6.2 Neutralisation Do you know what happens to the pH of an acid when an alkali is gradually added to it? The pH changes as shown in the diagram below. pH = 1 pH = 3 pH = 7 pH = 9 Acid + universal indicator Alkali When we add alkali to acid, the solution becomes less acidic and the pH of the solution increases. When we add more alkali, at one point, the alkali completely cancels out the effects of the acid. A neutral solution with a pH of 7 is produced. When we add more alkali to the neutral solution, it becomes more alkaline and the pH of the solution increases. The reaction between the right amounts of an acid and an alkali produces a neutral solution of salt and water, with a pH of 7. This reaction is known as neutralisation. Neutralisation takes place when the right amounts of an acid and an alkali react together. The reaction of different acids and alkalis produce different salts. For example: Acid + Alkali Salt + Water Hydrochloric acid + Sodium hydroxide Sodium chloride (salt) + Water Nitric acid + Potassium hydroxide Potassium nitrate (salt) + Water Sulphuric acid + Potassium hydroxide Potassium sulphate (salt) + Water Titration is the method used to carry out neutralisation. 176
Activity 6 Aim: To study acid-alkali neutralisation reaction through titration method Materials and apparatus: Phenolphthalein, hydrochloric acid (0.5 M), sodium hydroxide solution (0.5 M), burette, 25 ml pipette, 250 ml conical flask, retort stand with clamp, white tile, small filter funnel, evaporating dish, tripod stand and Bunsen burner Procedure: Pipette Conical flask Sodium hydroxide solution Phenolphthalein (indicator) Burette Hydrochloric acid Retort stand with clamp Sodium hydroxide solution + phenolphthalein White tile Conical flask 1 Add 40 cm3 of hydrochloric acid into a burette using a filter funnel. 2 Record the initial reading of the burette. 3 Add 25 cm3 of sodium hydroxide solution into a conical flask using a pipette. 4 Add three drops of phenolphthalein into the sodium hydroxide solution and shake the conical flask slowly. 5 Add hydrochloric acid drop by drop slowly from the burette into the sodium hydroxide solution while softly swirling the conical flask. Take note of any changes in colour of the content of the flask. 6 Stop adding the acid when the colour of the sodium hydroxide solution turns from pink to colourless. 7 Record the final reading of the burette. Calculate the volume of acid used. 8 Pour the solution in the conical flask into an evaporating dish and heat it until all the water has evaporated. What is the end product formed in the evaporating dish? Observation: Write down your observations. Discussion: 1 Name the reaction that occurs and write a word equation to represent the reaction. 2 Why should we stop adding the acid in step 6 when the colour of the sodium hydroxide solution turned from pink to colourless? 3 Name the method used in this activity. Conclusion: Write down your conclusion. Studying neutralisation reaction Solution Water evaporates Evaporating dish Heat End product Chapter 6 Acids and Alkalis 177
Applications of Neutralisation in Daily Life Some of our daily life activities make use of the neutralisation reaction. Here are some examples. Hydrochloric acid is produced in the stomach to aid in the digestion process and to kill germs found in food. However, when there is too much acid in the stomach, indigestion occurs, and antacids or indigestion tablets are generally recommended. This is because the antacids are alkaline. Antacids neutralise the excess acid to relieve any discomfort in the stomach. Neutralising excess acid in the stomach A wasp sting is alkaline in nature. Vinegar (ethanoic acid) which is acidic can be used to relieve the pain by neutralising the sting. A bee’s sting is acidic in nature, and it causes inflammation and pain around the infected area due to the release of formic acid. The pain can be eased by treating it with baking soda (bicarbonate of soda). When the bacteria in the mouth feed on the leftover food in between the teeth, acid is produced, and it can cause tooth decay. It is advisable to brush our teeth at least twice a day. This is because the alkali in toothpaste neutralises the acid present in the mouth. Thus, tooth decay can be prevented. Neutralising acidity in the mouth Treating wasp’s sting Treating bee’s sting 178
