Social Studies Grade 8 PDF

Social Studies Grade 8  Folding Faulting Volcanism Rift valley Earthquake Erosion 1.1.1. Internal or tectonic forces and resultant land forms What is internal force? Internal forces are som...

Social Studies Grade 8  Folding Faulting Volcanism Rift valley Earthquake Erosion 1.1.1. Internal or tectonic forces and resultant land forms What is internal force? Internal forces are sometimes called endogenous forces. They drive energy from the interior part of the earth. These forces create irregularities on the surface of the earth. Internal forces form the ups and downs on the earth’s crust by breaking and bending it. These ups and downs are the results of folding, faulting, earthquake and volcanism. A. Folding What is folding? How it is formed? Folding is one of the internal processes which occurs when two forces act towards each other from opposite sides. Due to this force, rock layers are bent into folds. The process of folding is the result of compressional force. In principle folding include the entire bend that ranges from microscopic wrinkling to domes and basin. PHYSICAL CHARACTERISTICS OF THE EARTH 2  Social Studies Grade 8 Figure 1.1 Folded Structure A fold arching upward is called an anticline and one arching downward is a syncline. The sides of a fold are called the limbs. Results of folding are the formation of different Fold Mountains in the world. Fold mountains are usually formed from sedimentary rocks and are usually found along the edges of continents. This is because the thickest deposits of sedimentary rock generally accumulated along the edges of continents. The resultant landforms can be formed as a result of folding are fold mountains. There are two types of Fold Mountains: young fold mountains (10 to 25 million years of age, example, the Atlas, Rockies and the Himalayas) and old fold mountains (over 200 million years of age, example, the Cape Range, the Urals in Russia and the Appalachians. These processes of mountain building are called Orogeny. PHYSICAL CHARACTERISTICS OF THE EARTH 3  Social Studies Grade 8 Figure 1.2 Sycline and Anticline B. Faulting What is faulting? What are the causes for faulting? Movements in the crust of the earth sometimes make cracks. These cracks are called faults. Faulting can be caused by either lateral or vertical forces, which can be either tension or compression. Tension causes a normal fault, and compression causes a reverse fault. Figure1.3 Reverse Fault | Figure1.4. Normal reverse fault PHYSICAL CHARACTERISTICS OF THE EARTH 4  Social Studies Grade 8 There are two major landforms that can be formed as a result of faulting they are 📕 Rift valley 📕 Block mountains I. Rift valleys What is rift valley? How rift valleys are formed? A rift valley is a linear shaped lowland area between highlands or mountain ranges created by geologic rifts or faults. The largest rift valley in the world is the East African Rift Valley. It extends from Syria to Mozambique, passing through the Red Sea, Eritrea, Ethiopia, Kenya, Tanzania, DR Congo, Rwanda and Burundi. The total length of the East African Rift Valley is about 7,200 km, of which 5,600 km is in Africa. The Ethiopian Rift Valley is a part of the East African Rift Valley. It extends from northeast to south west. Features found in the Rift Valley include active volcanoes, lakes, hot springs and fumaroles. PHYSICAL CHARACTERISTICS OF THE EARTH 5  Social Studies Grade 8 Figure 1.5 Rift valleys of Africa II. Block (Horst) Mountains How Block Mountains are formed? Block Mountains are formed when land between two parallel faults is pushed upward due to pressure from inside the earth. If there are two parallel faults, the crustal block between them may either rise to produce a Horst (block) mountain, or fall, to produce a rift valley. Examples: The Afar block mountain in Ethiopia and the Ruwenzori in West Africa. Figure1.6 Formation of block mountain (Horst) C. Volcanism What is Volcanism? Discuss the major volcanic areas in Ethiopia Volcanism is the process by which magma, gases, water vapor, ashes and other solid materials are forced out to the surface. It is another internal force which changes the surface of the earth, which is caused by internal movements within the earth. Inside the earth the temperature is very hot. This high temperature changes rocks into molten magma. When this magma reaches the surface, volcanic activity takes place. The out pouring of this liquid (molten) rock onto the surface is called lava. Magma is the molten rock when it is underground. PHYSICAL CHARACTERISTICS OF THE EARTH 6  Social Studies Grade 8 When the magma emerges on to the surface, it cools and hardens. Figure 1.7 a) Volcanism Magma reaches the earth’s surface through two kinds of holes. They are vents and fissures. Magma may force its way violently through a small hole called a vent. If lava emerges via a vent, it builds up into a volcano (cone-shaped mound), and if it emerges via a fissure, it builds up to form a lava plateau or lava flow. Magma may pour quietly through long cracks (fissures) onto the earth’s surface. If the magma flows to the surface through a vent, a crater is formed Sometimes a volcano erupts very forcefully. When this happens, the top part of the volcano is blown away. This forms a large crater called a caldera. Water collects in the crater or in the caldera and forms a lake. We call this a Crater Lake or caldera lake. In Ethiopia there are many crater lakes such as Zuquala, Wonchi and Dendi. If an eruption begins again in a caldera, a new small cone-shaped volcano is formed inside the caldera. These are known as caldera cones.In volcanic areas, where water sinks deep enough to be heated by the magma the superheated water carrying the dissolved minerals may flow quietly out of the earth’s crust to form a hot spring. When a hot water and steams are ejected through a vent, it is called a geyser Part of the magma may not reach the earth’s surface, and PHYSICAL CHARACTERISTICS OF THE EARTH 7  Social Studies Grade 8 when this magma cools, solidifies and forms rocks inside the crust, features such as batholiths, laccoliths, sills and dikes, are formed.  