Causes of gEOLOGIC hAZARDS.docx
- 1. There are different causes of landslide and sinkhole. Any of the triggers are only caused by a triggering element. A landslide can be caused by a natural or mechanical factor / human. Natural causes of landslides can be due to Climate, Earthquakes, Weathering, Flooding, Volcanic Activity, Forest Fires and Gravity while Humans triggered landslides triggered by mining or clear cutting of trees. In this lesson, we will discuss the causes of geological hazards such as landslides and sinkholes. Types of landslide will also be addressed with you so that you will be able to determine the various effects of rainfall-induced landslides and so that you will be able to determine the soil or soil response in such a situation. It is crucial that you know the various cause of landslide so that you can also process the interaction of these causes with each other in mind. CAUSES OF GEOLOGIC HAZARDS LANDSLIDE NATURAL CAUSES OF LANDSLIDES A. Climate Long-term climatic changes can significantly impact soil stability. A general reduction in precipitation leads to lowering of water table and reduction in overall weight of soil mass, reduced solution of materials and less powerful freeze-thaw activity. A significant upsurge in precipitation or ground saturation would dramatically increase the level of ground water. When sloped areas are completely saturated with water, landslides can occur. If there is absence of mechanical root support, the soils start to run off. B. Earthquakes Seismic activities have, for a long time, contributed to landslides across the globe. Any moment tectonic plates move, the soil covering them also moves along. When earthquakes strike areas with steep slopes, on numerous occasion, the soil slips leading to landslides In addition, ashen debris flows instigated by earthquakes could also cause mass soil movement. C. Weathering It is the natural procedure of rock deterioration that leads to weak, landslide-susceptive materials. It is brought about by the chemical action of water, air, plants and bacteria. When the rocks are weak enough, they slip away causing landslides. D. Erosion Erosion caused by sporadic running water such as streams, rivers, wind, currents, ice and waves wipes out latent and lateral slope support enabling landslides to occur easily. E. Volcanic eruption If an eruption occurs in a wet condition, the soil will start to move downhill instigating a landslide. Stratovolcano is a typical example of volcano responsible for most landslides across the globe. Volcanic gases partially dissolve in groundwater which turns into acidic hydrothermal systems that weakens rock by altering minerals to clay. – F. Forest fires
- 2. Burns vegetation that holds soil in place. Since it takes time for vegetation to re-establish on steep slopes, the risk of landslides prevails or up to 20 years in dry climates. Wildfire removes the protective ability of vegetation which increases runoff, erosion, and debris flow because the water cannot penetrate through the soil thus carrying the debris downslope. burrowing insects also play a role by aerating the soil destabilizing it during a wildfire because they tend to seek for cooler soil to survive. G. Gravity force can trigger a massive landslide. Steeper slopes can also cause the debris to move farther from the foot of the area. HUMAN CAUSES OF LANDSLIDES A. Mining Mining activities that utilize blasting techniques contribute mightily to landslides. Vibrations emanating from the blasts can weaken soils in other areas susceptible to landslides. The weakening of soil means a landslide can occur anytime. Earthwork alters the shape of a slope, imposing new loads on an existing slope. – B. Clear cutting Clear cutting is a technique of timber harvesting that eliminates all old trees from the area. This technique is dangerous since it decimates the existing mechanical root structure of the area. It changes the amount of water infiltrating the soil. Rainfall-Induced landslides are the most common and widespread damaging landslides in the Philippines, with an average of 20 typhoons each year due to prolonged or heavy rainfall. Most rainfall- induced landslides are shallow (less than a few meters deep), thin, and move quickly. Rainfall can be a cause for landslides, but soil conditions are also very critical. Researchers wanted to take into account a number of variables, including weather type, soil type and land-cover characteristics. However, there was no single source for the data they needed. The features of precipitation occurrences, including the accumulated volume of rainfall, length and severity, and the slope of the terrain show the greatest effect on the stability of the slope and the rate of occurrence of landslides and debris. In addition, the precipitation-induced landslide depends on the type of land covered and the type of soil. In short, the cause of rainfall-induced landslides still rely on the factors mentioned. Types of Landslide 1. Soil Creep Landslide is a very slow downslope movement of particles that occurs in every slope covered with loose, weathered material (Britannica dictionary). The level of creep induced by one rainfall
- 3. has a strong association with the amount of rainfall and changes in soil moisture. For the valley-head slope, the dirt creeps down and accumulates a hollow portion of the head at the bottom of the bottleneck, under which the dirt is compressed and forms a wave-shaped soil. 2. Slumping Landslide is a downward movement of rock debris, usually the consequence of removal of buttressing earth at the foot of a slope of unconsolidated material (Britannica dictionary). 3. Debris Flow Landslide happens when the slope becomes saturated with water, this then triggers a landslide of water-soaked mass of rock and soil that slides down the slope. Colluvial landslide in a debris flow valley is a common occurrence that is quickly caused by rainfall. The direct destructiveness of this type of landslide is limited, but if failure occurs, the resultant blockage of the channel can lead to a series of magnified secondary hazards. For this cause, it is important to explore the possible response of this type of landslide to rainfall. 4. Rock Fall landslides are sudden slides caused by heavy rain the rock on the slope loosens and then slides down the slope. SINKHOLE NATURAL CAUSES OF SINKHOLE – Dissolution of sedimentary rocks Sinkholes occur when sedimentary rocks are dissolved. When these rocks are dissolved, it can either form a void of water or air. A void filled with air forma a hole and a void filled with underground water makes a lake or a pond. Sinkholes can be in the form of holes or depressions on the ground. HUMAN CAUSES OF SINKHOLE – Underground water pumping Pumping water underneath the ground causes drainage to alter their flow which causes the soil to be eroded by water. Broken pipes underneath the ground can also cause internal erosion making a void underneath the ground. Altering drainage systems cause water to branch into other routes where there is soluble rock can cause erosion through the flow of water. Types of Sinkholes - Different types of sinkholes are made by various causes. 1. Cover Collapse Sinkhole Develop abruptly (over a period of hours) and thus cause catastrophic damages. They occur where the covering sediments contain a significant amount of clay. Over time, surface drainage, erosion, and deposition of sinkhole into a shallower bowl-shaped depression 2. Cover Subsidence Sinkhole Develop gradually where the covering sediments are permeable and contain sand. In areas where cover material is thicker or sediments contain more clay, cover-subsidence sinkholes are relatively
- 4. uncommon and therefore may not be seen frequently. They are smaller and thus may go undetected for long periods. 3. Dissolution Sinkhole Occur in areas where limestone is exposed at land surface or also is covered by thin layers of soil and permeable sand. Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface. Aggressive dissolution also occurs where flow is focused in pre-existing openings in the rock, such as along joints, fractures, and bedding planes, and in the zone of water-table fluctuation where groundwater is in contact with the atmosphere. Solution sinkholes are generally small in size and also slow to develop. 4.Artificial Sinkhole These types of sinkholes can be caused by various human activities, including groundwater pumping and construction activities. However, the most common activities that result in artificial sinkholes are mining, drilling, considerable changes in weight, as well as a tremendous increase in water flow, such as a construction of an artificial pond. Word Hunt Find eleven causes of landslides in the pool of letters below. Encircle your answer.