This rock formation located on the shores of Yeuliu Geopark, Taiwan is an example of honeycomb weathering that involves both physical and chemical weathering components. It is caused by salt crystals that expand and break fragments of rock creating holes that become larger over time. Erosion Some of the large rocks broken down into smaller and movable pieces during weathering are transported through the process called erosion. The transportation of all the brokendown rock material is assisted by natural agents, such as running water, waves, glaciers and wind. If the wind seems to be dusty, or the water or glacial ice seems to be muddy, it means that erosion is taking place. Pieces of weathered rock being transported eventually settle in another place as sediment. Landslides occur when the soil and bits of rocks on a slope are unstable and tumble down the slope. Landslides can change the surface of the Earth. Wave Rock is a granite cliff found in Australia that resembles a giant ocean wave. It is a type of rock formation that is caused by chemical weathering of the softer rock at the base by groundwater, followed by erosion from wind and rain that removed the weathered rock. Science Facts How are weathering and erosion different? Think About It Chapter 7 Structure of the Earth and Natural Disasters 195 ©Praxis Publishing_Focus On Science
Deposition Deposition occurs when sediments slow down or stop moving and deposited. This occurs when the agents that cause erosion lose their energy and slow down. Normally, sediments which are denser are deposited first, followed by the less dense ones. Examples of the landforms created due to deposition, on the surface of the Earth are the sand dunes in deserts and deltas at river mouths. A sand dune in Mui Ne, Vietnam formed from the deposition of sediments. 7.2 Movement of Tectonic Plates Tectonic plates are the extremely large pieces of the Earth’s crust and uppermost mantle that make up the lithosphere. These pieces float on a layer of partly liquid rock called the asthenosphere. There are two types of tectonic plates with different compositions, namely the oceanic plate and continental plate. Tectonic plates move above the molten mantle beneath them. There are major, minor and micro tectonic plates based on their size. There are a total of seven major tectonic plates covering nearly 95% of the Earth’s surface: African Plate, Antarctic Plate, Eurasian Plate, Australian Plate, North American Plate, Pacific Plate and South American Plate. They make up most of the seven continents and the Pacific Ocean. North American Plate Juan de Fuca Plate Pacific Plate Nazca Plate South American Plate Scotia Plate Antarctic Plate African Plate Arabian Plate Indian Plate Eurasian Plate Philippine Plate Pacific Plate Australian Plate Cocos Plate Caribbean Plate The major and minor tectonic plates on Earth’s surface Deltas formed from the deposits of sediments at river mouths. 196 ©Praxis Publishing_Focus On Science
In 1912, a German meteorologist Alfred Wegener suggested that the Earth’s continents were drifting and he named this movement continental drift. His suggestion was based on his observation on the coasts of western Africa and eastern South America that resemble puzzle pieces. He felt that maybe at one time, the pieces did fit together before drifting apart. Looking at the continents, Wegener came up with the theory that the continents had once been joined together as a single supercontinent 200 million years ago. He later named this supercontinent Pangaea. Unfortunately, Wegener’s continental drift theory was rejected due to lack of explanation for the cause of the drifting. Initially, he suggested that the continents moved due to the Earth’s rotation, but it turned out to be wrong. Since he was not a geologist but a meteorologist, geologists ignored his theory. Decades later, scientists recognised the existence of the ancient supercontinent called Pangaea before it broke apart about 200 million years later, as suggested by Wegener. They confirmed some of Wegener’s suggestions, which are now part of the widely accepted plate tectonics theory. However, in the plate tectonics theory, it is the tectonic plates and not continents that are moving. Science Facts Outer core Inner core Cold Hot Mantle Continental lithosphere Trench Mid-oceanic ridge Ocean Subduction Oceanic lithosphere The convection process in the mantle causes the movement of the tectonic plates. According to the plate tectonics theory, the movement of tectonic plates is responsible for the various landscapes and landforms on Earth’s surface. The movement of tectonic plates at a rate of a few centimetres per year, is due to the convection process in the mantle, where the hot magma separates the plates while the cooling magma joins the plates. There are three types of tectonic boundaries formed by the movement of the plates, namely the divergent boundaries, convergent boundaries and transform boundaries. Alfred Wegener suggested that about 200 million years ago, Earth’s continents formed a single, giant landmass. The present day Eurasia North America South America Africa India Antarctica Australia PANGAEA Chapter 7 Structure of the Earth and Natural Disasters 197 ©Praxis Publishing_Focus On Science