Batholiths: - are a very large mass of magma which accumulates in the crust. It is the largest structure.  Laccoliths: - are mushroom shaped bodies of intrusive igneous rock.Smaller than batholiths.  A dike: - is formed when magma solidifies in a vertical or near- vertical crack.  A sill: - is formed when magma solidifies horizontally or nearly horizontally along a bedding plane. Major landforms formed by volcanism If the magma comes out through vent, a crater is formed whereas when it erupts through fissures caldera can be formed. Caldera is larger than crater. When both are filled with water crater or caldera lakes are formed. When volcano erupted violently or explosively through a small hole volcanic mountain is formed. PHYSICAL CHARACTERISTICS OF THE EARTH 8  Social Studies Grade 8 Figure 1.8 Volcanic landforms Table 1.1: Type of Volcanoes Types Features Example Active vol- Erupts from time to time Erta’li, FentaleDubbi and Damiali cano Dormant volcanoHas not erupted for a long time but may erupt again in thefuture Tatali and Dabbahu Extinct Has not erupted within historic- Mt. Zuquala, Rasvolcano time Dashen and Batu Importance of volcanic eruptions: ✅ Provide fertile soil ✅ Creation of new land ✅ Provide hot springs (with medical value) PHYSICAL CHARACTERISTICS OF THE EARTH 9  Social Studies Grade 8 ✅ Generate geothermal energy and ✅ Help in the formation and concentration of minerals ✅ It can also provides information about internal forces of the earth. D. Earthquake What is an earthquake? How earthquakes do occurs? An Earthquake is shaking of the ground caused by the sudden (unpredictable) movements in the earth’s crust. They are caused by internal movements deep down inside the earth. Earthquakes are frequently associated with faults. They take place along fault lines where the earth’s crust is weak. When an earthquake occurs, vibrations from the Centre spread out in the form of waves in all directions. The point at which an earthquake originates is called the focus. The point on the earth’s surface immediately above the focus is called the epicenter. PHYSICAL CHARACTERISTICS OF THE EARTH 1  Social Studies Grade 8 Figure 1.9 An earthquake`s waves, focus and epicenter The intensity of an earthquake is measured by an instrument called a seismometer, commonly known as a seismograph. It records the vibrations produced by an earthquake. The scale which gives the magnitude is called the Richter scale. Scientists assign a magnitude rating to earthquakes based on the strength and duration of their seismic waves. A quake measuring to 5 would be considered minor or light; 5 to 7 is moderate to strong; 7 to 8 is major and 8 or more is great. Effects of Earthquakes Earthquake leads to loss of life and destruction of property, landslides and deep cracks in surface rocks, the rise or lowering of the sea floor, displacement of parts of the earth’s crust vertically or laterally and the devastation of cities, fires and diseases. PHYSICAL CHARACTERISTICS OF THE EARTH 1  Social Studies Grade 8 Figure 1.10 Destruction caused by earthquakes Earthquakes take place in many parts of the world. However, frequent occurrences are observed along certain belts. 80% of all seismic energy generated around a belt that is found at the east- west border of the Pacific Ocean. This place is called the pacific ring of fire. PHYSICAL CHARACTERISTICS OF THE EARTH 1  Social Studies Grade 8 Figure 1.11: Major Zones of Earthquake Activity 1.1 Being in group discuss on the following questions 1. Discus the following issues and questions in pairs. I) Folding IV) Volcanism II) Faulting V) Earthquakes III) Rift valley 2. Explain the characteristics of I) Active Volcano III) Dormant Volcano. II) Extinct volcano 1.1.2. External or gradational forces and resultant land forms. What are external forces? External forces are the forces that act upon the earth’s surface from outside. They are also known as exogenous processes. These include weathering, mass movement, erosion, transportation and deposition. These forces are responsible in the formation of different land features on the surface of the earth. These forces level the ups and downs of the earth’s surface and this process is called denudation. They also raise the level of the land by deposition. Denudation consists of weathering and erosion. PHYSICAL CHARACTERISTICS OF THE EARTH 1  Social Studies Grade 8 A. Weathering Weathering is the disintegration or decomposition of rocky materials of the earth’s crust. It results in the formation of a covering of rock wastes which sometimes are called regolith. Generally, the term weathering refers to the processes that change the physical and chemical character of rock at or near the surface. Weathering is a relatively long, slow process. Types of weathering I. Physical (Mechanical) Weathering Discuss physical weathering and main agents of physical weathering? Mechanical weathering is the