Earthquakes in East Asia PDF

1 Refer to p. T-4 for the answers. 1 Photos a and b show Opportunities and risks— Is it rational to live in hazard-prone areas? the landscape and destruction caused by volcanism respectively. a Refer to P...

1 Refer to p. T-4 for the answers. 1 Photos a and b show Opportunities and risks— Is it rational to live in hazard-prone areas? the landscape and destruction caused by volcanism respectively. a Refer to Photo a. What are the attractions brought by the landscape? b Refer to Photo b. What destruction was caused? 2 If you were living in a place near a volcano, would you choose to stay? Why? a Grand Prismatic Spring, the largest hot spring located in Yellowstone National Park, USA b Houses covered with volcanic ash during a volcanic eruption of Mount Ontake in Japan Contents Refer to p. T-4 for the teaching note of this part. 1.1 1.2 1.3 1.4 Why is our earth What are the major What are the How do plate unstable? 3 internal forces and major landforms movements create processes affecting found at the plate tectonic hazards? 57 the earth? What boundaries? 33 are the resultant landforms/features?  14 1.5 1.6 What destruction Is it rational to do tectonic hazards choose to live in cause? How can we hazard-prone areas? deal with them? 75  104 Skills box How can we identify a volcano and related features using a cross-section? 26 Case study The Tohoku earthquake and tsunami in Japan, 2011 90 What has Mexico done to reduce the losses of earthquakes?100 Field study Guide to writing A field study at Ma Shi Chau 27 short essays 63 List of IT resources Tackling DSE Scan the QR code to access questions the multimedia resources such as audio files, videos, flipped 71 classroom tools, animations and games for the series (indicated by icons inside the book). Practice questions 115 1.1 Why is our earth unstable? Refer to p. T-4 for the To arouse students’ interest, places that are near Hong Kong and students are teaching note of this unit. familiar with (Japan and Taiwan) are used in this activity. This can bring out the 1.1 fact that many of our neighbouring countries and places in Asia are affected by earthquakes from time to time. Besides, through this activity, students should easily recognise the spatial relationship Is the earth we live on stable? between earthquakes and plate boundaries. Figure 1 shows the distribution of earthquakes between 2010 and 2017 in East Asia. Plate boundary Earthquake (2010–2017) International North boundary Korea Japan N Dong China Hai Hong Kong Taiwan 0 600 km The map shows the distribution of earthquakes with a magnitude of 6 or above on the Richter scale. Tohoku earthquake (Magnitude 9.0) (March 2011) Triggered a huge tsunami About 16,000 deaths, more than 6,200 injuries About 1,163,000 buildings damaged or collapsed *Most of the deaths and damage were caused by the tsunami. Tainan earthquake (Magnitude 6.4) (February 2016) 117 deaths, more than 550 injuries Many buildings damaged and collapsed Source: USGS and various online news 1 The earthquakes caused loss of life and property, and injuries of people. One of them Figure 1 (the Tohoku earthquake) even triggered a tsunami (a secondary hazard of earthquake). 1 Refer to the photos in Figure 1. What damage did the earthquakes cause? 2 The distribution of earthquakes shows a clustered pattern. They are mostly found near/along plate boundaries, especially off 2 Refer to the map in Figure 1. Is there any spatial pattern for earthquakes? the eastern coast of Japan. In addition, they are found both on land and in seas. earthquake 地震 magnitude 強度 tsunami 海嘯 Dong Hai 東海 plate boundary 板塊邊界 1-3 1 Yes, Hong Kong has experienced earthquakes, but very minor ones. In the future, it is not likely for Hong Kong to experience strong earthquakes. This is because Hong Kong is located quite far away from the plate boundary (Hong Kong is over 600 kilometres away from the Circum-Pacific seismic belt). 3 a Mark the location of Hong Kong on the map in Figure 1 on p. 3. Refer to the map on p. 3 for the answer. b Did Hong Kong experience any earthquakes? Will Hong Kong experience strong earthquakes in the future? Explain your answer. c If an earthquake of a magnitude similar to that of the earthquake in Tainan in 2016 happened near Hong Kong, would Hong Kong suffer even more serious damage? Explain you answer. Yes, Hong Kong would probably suffer more serious damage because the population and building density in Hong Kong are higher than those in Tainan. Besides, there are no earthquake-proof designs in most buildings in Hong Kong. Furthermore, people in Hong Kong have low earthquake awareness. They may not know how to evacuate during strong earthquakes. [Note to teachers: Although buildings in Hong Kong are designed to withstand shaking/swaying caused by typhoons, they are not built with earthquake-proof designs intended for earthquakes.] In Hong Kong, we rarely experience earthquakes and the earth seems stable to us. But from the above activity, we know that many places, such as Japan, are not so lucky. They are struck by earthquakes of different magnitudes from time to time. Earthquakes are