Divergent Boundaries A divergent boundary is formed when two tectonic plates move away from each other. This occurs as magma, or molten rock, spreads underneath Earth’s surface. A divergent boundary is considered as a constructive boundary because new landforms are formed along this type of plate boundary. Mid-ocean ridges and continental rift valleys are the outcomes of divergent boundaries. A mid-ocean ridge, or underwater mountain range is formed along the ocean floor due to a divergent boundary that occurs on the ocean floor. The ridge is formed when magma rises to the surface as the plates move away from one another. The magma forms a new layer of rock upon cooling. The Mid-Atlantic ridge is an example of a divergent boundary. Divergent boundaries that form on land create rifts in the Earth. Sometimes, magma rises to the Earth’s surface, creating a volcano. Mid-ocean ridge Lithosphere Asthenosphere A divergent boundary model Convergent Boundaries A convergent boundary is formed when two tectonic plates move towards each other. Convergent boundary movement is divided into two types based on the density of the plates involved, namely the subduction and collision. Trench Volcanic arc Overriding plate Downgoing plate A convergent boundary (subduction) Silfra rift in Thingvellier National Park, Iceland is an example of divergent boundary between the North American and Eurasian tectonic plates. 198 ©Praxis Publishing_Focus On Science
Subduction occurs when the denser crust (downgoing plate) sinks beneath the less-dense crust (overriding plate), into the mantle, creating a trench. The area where subduction takes place is known as the subduction zone. As the sinking crust moves down into the mantle, the intense heat and pressure causes the melting of the crust. Most of the time, the energy produced causes earthquakes. A convergent boundary is considered a destructive boundary because the crust gets destroyed in the subduction process. Collision occurs when the continental plates converge without subduction. This collision causes the Earth’s crust to fold, forming mountain ranges, for example the Himalayan mountain range. The Himalayan mountain range consists the highest peak in the world, Mount Everest. The range stretches 2900 km along the border between India dan Tibet. The Himalayan moutain range is the result of convergent boundary. It was formed due to the collision between the Eurasian Plate and the Indian Plate 50 million years ago and continues today Indian Plate Eurasian Plate Land folded upwards Plate boundary An oceanic trench is a feature of a convergent boundary. The deepest oceanic trench in the world is Mariana Trench located in the Pacific Ocean. It is formed when the denser Pacific plate subducts beneath the less-dense Philippine plate. Jacques Piccard and US Navy Lieutenant Don Walsh were the first scientists to travel and explore the bottom of the Mariana Trench in 1960. It is crescent in shape, measuring about 2550 km in length and 69 km in width. The deepest point of Mariana Trench called Challenger Deep, is 11 035 m deep below sea level, which is higher than Mount Everest which is 8848 m above sea level. Therefore, if Mount Everest was dropped into the Mariana Trench, its peak would still be 2187 m below sea level. Science Facts Lithosphere Oceanic trench Rising magma Oceanic crust Subduction zone Asthenosphere Challenger Deep 11 035 m below sea level Mariana Trench Mount Everest (8848 m above sea level) 2187 m Chapter 7 Structure of the Earth and Natural Disasters 199 ©Praxis Publishing_Focus On Science