Neutralisation plays a major role in farming. For crops to grow well, the soil should not be too acidic. However, the excessive use of pesticides and fertilisers makes the soil too acidic. Quick lime (calcium oxide) or slaked lime (calcium hydroxide) can be added to the soil to neutralise the acid. Soil treatment Most of the waste generated by factories are toxic and acidic in nature. The waste are normally discharged into the nearby flowing water bodies such as the rivers. This is harmful to the aquatic plants and animals. To minimise the harmful effect of the waste, it must be neutralised to minimize its toxicity before it enters the water. Some of the substances that are used for neutralisation are sodium bicarbonate, magnesium hydroxide, calcium oxide and calcium carbonate. Industrial waste treatment When fossil fuels in power stations are burnt, the acidic gas sulphur dioxide is produced. This gas can dissolve in rainwater to form acid rain. However, in some power stations, this gas is removed through a process known as scrubbing. In this process, an alkaline substance or solution such as limestone or sodium hydroxide, is used to neutralise the acidic sulphur dioxide gas. Smoke without sulphur dioxide is then released through the chimneys. Reducing air pollution Chapter 6 Chapter 6 Acids and Alkalis Acids and Alkalis 179
The laundry detergent that we use to wash our clothes is alkaline in nature. Fabric softeners which are mildly acidic, are used to neutralise the detergent to bring the texture and feel of the fabrics back to its original condition. Fabric softeners also reduces the risk of skin irritation which may be caused by alkaline residues. Fabric softeners Face cleansers which are alkaline can make the skin on the face becomes dry. Therefore, acidic toners are used to neutralize the slightly alkaline skin after cleansing. Toners After washing with shampoo, usually the hair feels rough and becomes tangled because the shampoo is alkaline in nature. A hair conditioner which is a mild acid can be applied to the hair to neutralise the alkali in the shampoo and make the hair smoother. Shampoos and conditioners 180
1 Acids are substances that dissolve in water to form solutions. 2 Alkalis are substances that dissolve in water to form solutions. 3 Acid solutions are composed of ions while alkali solutions are composed of ions. 4 acid is found in tea. 5 Some bacteria react with sugars and carbohydrates from the foods we consume to produce acid. 6 with the chemical formula KOH, is found in alkaline batteries. 7 Acids and alkalis are electrical conductors. 8 change the colour of blue litmus paper to red whereas change the colour of red litmus paper to blue. 9 Acids taste while alkalis taste . 10 Without the presence of , an acid or an alkali does not show its properties. 11 are substances that change colour according to the acidity and alkalinity of the solutions. 12 Red cabbage extract changes its colour to red in solutions. 13 A solution is acidic if the pH value is 7. 14 A solution is alkaline if the pH value is 7. 15 A solution is neutral if the pH value is . 16 A solution with a pH of 2 is acidic than a solution with a pH of 6. 17 A solution with a pH of 9 is alkaline than a solution with a pH of 13. 18 When milk is kept too long at room temperature, it becomes sour because the bacteria found in milk convert lactose in the milk to acid. 19 takes place when the right amounts of an acid and an alkali react to produce salt and water. 20 A wasp sting which is can be treated with vinegar. 21 can be added to the soil to treat soil which is too acidic. RECALL Fill in the missing words. Chapter 6 Acids and Alkalis 181
THINKING CAP Put on your 1 The diagram shows the inner part of an alkaline electric cell. Which property of the potassium hydroxide used in the cell makes it suitable as a component of the cell? 2 What happens to the pH of a solution with a pH of 13 if you keep adding a lot of water to it? Give a reason for your answer. Positive terminal Zinc case Potassium hydroxide (alkali) Carbon rod Negative terminal 3 You are given 100 ml of a solution of pH 5. Explain how you could prepare a solution of pH 6 from the solution given. 182
Project Investigating pH of Soils Activity objective: To determine the pH of different types of soil and the plants that grow well in them. Problem statement: Soil pH is important for the healthy growth and yield of plants. There are many factors that affect the pH of soil. Not all plants need the same soil conditions. The type of plant which suits a certain soil condition must be determined for maximum growth. Concept applied: Method of testing the pH of soil Procedure: 1 Divide the students in the class into several groups. 2 Each group should carry out the following tasks: (a) Collect different soil samples in the surroundings. (b) Identify the methods used to test the pH of the soil samples. (c) Determine the apparatus and materials to use. (d) Test the pH of the soil samples. Solution: Identify the pH of the soil and conclude which plants will grow well in each type of soil. Explain the advantages of the method used to test the pH of the soils. Presentation: Present the finding using Microsoft PowerPoint. Chapter 6 Acids and Alkalis 183