natural hazards. A hazard is an event that threatens or causes loss of life and property. If a hazard is caused by earth movement, it is known as a tectonic hazard. Earthquakes, volcanic eruptions and tsunamis are tectonic hazards. Why is the earth unstable? The answer is related to the structure of the earth. What is the structure of the earth? Structure of the Our earth can be divided into three layers based on their composition. From earth (with English the surface to the centre, these layers are the crust, the mantle and the core subtitles) (animation) Remind students that (Figure 2). They are arranged according to density. In general, layers with Figure 2 is not drawn lighter, less dense minerals are on top of layers with heavier, denser minerals. to scale. Although geologists have different views on the radius Oceanic crust (6–8 km thick) of the core, it is believed that The crust 1 (3,470 + 3,470) km = 6,940 km Continental crust 70 it lies between 3,100 km (25–70 km thick) 2 a = 1 (1:50) 3,470 50 and 3,700 km. The mantle b The crust is very thin. (2,900 km thick) 1 What is the diameter of the core? 2 a What is the ratio of the thickness The core (3,470 km thick) of the crust to the core? Outer core b What do you notice from the ratio? Inner core Average density Layer Composition of the three layers Crust Mainly silica 2.7 g/cm3 Mantle Rich in iron and magnesium 3.3 g/cm3 Core Rich in iron and nickel 11 g/cm3 Figure 2 The structure of the earth Note: The crust is exaggerated in scale. 1-4 tectonic hazard 構造災害 volcanic eruption 火山爆發 crust 地殼 mantle 地幔 core 地核  Factors leading to urban Structure growth andof the earth urbanization 1.1 The coastal waters of continental shelves are shallow, less than 200 metres deep. They are often the major fishing grounds in the world 1 Crust because sunlight can penetrate the shallow water. This favours the rapid growth of sea plants and planktons, which are the major food to fish. The crust is the solid outer layer we live on. There are two types of crust: Continental crust: This makes up the continents and the continental shelves, which are gently sloping plains bordering the continents. Continental crust is thick and light, and the age of rock is old. Oceanic crust: This is found under the oceans. It is thinner and denser, and the age of rock is generally younger than continental crust. 2 Mantle The mantle is denser than the crust and therefore it lies below the crust. It can be divided into three parts: Rocks in the asthenosphere are Uppermost part of the mantle: It exists in a solid state and is attached to under great pressure and very high temperature. They the bottom of the crust. The uppermost part of the mantle together with are soft, weak and easily the crust form the lithosphere. deformed, like butter or modelling clay, and exist in Asthenosphere: It lies below the lithosphere. The rocks in this layer are a plastic, solid state. in a plastic, solid state. They can flow very slowly under pressure. Energy in the earth’s Lower mantle: It exists in a solid state under high pressure. interior mainly comes from the residual heat from the formation of the planet and radiant energy produced 3 Core through radioactive decay. Radiant energy is constantly This is the hottest and densest part of the earth. It can be subdivided into released from the radioactive elements such as uranium-238 two layers: and thorium-232. It is absorbed and changed into Outer core: It is very hot. The material has melted and is in liquid form. heat energy, causing high Inner core: It is under the greatest pressure. The material is kept in solid temperature of the core (5,500°C). form. Great heat released from the core makes the temperature to increase with depth from the earth’s surface. Heat from the earth’s interior can be used as a power resource known as geothermal power. Refer to p. 109 for more details. Sea 0 km 0 km Lithosphere (solid) Continental crust Crust Crust 100–200 km 6–70 km t p ar t of the mantle Solid crust U p per m o s Pla sti Mantle c, so Solid Solid uppermost Asthenosphere lid lower mantle part of the mantle (plastic, solid) as the Upper mantle Liquid nos outer core pher Core 350–500 km Solid e inner Lower mantle (solid) core Lower mantle (Not drawn to scale) (Not drawn to scale) Figure 3 Layers of the earth continental crust 大陸地殼 continental shelf 大陸架 oceanic crust 大洋地殼 lithosphere 岩石圈 asthenosphere 軟流圈 1-5 1 Structure of the earth Note that the major mineral found in the crust, no matter continental crust or oceanic crust, is silica. The presence of other minerals in specific crusts provide clues to the density of these crusts. For example, more iron and magnesium in the oceanic crust results in higher d ensity of the crust. This helps explain why oceanic Table 1 Characteristics of the different layers of the earth crust is always the crust that subducts. Division Density Temperature (based on Layer Composition and State Description and depth the state of pressure materials) 0 km Lowest Continental Thicker and lighter 70 km crust With more aluminium, Mantle Crust Crust Mainly silica Solid potassium and sodium 7) are found along