Transform Boundaries A transform boundary is formed when two tectonic plates slide past each other in opposite directions. This movement results in the formation of a transform fault. Usually, earthquakes occur along such boundaries, but there are no volcanoes or probably just a few of them. Earthquakes occur when the plates that slide past each other shift or break off. Although transform boundaries are very common around mid-ocean ridges, they also occur in the continental crust. San Andreas Fault in California is an example of transform boundary between two major plates of the Earth’s crust: the Pacific Plate and the North American plate. The Pacific plate slides laterally past the North American plate in a northerly direction. The fault which extends from the Mexican border to north of San Francisco was responsible for the 1906 San Francisco earthquake. Type of tectonic boundary Characteristic Divergent boundary Transform boundary Convergent boundary Type of motion Two tectonic plates move away from each other Two tectonic plates slide past each other in opposite directions Two tectonic plates move towards each other (subducts/collides) Type of effect Constructive Conservative (neither created or destroyed) Destructive What is the landform created? Ridge/Rift Fault Trench/Mountain range Does it cause volcanic activity? Yes No Yes Transform fault A transform boundary model Los Angeles San Francisco San Andreas Fault North American Plate Pacific Plate Pacific Ocean The table below shows the differences between all three types of tectonic boundaries. San Andreas Fault, California Asthenosphere Divergent boundary Transform boundary Convergent boundary 200 ©Praxis Publishing_Focus On Science
Activity 2 Constructing models of tectonic plate boundaries Strawberry jam Cream cracker Part 2: Convergent Boundaries (Subduction) 1 Place a scoop of strawberry jam on a paper plate. 2 Take two new cream crackers and dip one end of each of the cream crackers into a cup of water. 3 Quickly remove the crackers from the cup to prevent the crackers from falling apart. 4 Lay the crackers on the paper plate with the wet edges nearly touching. 5 Push the two crackers slowly towards each other. One cracker will ride up over the other. Discussion: 1 What do the cream crackers and strawberry jam represent? 2 In Part 2 of the activity, what do the resulting ends of the wet crackers represent? 3 In Part 3 of the activity, initially nothing happend, but due to the increase in pressure and the cream crackers getting caught on one another, they will eventually break. What does the breaking of the cream crackers represent? Part 3: Transform Boundaries (Sliding) 1 Place a scoop of strawberry jam on a paper plate. 2 Place two cream cracker pieces side by side on top of the strawberry jam. 3 Place one hand on each of the cream crackers. At the same time, push one of the crackers away from you while pulling the other towards you. Cream cracker Strawberry jam Aim: To construct models of tectonic boundaries Materials: Cream crackers, strawberry jam, paper plate, spoon and water Procedure: Strawberry jam Cream cracker Part 1: Divergent Boundaries (Move apart) 1 Place a scoop of strawberry jam on a plate. 2 Lay two pieces of cream crackers side by side on top of the jam. 3 Press down on the crackers as you slowly pull them apart in opposite directions. Chapter 7 Structure of the Earth and Natural Disasters 201 ©Praxis Publishing_Focus On Science
7.3 Earthquakes An earthquake is the movement (shaking and vibration) of Earth’s surface due to the sudden release of energy stored in the Earth’s lithosphere. Most earthquakes occur along any type of plate boundary. The movement of the tectonic plates causes friction on one another. Therefore, there will be a build-up of pressure at their boundaries and energy is stored up in Earth’s crust. At one point, the pressure can no longer be contained, thus energy is released in a form of shock waves through the crust and onto Earth’s surface. An earthquake occurs when the rocks break up and move in a series of sudden jerks. The strength of an earthquake depends on the focus and the epicentre of the earthquake. The focus is the point inside Earth’s crust where energy is released, while the epicentre is a point on Earth’s surface directly above the focus. Earthquake energy is released in seismic waves that spread out from the focus. The waves are very strong at the epicentre and become less strong as they travel further away. Typically, severe damages caused by an earthquake occur close to the epicentre. Usually, the nearer the focus is to Earth’s surface, the stronger the earthquake. This is because the vibrations take a shorter time to reach Earth’s surface. Magnitude is used to describe the size of an earthquake. It is expressed in whole numbers and decimal fractions. Richter magnitude scale, also known as Richter scale, was invented by a seismologist, Charles Richter. It is a measuring system used to rate the magnitude of an earthquake or the largest wave of energy produced by earthquakes through information gathered by a