mechanical fragmentation of rocks which is accomplish by physical forces which broke the rocks into smaller pieces without changing the mineral composition. Its main agents (causes) are temperature changes, frost action and the action of plants and animals. The effects of temperature changes: The temperature variation between day and night causes rock to expand and to contract. This process causes cracks to develop. In time, the cracked layer peels off and falls to the ground this process is called exfoliation. PHYSICAL CHARACTERISTICS OF THE EARTH 1  Social Studies Grade 8 Figure 1:12 Exfoliation domes The effects of frost action: Due to frost action, rock breaks up into pieces and these fragments accumulate around the lower slopes of the rock. Frost action is very common in the winter season in the temperate zone and in some high mountains all year round. Water -filled crack Freezes to ice Breaks Rocks PHYSICAL CHARACTERISTICS OF THE EARTH 1  Social Studies Grade 8 Figure 1.13 the breaking of rocks by the action of frost The effects of plant and animal action: Plants and animals also cause weathering. For example, seeds may fall in cracks of rocks. If water collects there, it forms suitable conditions for the seeds to germinate and grow. As plants develop their roots may push the rock apart. Some animals burrow, and this also helps to break up rocks. Figure 1.14 breaking of rock by a plant II. Chemical Weathering Discuss chemical weathering and main agents of Chemical Weathering? Chemical weathering is the process by which rocks and minerals undergo changes in their composition. Agents of chemical weathering include water, oxygen, carbon dioxide, and acid precipitation. The interaction of these agents with rock can cause some substances to dissolve, and some new minerals to form. B. Erosion and deposition Erosion Define the term erosion? What are the major agents of erosion? PHYSICAL CHARACTERISTICS OF THE EARTH 1  Social Studies Grade 8 Erosion is the movement of broken rock and soil particles from one place to another by the different agents like running water, stream, wind, and glacier sea waves. Causes of Soil Erosion What are the causes of soil erosion?  Physical causes: - involve a combination of the following physical factors: steepness of slope, the intensity, duration and seasonality of rainfall, soil type (example, texture) and vegetation cover.  Human causes: - have to do with human interference with natural processes, including deforestation, overgrazing and bad farming practices which deplete plant cover, leaving the land exposed and vulnerable to erosion. A. Erosion by Running Water Rivers are the most important of all natural agents which help in shaping the earth’s surface. The work of running water includes eroding, transporting and depositing eroded material. Running water results three types of erosion. These are; PHYSICAL CHARACTERISTICS OF THE EARTH 1  Social Studies Grade 8  Sheet erosion develops when running water erodes the surface uniformly. Fig 1.15 Erosion by sheet flow  Rill erosion develops when running water cuts small channels. Figure 1. 16 Rill Erosion  Gully erosion is formed when a rill are more widened and deepen. PHYSICAL CHARACTERISTICS OF THE EARTH 1  Social Studies Grade 8 Fig 1.17 Gully erosion There are three main types of processes that occur in a river. These are erosion, transportation and deposition. These processes depend on the amount of energy of the river and acts on the different courses of the river. Accordingly, different landforms can be formed. The followings are the major landforms that can be formed at three courses of river. Figure 1.18 three stages of a river PHYSICAL CHARACTERISTICS OF THE EARTH 1  Social Studies Grade 8 Upper Course: - In this stage the river water is usually small in volume. As the river flows very fast down, steep-slopes, a V-shape valley, waterfalls and deep gorges are formed. The fast flow of the river causes vertical erosion and destruction. The V-shape valley has steep sides and a narrow floor. The fast flowing river cuts down deeply into the land. Example:-Tis Abay water fall. Figure 1.19 Features of the Upper courses of a river The Middle Course: - during a river’s middle course, the river valley becomes wider and larger. The river may receive waters of many tributaries, which increase the volume of water. Wide-floored valleys with gentle slopping sides are the main features of the middle course of the river. Instead of taking the most direct course possible, the river begins to meander. Meanders are pronounced curves in the course of a river. PHYSICAL CHARACTERISTICS OF THE EARTH 2  Social Studies Grade 8 Figure 1.20 Features of the Middle courses of a river The Lower Course:-The river flows fast, meandering over wide plains, and makes widespread deposition. The load is so large that deposition occurs. Flat flood plains, big meanders, levees, ox-bow lakes and deltas are the main features of this course. Figure 1.21 Features of the lower courses of a river PHYSICAL CHARACTERISTICS OF THE EARTH 2  Social Studies Grade 8 Figure 1.22 Rivers profile Wind Erosion: - is the most common form of erosion in dry and treeless areas where the soil is exposed. The dry and unprotected topsoil from fields is blown away. Wind forces are capable of removing all of an area’s topsoil and transporting it several thousand kilometers away. Wind erosion is common in desert and semi-desert area where there is no vegetation cover. Figure 1.23 Wind Erosion PHYSICAL CHARACTERISTICS OF THE EARTH 2 2