Lesson 5 - Exogenic Process PDF

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Lesson 5...

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LESSON 5 – EXOGENIC PROCESSES OBJECTIVES At the end of the lesson, you should be able to: 1. Differentiate exogenic and endogenic process; 2. Compare and contrast the different exogenic processes; 3. Infer the exogenic process that took place in a given rock or land formation. SUBJECT MATTER In the previous lesson, we discussed the processes occurring in rocks. In this lesson and the next, we are going to discuss about processes that affects larger scales such as rock and land formations. We call these processes geological processes. Geological processes are classified into exogenic and endogenic processes. Exogenic processes include weathering, erosion, mass wasting, and deposition. Together, exogenic processes shape the Earth’s landscape. Weathering You may have encountered the concept of weathering as early as your elementary school days. We are going to recall this concept and provide some examples of weathering in everyday action. Weathering is simply defined as a process that leads to breaking down of rocks into smaller and smaller pieces. One particular importance of this process is that it produces soil for our plants to survive as well as release nutrients such as potassium and phosphorus, which are found mainly from broken down rocks. Weathering is divided into two types: physical and chemical weathering. Physical Weathering Physical weathering is where rock is broken down without altering its mineral composition. Agents of physical weathering include ice, temperature, pressure release, and plants. Frost Wedging Ice can cause weathering because of freeze-thaw cycle, a process that occurs when temperature changes between below and above freezing point. This phenomenon is common in high mountainous regions where rocks and ice are both present. During the day, for example, temperature is above freezing point. Water can then seep into small spaces inside rocks. During the night, temperature may go below freezing point, turning water into ice. One unique property of water is that it expands as it freezes. The expansion will generate a force that pushes the surrounding rock aside, creating cracks Figure 5.1. Expansion of water upon and effectively breaking the rock. Because this type freezing generates a force enough to break rocks slowly. of weathering results in wedge-shaped rocks caused by ice, this process is also called frost wedging (Figure 5.1). Insolation Weathering Temperature can also break a rock because of extreme temperature changes. When a material is heated, it expands. When it is cooled, it contracts. Rocks that are exposed on the surface is subject to the changing temperature of day and night. During the day, rocks expand due to the fact that they are exposed to the sun. during the night, the temperature drops, and the rocks contract. The repeated expansion and contraction creates stress that eventually breaks the rock. In other terms, this type of weathering is called insolation weathering since it is the sun’s radiation which heats the rock (Figure 5.2).

Figure 5.2. Repeated expansion and contraction due to extreme temperature can stress a rock and cause it to break

Pressure Release In some not-so-familiar cases, rocks can also be broken down by pressure release. This is particularly evident in plutonic rocks such as granite domes. Recall from Lesson 4 that plutonic rocks form beneath the Earth’s surface. Thus, they are subjected to pressure caused

by overlying rocks. If these overlying rocks are carried away, the pressure is released, expanding the rock formation and creating fractures in the process (sheeting). This type of weathering is also called unloading (Figure 5.3). Biological Weathering Our last example of physical weathering might be the most familiar to you—biological weathering (Figure 5.4). Simply put, this is a weathering mechanism exhibited by plants (and sometimes animals). When plant roots seep through the soil or a rock, the roots act like wedges to gradually break the rocks.

Figure 5.3. Pressure release due to erosion of overlying rock causes the plutonic rock to undergo expansion and sheeting.

Chemical Weathering Unlike physical weathering which only involves breaking down of rocks into smaller pieces, chemical weathering, involves altering the rock’s mineral composition; in other words, it involves chemical change. Practical examples of chemical weathering are carbonation and oxidation.

Figure 5.4. Plant roots breaking the pavement.

Carbonation Carbonation is the dissolution of limestone in acidic water. In chemical terms, the calcium carbonate (CaCO3) present in limestone reacts with the acidic water to form carbonic acid (H2CO3). Carbonic acid is unstable, leaving behind water (H2O) and carbon dioxide (CO2). This type of weathering is particularly responsible for the formation of stalactites and stalagmites in limestone caves (Figure 5.5). Oxidation Oxidation is evident in rocks rich in iron such as hematite. As you have learned in your previous years, rust is formed by reaction of iron with oxygen in the atmosphere (or sometimes in water). Rocks rich in iron undergo chemical weathering to form a characteristic color that indicates rusting (Figure 5.6). In this case, the mineral iron (Fe) turns into ferric oxide (Fe2O3).

Figure 5.5. Limestone caves are formed by dissolution of limestone in acidic water.

Erosion Weathering alone is not enough to shape the Earth’s landscape. The weathered particles, or rock debris, are removed from the point where they were broken down by weathering and then transported elsewhere. The process of removing these rock debris loose from their point of origin and transporting them to another location is called erosion. Erosion can be classified in two ways: (1) depending on the agent; and (2) depending on the type of transportation. We will discuss both classifications.

Figure 5.6. Hematite rusting as evidence of oxidation of iron mineral....