seismograph. Seismographs make zig-zag lines that show the magnitude of an earthquake’s seismic waves. The longer the wave, the larger the seismic wave. The Richter scale was then replaced with the moment magnitude scale. However, Richter scale is used until today to measure small earthquakes while moment magnitude scale is used for most large earthquakes. Richter’s equations are still used for forecasting future earthquakes. Richter magnitude Features 0 to 1.9 Cannot be felt, can only be detected by seismograph 2 to 2.9 Slightly felt, may cause hanging objects to swing 3 to 3.9 Can be felt if near the epicentre 4 to 4.9 May cause small or unstable objects to fall especially if near the epicentre 5 to 5.9 May cause furniture to move and damage to weak buildings especially if near the epicentre 6 to 6.9 Causes severe damage to weak buildings especially if near the epicentre 7 to 7.9 Causes severe damage such as buildings displaced from foundation, pipes break 8 to 8.9 Causes death and massive destruction such as collapse of bridges Above 9 Rarely but if it occurs, can cause total destruction Epicentre Plate movement Seismic waves Focus Tectonic plate The occurrence of an earthquake 202 ©Praxis Publishing_Focus On Science
The “earthquake belt” is a geographic zone on Earth’s surface where the activity of tectonic plates causes earthquakes. Although earthquakes can occur at any time and in any area, they follow the same common trends year after year, mainly in three significant zones of the earth: the circum-Pacific seismic belt, Alpide earthquake zone and underwater mid-Atlantic Ridge. A seismograph, or seismometer, is a tool to detect and record seismic waves, the propagating vibrations that carry energy from the source of an earthquake outwards in all directions through the interior of the Earth. Seismographs have been set up by scientists all over the world to track the movement of the Earth’s crust. Earthquake belt • The “circum-Pacific seismic belt” that runs along the Pacific Ocean’s rim is responsible for approximately 81% of Earth’s major earthquakes. It has acquired the title “Ring of Fire”. • The “Alpide earthquake zone” that stretches from Java to Sumatra, across the Himalayas, the Mediterranean, and then into the Atlantic is responsible for approximately 17% of the Earth’s major earthquakes. One of the most devastating earthquakes is the 2004 M9.1 Indonesian earthquake that caused a tsunami affecting over 230 000 people. • The “underwater mid-Atlantic Ridge” is remote from human activities. Iceland, which is located on top of the mid-Atlantic Ridge, has experienced earthquakes as large as M6.9. Why do earthquakes mainly occur along plate boundaries? Think About It Chapter 7 Structure of the Earth and Natural Disasters 203 ©Praxis Publishing_Focus On Science
Tsunami A tsunami is a series of huge waves created due to earthquakes under the sea. It washes against the coast with enormous speed and force as it travels. Tsunami waves can be as far as hundreds of kilometres apart. A tsunami starts with an earthquake under the sea. As the earthquake occurs, a massive amount of water is displaced quickly, generating a massive amount of energy. This energy is responsible for a series of progressive waves that move across the ocean surface at speeds of up to 800 km per hour. In deep water, the tsunami moves very fast and has a long wavelength, but a small amplitude. As it approaches the shoreline, it collides with the rising seabed and friction causes the waves to slow down. As a result, the wavelength decreases, and the large amount of water in the wave must rise higher within the shorter wavelength. Thus, as the amplitude of the waves increases, the waves become gigantic and collide with the seashore causing massive damage. Trees are uprooted; people and property are washed away. An example of an undersea earthquake which triggered off a massive tsunami occurred in the Indian Ocean near Sumatra, Indonesia on 26 December 2004. Huge waves travelled for thousands of kilometres before hitting the west coast of Sumatra and many other nearby countries. About 240 000 people died in 14 different nations due to this tsunami. Tsunami hits the coast Epicentre of an earthquake The occurrence of a tsunami Giant waves travelling across the sea 204 ©Praxis Publishing_Focus On Science