Have you ever wondered what is beneath the soil that we step on daily? Do you know how many layers are there in our Earth? CHAPTER 7 What will you learn? Describe the structure of Earth and Earth’s surface processes Explain the movement of tectonic plates Describe the different plate movements Describe what an earthquake is Describe the types of volcanoes Respond appropriately during natural disasters such as earthquakes and volcanic eruption Structure of the Earth and Natural Disasters
7.1 Structure of Earth Earth’s surface is uneven because of a variety of landforms found on it. Landforms 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. Some landforms are created in a very short period, while others take millions of years. There are many types of landforms on the Earth’s surface such as mountains, valleys, deserts, dunes, islands, plains, rivers, oceans and deltas. Some of the landforms found on Earth’s surface around the world Mount Agung in Bali, Indonesia Flores Island in Indonesia Nile River in Egypt Sahara Dessert in North Africa River valley in Sumbawa, Indonesia Atlantic Ocean Chapter 7 Structure of the Earth and Natural Disasters 185
Characteristics of the Earth’s Layers The Earth can be divided into layers based on chemical composition and physical characteristics. Based on chemical composition, our Earth is made up of three main layers, namely the crust, mantle and core. Based on physical characteristics, the Earth can be divided into five distinct layers, namely the lithosphere, asthenosphere, mesosphere, outer core and inner core. Crust Mantle Outer core Inner core Lithosphere Asthenosphere Mesosphere Earth’s layers based on chemical composition and physical characteristics Chemical Layers Crust The Earth’s crust is the outermost layer of the Earth which we live on. It is 5 to 70 km in thickness. Earth’s crust is divided into two types: oceanic crust and continental crust. The thickness of the continental crust which makes up the land on Earth ranges from 35 km to 70 km while the thickness of the oceanic crust beneath the ocean floor ranges from 5 km to 7 km. The average density of the Earth’s layers increases from the crust to the core. Thus, the Earth’s crust has the lowest density compared to other layers. The crust makes up less than 1% of the Earth’s mass. It consists of soil, rocks that are easily broken and various elements. There are eight elements that make up 99% of the Earth’s crust. They are oxygen, magnesium, aluminium, silicon, calcium, sodium, potassium and iron. The temperature of Earth’s crust varies considerably based on location and time it is measured. Its average surface temperature depends on whether it is being taken on land or beneath the sea. Since most of the Earth’s crust lies beneath the oceans further from the sun, the temperature ranges from as low as 0°C to 3°C. On the other hand, the deeper you dig into the continental crust, the higher the temperature gets. For example, the temperature at Mponeng gold mine in South Africa, which is 4 km deep from the ground, can reach up to 60°C. Chemical layers Physical layers Resource 186
The crust is mostly covered by water, sand, soil and ice. Underneath the loose layer of soil, sand and rock particles, is the solid rock known as the bedrock as shown here. Organic layer A layer that consists of organic matter such as dried leaves, grasses, dead leaves and twigs, and other decomposed organic matter. 1 2 3 4 5 Topsoil A layer that consists of minerals, decomposed organic matter and organisms such as earthworms and bacteria. Most plant roots grow here. Subsoil A layer that consists of sand, silt and clay with less organic matter. Weathered rock A layer that consists of rock particles. Bedrock A layer that consists of solid rocks. The different layers in the Earth’s crust Mantle The Earth’s mantle is the layer underneath the crust that goes down almost halfway to the centre of Earth. The mantle is divided into two zones, namely the upper mantle and lower mantle. The mantle is mostly solid with a small amount of molten material between the crust and the uppermost part of the mantle. The mantle is the Earth’s thickest layer with a thickness of about 2900 km. It makes up about 68% of the Earth’s mass. It is made up of silicon, magnesium, oxygen, aluminium and iron. Its temperature is between 500°C and 2000°C. The upper mantle consists of huge slabs of rock, called tectonic plates which are brittle and moving constantly. The movement of tectonic plates is extremely slow and when they rub against each other, earthquakes and volcanic activities occur. The lower mantle consists of rocks which are soft but not in the molten form. However, when rocks in the lower mantle are subjected to forces, they flow instead of breaking. Core Upper mantle Lower mantle 2900 km Mantle Cross section of the Earth showing two different zones of the mantle: upper mantle and lower mantle. Chapter 7 Structure of the Earth and Natural Disasters 187
