Lecture 3 Geological Structure PDF

Geological Structure Faculty of Civil Engineering Lecturer: ROTH CHANRAKSMEY (M. ENG) TELEGRAM: 061411010 Nov 2024 ...

Geological Structure Faculty of Civil Engineering Lecturer: ROTH CHANRAKSMEY (M. ENG) TELEGRAM: 061411010 Nov 2024  2 Geological Structure -Geological structures refer to any forms or arrangements of rock layers that result from deformational processes. Understanding geological structures is crucial for civil engineers to ensure the stability and longevity of structures like buildings, bridges, and tunnels. -Structural geology is the study of the deformation of rocks (folds and faults) and other structural features of rocks. -Folds and Faults are geologic structures formed due to the deformation of rocks Major Types of Geological Structures Folds: Form when rocks are deformed plastically under compressional stress. Faults: Breaks in the Earth's crust where significant displacement has occurred. Joints: Fractures in rocks along which there has been no significant movement.  3 Geological Structure 1. Stress and Strain in Rocks Stress: The force applied per unit area within rocks, which causes deformation. Types of stress include compressional, tensional, and shear. Strain: The change in shape or size of rocks as a result of applied stress. Strain can be elastic, plastic, or brittle. Types of deformation Elastic: Temporary change in shape or size that is recovered when the deforming force is removed Ductile (plastic):Permanent change in shape or size that is not recovered when the stress is removed Occurs by the deforming material, without loss of cohesion (folding) Brittle(rupture): Loss of cohesion of a body under the influence of deforming stress (faulting)  4 Folds Folds are types of geological structures that arise due to the curving or bending of Earth’s crust. Folds are secondary tectonic structures that have formed after the deposition of rocks in response to compressional stress. Horizontal layers subjected to compressional stress, which is indicated by the arrows. Fold’s structural elements Link to watch fold simulation of earth‘s fold. https://www.youtube.com/watch?v=6GSqottRpXo https://www.youtube.com/watch?v=l2_nX_ac3dk  5 Folds Stress and strain to create fold in the earth’s crust.  6 Folds  7 Folds Folds can occur in a variety of geological settings, not just in mountainous regions. They are a common feature in areas where the Earth's crust has been subjected to compressional forces, which cause the rock layers to buckle and fold. Here’s an overview of where folds typically form and how they manifest in different environments: Locations and Environments Where Folds Can Occur: 1. Mountain Ranges: Formation: Most notably, folds are associated with mountain ranges. Here, the intense compressional stresses resulting from tectonic plates colliding can fold rock layers extensively. Example: The Himalayas, the Andes, and the Rockies all contain numerous examples of folds that were formed during the orogeny (mountain-building periods) associated with the convergence of tectonic plates. 2. Sedimentary Basins: Formation: Folds can also occur in sedimentary basins which are not necessarily mountainous. These basins may experience subsidence and subsequent compressional forces that result in folding. Example: The Appalachian Basin in the United States contains folded sedimentary rocks formed during the Appalachian orogeny. 3. Continental Interiors: Formation: Even within stable continental interiors away from current tectonic activity, ancient folds can be found. These folds might have formed during earlier orogenic events and are now preserved as part of the geological record. Example: The cratonic areas of continents like the shields in Canada and Scandinavia display evidence of very old folds from events that occurred hundreds of millions to over a billion years ago.  8 Folds 4. Oceanic Settings: Formation: Folds are not confined to continental settings; they can also occur in oceanic plates, particularly in subduction zones where oceanic crust is being compressed and folded before it descends into the mantle. Example: The accretionary wedges in subduction zones are often characterized by folds formed as sediments are scraped off the subducting plate and compressed against the overriding plate. 5. Fault Zones: Formation: Rock layers near major fault zones can be folded due to localized compressional stresses associated with fault movements. Example: Near the San Andreas Fault, minor folding can be observed in association with the complex stress fields generated by the movement of the fault. Understanding Folds: Visual Identification: In the field, folds can often be recognized by their characteristic wavy or layered appearance in exposed rock formations. The layers may bend, twist, or loop due to the deformation. Geophysical Studies: Folds that are buried beneath the surface can sometimes be detected using geophysical methods such as seismic reflection, which can image the subsurface layers and reveal their folded structures.  9 Folds https://www.nagwa.com/en/explainers/924150408952/  10 Folds https://www.nagwa.com/en/explainers/924150408952/  11 Folds https://geologylearn.blogspot.com/2015/08/is-there-oil-beneath-my-property-first.html  12 Why is it important to understand fold phenomena ? Understanding folds in the Earth’s layers is important for several reasons: 1.Finding Resources: Folds often trap valuable resources like oil, gas, and minerals. Knowing where and how folds happen helps us locate these resources more easily. 2.Earthquake Safety: Folds are related to movements in the Earth’s crust, which can lead to earthquakes. By studying folds, scientists can identify areas where earthquakes might happen, helping to keep people safe. 3.Building Safely: When engineers build things like tunnels, dams, or buildings, they need to know if the ground is stable. Folded rocks can be weaker or more likely to move, so engineers study folds to make sure they build on solid ground. 