7.4 Volcanoes Magma is the molten rock that makes up the upper part of Earth’s mantle. New landforms are formed when magma emerges onto Earth’s surface and then cools down and solidifies. An example of such landform is a volcano. Volcanoes can be classified as active, dormant and extinct. • Active volcanoes have erupted recently (in the past 10 000 years) and are likely to erupt again soon. • Dormant volcanoes have not erupted in a long time (in the past 10 000 years) but likely to erupt again in the future. • Extinct volcanoes have not erupted in a long time (in the past 10 000 years) and are unlikely to erupt in the future. They no longer have access to lava supply. Mount Batur in Bali, Indonesia which is an active volcano. Crust The upper part of the Earth Magma chamber An underground area containing molten rock Lahar of mudflow A mixture of water and volcanic ash Central vent A channel in the neck of a volcano, where magma erupts Crater A depression around the mouth of a volcano Bomb A lump of molten lava ejected from the volcano Ash and cinders Tiny pieces of rock and lava Lava Molten rock that flows Fissure A large crack where the lava can flow through There are four types of volcanoes according to their shapes, namely the shield, composite, lava dome and cinder cone volcanoes. Parts of a volcano What is the difference between magma and lava? Think About It One of the most catastrophic volcanic eruptions of all time was that of August,1883 from Krakatau, a volcanic island in the Sunda Strait, between the islands of Java and Sumatra, Indonesia. This eruption was heard more than 3000 miles away, spewed a huge amount of ash into the atmosphere, and generated a series of large tsunami waves killing more than 36 000 people. Science Facts Chapter 7 Structure of the Earth and Natural Disasters 205 ©Praxis Publishing_Focus On Science
Shield Volcanoes A shield volcano is formed almost entirely of basic lava, which is highly fluid and spreads widely over great distances before it solidifies. It has a gentle slope that is flat near the top. The eruption of a shield volcano is mostly quiet and non-violent. An example of a shield volcano is the Mauna Kea and Mauna Loa volcanoes in Hawaii, the largest active volcanoes in the world, rising over 9 km above the sea floor. Crater Side vent Narrow base with steep slope Solidified lava layers Fluid lava Magma chamber Central vent Composite volcano Crater Wide base with gentle slope Solidified lava layers Fluid lava Central vent Magma chamber Shield volcano Lava Dome Volcanoes When the erupting lava is too thick to flow, it does not flow far from the vent. The lava piles over and around the vent, forming a steep-sided mount known as a lava dome. Lava domes can also form within the crater of a previous volcanic eruption. Crater Magma chamber Central vent Lava dome volcano The viscous lava rises up the central and side vents and solidifies during an eruption. This prevents the release of magma and gases, causing a build-up of pressure. The increase in pressure produces a violent eruption of ash and cinder, followed by lava. The lava covers the ash and cinder to form layers of a composite volcano. The layers stack on each other with every eruption that occurs. Composite Volcanoes A composite volcano, also known as stratovolcano, has relatively steep sides and a narrow base. It is more cone-shaped compared to a shield volcano. The composite volcano is made up of lava that does not flow easily, ash and cinder. Mount St. Helens is an active stratovolcano located in Washington. A large lava dome can be found inside the summit crater of Mount St. Helens. Mount Merapi in Indonesia, an example of an active composite volcano. 206 ©Praxis Publishing_Focus On Science
Cinder Cone Volcanoes Cinder cones are the most common type of volcano in the world. A cinder cone has steep sides with a prominent bowl-shaped crater at the summit. It only grows to a height of about a thousand feet, the size of a hill. It is usually formed after a violent eruption blows lava fragments into the air, which will then solidify and drop as cinders around the volcanic vent. Natural Disaster Response During natural disasters such as earthquakes and volcanic eruptions, it is very important to respond appropriately. Some of the recommendations are listed below. Crater Magma chamber Central vent Cinder cone volcano Mount Bromo, an active volcanic cinder cone in Java, Indonesia. Activity 3 Investigating land use by people living close to volcanoes 1 Work in a group of five. 2 Select a volcano in your country which has erupted before and find out how people living close to the volcano use the surrounding land to benefit them. 3 Make a poster and present your findings in the class. • Follow instructions from the authorities on road closures, evacuations or shelter orders. • Pay attention to updates from the authorities on hazard zones or any new information on the eruption. • Stay safe and help others in