers with respect to sea level or some other datum are widely used, as are contour maps that represent thickness variations.
Strain analysis is another important technique of structural geology. Strain is change in shape; for example, by measuring the elliptical shape of deformed ooliths or concretions that must originally have been circular, it is possible to make a quantitative analysis of the strain patterns in deformed sediments. Other useful kinds of strain markers are deformed fossils, conglomerate pebbles, and vesicles. A long-term aim of such analysis is to determine the strain variations across entire segments of mountain belts. This information is expected to help geologists understand the mechanisms involved in the formation of such belts.
A combination of structural and geophysical methods are generally used to conduct field studies of the large-scale tectonic features mentioned below. Field work enables the mapping of the structures at the surface, and geophysical methods involving the study of seismic activity, magnetism, and gravity make possible the determination of the subsurface structures.
The processes that affect geologic structures rarely can be observed directly. The nature of the deforming forces and the manner in which the Earth’s materials deform under stress can be studied experimentally and theoretically, however, thus providing insight into the forces of nature. One form of laboratory experimentation involves the deformation of small, cylindrical specimens of rocks under very high pressures. Other experimental methods include the use of scale models of folds and faults consisting of soft, layered materials, in which the objective is to simulate the behaviour of real strata that have undergone deformation on a larger scale over much longer time.
Some experiments measure the main physical variables that control rock deformation—namely, temperature, pressure, deformation rate, and the presence of fluids such as water. These variables are responsible for changing the rheology of rocks from rigid and brittle at or near the Earth’s surface to weak and ductile at great depths. Thus, experimental studies aim to define the conditions under which deformation occurs throughout the Earth’s crust.
Analysis of Related Works/Research
It is important for a structural geologist to analyze and record data of structures (e.g. Faults, folds and boudins, foliations etc.) so that it can give them information about finite and increment strains, and to know how much maximum stress a material can handle.
Sosio De Rosa, S., Shipton, Z.K., Lunn, R.J., Kremer, Y., Murray, T., Along-strike fault core thickness variations of a fault in poorly lithified sediments, Miri (Malaysia), Journal of Structural Geology (2018), the researchers compiled a structural map of outcrops in different sites using high resolution cameras taken by a drone. The maps included all structures that are over 10 cm in length, and the main fault is logged with a measuring tape along its length at every 0.5cm. Some of it can be seen in the figure below.
The fault core is mostly comprised of continuous, low-pervious clay which were present in the map and it varies from 60 cm thick to a layer of 1 – 2 cm thick. The clay rich fault core loses most of its structure and even becomes more chaotic at some locations. Two sections of the fault core are mostly sand-dominated and composed solely of anastomosing sandy shear zones. These sections represent high-permeability zones or otherwise low permeability fault core that is rich in clay.
The study of structural geology has a primary importance in economic geology, both petroleum geology and mining geology. The main target of structural geology is to use measurements to understand the stress field that resulted in the observed strain and geometries. We can also understand the structural evolution of a particular area due to plate tectonics (e.g. mountain building, rifting).
An essential importance of structural geology is to know areas that contain folds and faults because they can form traps in which the accumulation and concentration of fluids such as oil and natural gas occur. Environmental geologists and hydrologists need to understand structural geology because structures are sites of groundwater flow and penetration which may have an effect on leakage of toxic materials from waste dumps or leakage of salty water into aquifers.
Sosio De Rosa, S., Shipton, Z.K., Lunn, R.J., Kremer, Y., Murray, T., Alongstrike fault core thickness variations of a fault in poorly lithified sediments, Miri (Malaysia), Journal of Structural Geology (2018), doi: 10.1016/j.jsg.2018.08.012.
Russell, William (1955). Structural Geology for Petroleum Geologists. New York.
Davis, G. H. (2012). Structural Geology of Rocks and Regions, 3rd Edition. Vitalsource. Retrieved from https://bookshelf.vitalsource.com/#/books/9781118215050/
Babaie, H., April 14, 2010, Force, traction, and stress: http:// www.gsa.edu/~geohab/pages/geo/4013/lectures.htm.