Core The core is the centre of the Earth and about 2900 km below Earth’s surface. It makes up about 31% of the Earth’s mass. The ball-shaped core layer is divided into two parts, namely the outer core and inner core. The outer and inner cores make up about half of Earth’s diameter. There is a liquid-solid boundary between the inner and outer cores which is the hottest part of the core, as hot as the surface of the Sun. The outer core is beneath the mantle and mainly composed of molten iron and nickel. Both these metals are magnetic. Its thickness is about 2200 km, and the temperature is between 4000°C and 6000°C. The inner core is the densest part of the Earth. It is mainly composed of solid iron and nickel. The inner core is in solid form because the intense pressure it experiences prevents the iron from melting. Its thickness is about 1270 km and the temperature is between 5000°C and 7000°C. Activity 1 1 Work in groups. 2 Find out more about the Earth’s layer based on physical characteristics. You may use various source such as the books from your library, internet or videos. 3 Each group will present their findings on a poster. Researching the Earth’s physical layers The Kola Superdeep Borehole is the deepest manmade hole ever dug, purely sciencedriven, by the Soviet Union near the Russian border with Norway. Soviet scientists wanted to investigate more about the Earth’s crust, to get a better understanding on how that crust was formed and evolved. They wanted to drill as deep as possible into the Earth’s crust. The hole was finally drilled into a depth of 12 262 m, a depth that took about 20 years to reach. It is deeper than the deepest point in the ocean, the Mariana Trench, that has a depth of 11 035 m. Drilling of the Kola borehole began in May of 1970 but was abandoned in 1992 when drillers encountered extremely high temperatures as they dig deeper. Science Facts Mariana Trench Kola Superdeep Borehole 12 262 m 188
Earth’s Surface Processes Do you know the processes that cause changes to the Earth’s surface? They are known as geomorphic processes, which are important in the creation of landforms on Earth’s surface. There are two geomorphic processes, namely the endogenic process and exogenic process. The endogenic process is caused by forces that originate from within the Earth. Endogenic forces include earthquakes and volcanic activities. Their impact can be observed immediately. Mantle convection process The Earth’s crust is composed of pieces of moving plates, according to the Plate Tectonics theory. Due to the constant movement of these plates, collision and divergence occur. Thus, various landforms are formed on Earth. Movement of Earth’s crust A volcano is a vent on the Earth’s crust. Molten magma flows out through the vent in a strong eruption. The erupted materials deposited around the slope of the vent form the hillshaped landform known as the volcanic cone. Magma activity Mantle Asthenosphere Outer core Inner core Convection currents are produced in the asthenosphere layer due to the high temperatures in the mantle and core of Earth. The convection currents cause the movement of the Earth’s crust. Endogenic Processes Movement of Earth’s crust Lithosphere Vent Lava (molten rock erupted out of the Earth’s surface) Cone Magma (molten rock beneath Earth’s surface) Chapter 7 Structure of the Earth and Natural Disasters 189
The exogenic process is caused by forces that originate from the Earth’s surface or Earth’s atmosphere. Exogenic forces include the tidal force of the moon and erosion. Their impact can still be observed thousands or millions of years later. Exogenic Processes Weathering A process of fragmentation and decomposition of rocks on Earth’s surface caused by various factors such as changes in temperature, rain, wind and organisms. Mass wasting The movement of rocks and soil from the top of a slope downwards due to gravitational force. Erosion A process in which the materials on Earth’s surface are worn away by natural forces such as water, wind and waves. Transportation and sedimentation The movement of weathered or corroded materials by agents such as water, wind and glaciers. Eventually, the materials are sedimented when the agents slow down. 190