need. • Reduce the exposure to volcanic ash by staying indoors and covering ventilation openings. • If you need to be outdoors, use a face mask and wear protective clothing to avoid irritation to the eyes and skin due to the ash. Recommended response to volcanic eruptions • Go to a safe place such as under a sturdy table and cover your head and neck with your arms and hands. Stay there until there are no more shakings. • Check yourself for any injuries. • When the shaking stops, quickly leave the place but be on the lookout for things that have fallen or broken and fires. Grab something to shield your head. • If you need to leave a building, use the stairs and once you are outside, move away from the building. Recommended response to earthquakes Chapter 7 Structure of the Earth and Natural Disasters 207 ©Praxis Publishing_Focus On Science
1 are the natural physical features found on Earth’s surface which were formed due to various forces of nature such as wind, water and ice. 2 Based on chemical composition, our Earth is made up of three main layers, namely the crust, and core. 3 Based on physical characteristics, the Earth can be divided into five distinct layers, namely the , asthenosphere, mesophere, outer core and inner core. 4 Earth’s crust is divided into two types: oceanic crust and crust. 5 The in the Earth’s crust is a layer that consists of solid rocks. 6 The is the Earth’s thickest layer with a thickness of about 2900 km. 7 Earth’s crust and upper is made up of tectonics plates. 8 The Earth’s outer core is mainly composed of iron and nickel. 9 The Earth’s inner core is mainly composed of iron and nickel. 10 There are two processes, namely the exogenic process and endogenic process. 11 Convection currents that cause the movement of the Earth’s crust are produced in the layer due to the high temperatures in the mantle and core of Earth. 12 There are three types of weathering, namely physical, biological and weathering. 13 weathering is the breaking down of rocks without involving any changes to the chemical composition of the rocks. 14 In very cold areas, water can cause physical weathering through a process known as ice that involves repeated freeze and thaw cycles. 15 weathering is caused by the movements of living things such as plants, animals and microbes. 16 weathering of rocks involves chemical reactions which change the mineral compositions of rocks. 17 Pieces of weathered rock being transported eventually settle in another place as . 18 There are three types of tectonic boundaries formed by the movement of the plates, namely the divergent boundaries, boundaries and transform boundaries. 19 A divergent boundary is considered as a boundary because new landforms are formed along this type of plate boundary. 20 occurs when the denser crust sinks beneath the less-dense crust, into the mantle, creating a trench. 21 An is the movement of Earth’s surface due to the sudden release of energy stored in the Earth’s lithosphere. 22 is a series of huge waves created due to earthquakes under the sea. 23 volcanoes are the ones that have erupted recently and likely to erupt soon. RECALL Fill in the missing words. 208 ©Praxis Publishing_Focus On Science
THINKING CAP Put on your 1 Which of the following weather faster, (a) large rocks or small rocks? (b) rocks on steep slopes or rocks on level ground? Explain your answer. 2 How does the magma in a shield volcano differ from the magma in a composite volcano? 3 Why are some mountains volcanic, but others are not? Chapter 7 Structure of the Earth and Natural Disasters 209 ©Praxis Publishing_Focus On Science
Project Create Earth’s Model Activity objective: To design and create a model of the Earth to scale, in order to explain its structure more clearly. Problem statement: The Earth can be divided into layers of different thickness based on chemical composition and physical characteristics. To understand these layers better, a model of the Earth to scale can be created. Procedure: 1 Divide the students in the class into several groups. 2 Each group should search for information regarding the structure, shape, layers and surface of the Earth. Action plan: (a) Sketch out the layers of Earth’s model. (b) Decide the scale that can be used to create the model. (c) Decide the best material that can be used to make the model. For example, clay, playdough, styrofoam and mashed paper. Solution: Create the Earth’s model to scale with the layers clearly shown based on chemical composition and physical characteristics. Presentation: Present the model to the class. 210 ©Praxis Publishing_Focus On Science
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