4.Learning Earth’s History: Folds tell us about how the Earth’s surface has changed over millions of years. By studying folds, scientists can learn about past events, like mountain formation or collisions of Earth’s plates. 5.Managing Water Resources: Folds can affect how water flows underground. Some types of folds hold water, while others block it. This knowledge helps us find and manage water supplies, especially in areas that rely on groundwater. 6.Preventing Landslides: In hilly areas, folds can make slopes more or less stable. Knowing about folds helps us understand landslide risks and plan better to avoid them.  13 Faults Fault: A fault is a fracture or zone of fractures in the Earth's crust where two blocks of rock have moved relative to each other. This movement can occur horizontally, vertically, or in both directions. Faults are typically caused by tectonic forces and can be classified by their movement type, such as normal, reverse, and strike- slip faults. Fault Line: A fault line, or fault trace, is the visible surface expression of a fault. It’s where the fault intersects with the Earth's surface. Fault lines are often where earthquakes are most likely to occur because they represent points of weakness in the Earth's crust where tectonic plates or rock layers have fractured and moved. https://www.youtube.com/watch?v=l1TCp4bCvEo  14 Faults The World Fault Line Map shows the major fault lines across the globe. The earth is constantly moving because of which there is a continuous movement of the rocks. This movement of the rocks creates fractures or discontinuity which is better known as a fault. The tectonic forces at work within the rocks create large faults resulting in the release of energy that consequently leads to the eruption of volcanoes and earthquakes. The surface trace of a fault is called a fault line. When a continental/oceanic plate or two continental/oceanic plates or a continental and an oceanic plate move apart, a fault line is created; while when the plates head-on, a fold is created. For instance, as shown in the map, when the Nazca Plate and the South American Plate move apart, a fault line is created that leads to the formation of the Andes mountain range. Similarly, many mountains and other formations are created owing to the movement of other continental and oceanic plates. Types of plate boundary There are three types of plated boundary: divergent: plates moving apart convergent: plates coming together transform: plates moving past each other Boundaries between tectonic plates are made up of a system of faults.  Faults 15 From the Civil engineering point of view, faults are the most unfavorable and undesirable geological structures at the site for any given purpose, i.e. for location of reservoir; as foundations site for construction of dams, importance bridges or huge buildings, for tunneling; for laying roads, railways tracks, etc. Structurally, faults may be described as fractures along which relative displacement of adjacent blocks has taken place. If such relative displacement does not take place on either side of fracture plane, it is called a joint. Thus both joint and faults are fractures in rocks but with difference in the kind of displacement. Joints may be described as a set of aligned parallel cracks or openings in geological formations. https://www.youtube.com/watch?v=l1TCp4bCvEo  16 Faults Fault Terminology Hanging Wall: The block of rock that lies above the fault plane. Footwall: The block of rock that lies below the fault plane. Dip: The angle at which the fault plane is inclined relative to the horizontal. Strike: The direction of the fault line along the Earth's surface, typically measured as a compass bearing.  17 Faults  18 Faults https://www.geologyin.com/2024/09/types-of-faults-with-photos.html  19 Faults  20 Faults  21 Faults  22 Faults Thrust fault https://formontana.net/chief.html  23 Faults  24 Faults Horst and Graben Horst and Graben: this refers to a type of #topography created when the earth's crust is pulled apart. This process, called extension, can stretch the crust up to 100% of its original size. As the crust is strained in this way, normal faults develop and blocks of the crust drop down to form grabens, or valleys. The end result of this is a vast landscape of alternating valleys and ridges. The western United States is an example of this, in the physiographic province known as the Basin and Range.  25 Why is it important to understand fault? Earthquake Prediction and Safety: Faults are where earthquakes often happen. When there’s movement along a fault, it can cause the ground to shake. By studying faults, scientists can figure out which areas are more likely to have earthquakes, helping people prepare and stay safe. Building Safe Structures: Engineers need to know where faults are to build safe buildings, bridges, and roads. If a fault runs through or near a construction site, special designs are needed to make sure the structure can handle possible ground movements. Resource Location: Like folds, faults can trap resources like oil, gas, and minerals. Understanding faults helps us find these resources more easily, as they often form in specific fault zones. Water Management: Faults can also impact the flow of groundwater. In some cases, faults create pathways for water to move, while in other cases, they block it. Knowing where faults are helps in planning wells and managing water supplies. Understanding Earth’s History: Faults show us how the Earth’s crust has moved over time. Studying faults helps geologists learn about the history of tectonic plate movements, mountain formation, and other big changes in the Earth’s surface.  26 Thank you for your attention Faculty of Civil Engineering