Slow Processes that Change the Surface of the Earth Some changes on the Earth’s surface are caused by slow processes that have occurred over billions of years ago. The slow processes involved are weathering, erosion and deposition. Weathering Weathering is the process where rocks on the Earth’s surface break into smaller pieces and gradually wear away. There are three types of weathering, namely physical, biological and chemical weathering. Physical Weathering Physical weathering, also known as mechanical weathering, is the breaking down of rocks without involving any changes to the chemical composition of the rocks. Temperature changes between day and night Cracks at outer layer of rock Thermal stress Physical weathering can occur due to temperature changes in a process called thermal stress. During the day when it is hot, the outer layer of a rock becomes hot while its centre is cold. So, the outer layer of the rock expands slightly. At night, when it is cold, the outer layer cools faster than its centre and contracts slightly. As the expansion and contraction occur repeatedly, over time the rock weakens. Cracks may form at the outer layer and pieces of the rock will fall off. Water enters the rock Water freezes and expands The rock breaks apart The effect of ice wedging can be seen at Bryce Canyon National Park, USA. The rocks go through freeze and thaw cycles repeatedly until they break apart. Freeze and thaw cycles occur hundreds of times each year, thus breaking the rocks into unusual shapes. In very cold areas, water can cause physical weathering through a process known as ice wedging that involves repeated freeze and thaw cycles as shown above. Rainwater or melting snow seeps into the crevices of a rock and when the water or melted snow freezes, it expands. This expansion pushes the crevices further apart. The freeze and thaw cycle occurs repeatedly and over time, the expanding ice widens the crevices, and the rock breaks apart. Ice wedging Chapter 7 Structure of the Earth and Natural Disasters 191
It is very cold on top of mountains, especially in the evenings and at night. The freezing and expansion of water in the rocks can break apart the rocks into fragments causing rockfalls. For this reason, mountain climbers start their journey early in the morning and they try to complete it by midday. The action of wind, gravity, rain and waves can also cause physical weathering. When there is strong wind, it carries sand particles that hit against rocks and breaks off the softer parts. These pieces will eventually fall off due to gravity. Over time, the sand in the wind sculpts large rocks into interesting shapes such as this arch found in Arches National Park, USA. The movement of water is a major force in physical weathering. Coastal rocks will gradually wear down due to the persistent crash of waves against them. 192
Biological Weathering Biological weathering is caused by the movements of living things such as plants, animals and microbes. Animals such as rabbits, chipmunks and badgers can burrow into a crack in a rock, thus widening it and eventually splitting the rock. A plant may grow in the crack of a rock and, as its roots grow, they wedge the rock apart. Sometimes humans contribute to biological weathering. This occurs when they walk over the same patch of rock which will gradually cause the rock to wear down. Humans wearing hiking shoes or boots to walk on rocks can contribute to weathering of the rocks. The pressure created in the rocks by the growing roots of plants can wedge the rock. When burrowing animals such as badgers dig tunnels underground, water may enter the rocks causing them to break apart over time. Chapter 7 Structure of the Earth and Natural Disasters 193
Chemical Weathering Chemical weathering of rocks involves chemical reactions which change the mineral compositions of rocks. Chemical weathering occurs due to agents like rain and organisms. The burning of fossil fuels such as coal, oil and gasoline releases carbon dioxide, nitrogen and sulphur into the atmosphere, which dissolve in rainwater to form carbonic, nitric and sulphuric acids respectively. This is what makes rain slightly acidic. Limestone is a type of rock that reacts with acidic rainwater, also known as acid rain, making the rock weathered. Organisms such as lichens also contribute to chemical weathering of rocks by excreting various organic acids, particularly oxalic acid, which breaks down rocks. Lichens are one of the organisms responsible for the weathering of rocks by causing the break down of rocks over time. Limestone is a type of rock mainly composed of calcium carbonate. When rainwater seeps through cracks in the roofs of limestone caves, the rainwater loses some of its dissolved carbon dioxide and deposits part of its dissolved calcium carbonate. Some of the deposits form stalactites that hang downwards from the roofs and some form stalagmites that grow up from the floor. When limestone reacts with acid rain, a soluble substance formed during the reaction weathers the limestone as the substance washes away. 194
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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