convergent plate boundaries. Earthquakes along divergent plate boundaries are mostly of lower magnitude. c Unclick the layer ‘Significant Earthquakes’ to clear the map. 2 Volcanic eruptions a Tick the layers ‘Volcanoes’ and ‘Significant Volcanic Eruptions’ on the left panel. Complete Column B in Table 1 about the distribution of volcanic eruptions. Questions: Which region has the highest density of volcanic eruptions? Do all volcanic eruptions The region roughly surrounding the Pacific Ocean has the occur along plate boundaries? highest density of volcanic eruptions. b Unclick the two layers on the left panel to clear the map. No, some of the volcanic eruptions occurred far away from Figure 1 on p. 57 shows the screen capture plate boundaries, such as in the centre of the Pacific Plate. 3 Tsunamis of the distribution of tsunamis. a Tick the layer ‘Tsunami Events’ on the left panel. Select ‘Green Squares’ to show the locations of tsunami events. Complete Column C in Table 1 about the distribution of tsunamis. b Select ‘Symbols for Cause/Fatalities’ to show the causes of the tsunami events (click the legend for details). Questions: What are the possible causes of tsunamis? Which is the most common cause of tsunamis? The possible causes of tsunamis are (submarine) volcanic eruptions, landslides and earthquakes. The most common cause of tsunamis is earthquakes. Table 1 Relationship between tectonic hazards and plate boundaries Column A Column B Column C Earthquakes Volcanic eruptions Tsunamis Occurring along convergent   ✓ Frequent   ✓ Frequent   ✓ Frequent plate boundary   Few   Few   Few   None   None   None Occurring along divergent   Frequent   Frequent   Frequent plate boundary   ✓ Few   ✓ Few   ✓ Few   None   None   None Occurring along transform   ✓ Frequent   Frequent   Frequent plate boundary   Few   Few   ✓ Few   None   ✓ None   None Hawaiian Islands, eastern Occurring away from the and western Africa, Australia, Hawaiian Islands and Africa Hawaiian Islands, coasts of Norway, Africa and India, plate boundaries (Briefly on the ocean floor of the eastern coast of North describe locations) Pacific Ocean and Indian America and Greenland Ocean, inland Asia, and eastern part of North America 1-58  Factors leading to urban Earthquakes growth and urbanization 1.4 As seen in Table 1 (p. 58), tectonic hazards are mostly concentrated in belts at plate boundaries. Is it just a coincidence, or is there an explanation? Try this experiment How do plate movements create simulating the tremors of earthquakes: Student A hits one end of a desk with a hammer while Student B presses his/her hand on the earthquakes? other end. Student B will feel the vibrations set up by the What an earthquake is energy of the blow. The harder the blow, the stronger An earthquake is the sudden shaking of the land. It is caused by the the vibrations. Why can Student B feel sudden release of energy stored in the rocks beneath the earth’s those vibrations? This is surface. because the energy passed by the hammer is transferred When the stress (or force) is applied to rocks, energy builds up in the to his/her hand by tremors travelling through the solid rock layer and a fracture is formed. When the energy exceeds the strength wood. In the crust, a sudden of the rock, the rock suddenly breaks and displaces along the fracture. slip of rock masses along the fault planes will result in similar tremors. a Energy builds up in the b Energy is released and the Stress/Force rock and a fracture forms rock breaks along the fracture applied to Seismic waves the rock penetrate the earth either as ‘body waves’ or ‘surface waves’. They shake the ground in different ways. Visit https://digital.oupchina. com.hk/ssgeog3e_rma/ Earthquake p1_59e_q.html or scan the occurs here QR code below for an Figure 2 The formation of earthquakes animation showing the movement of these waves. Earth’s surface The energy released from the breaking of rock transmits as shockwaves, also Seismic known as seismic waves. They move wave outward in all directions and shake the Seismic wave ground. Seismic waves are generated from a focus. The point on the crustal surface directly above the focus is the epicentre. It is nearest to the source of the seismic Epicentre waves. Thus, it is the point on the surface where earthquake is most strongly felt. p. 63 Q2 Se ism The website of the USGS provides various information ic wa of recent and historic earthquakes, visit https://digital. ev oupchina.com.hk/ssgeog3e_rma/p1_59e_c.html or scan the QR code below. Focus Figure 3 The focus and epicentre of an earthquake seismic wave 地震波 focus 震源 epicentre 震央 1-59  1 Earthquakes Ways of measuring earthquakes The severity of an earthquake can be expressed in terms of both intensity The Richter scale is not and magnitude. commonly used anymore because it only works best for Intensity is based on the damage caused by an earthquake. It is earthquakes in California that are within 600 km of measured in Modified Mercalli Intensity Scale (Figure 5). The scale ranges seismographs. Nowadays, the from I (not felt at all) to XII (total destruction). universal measurement is the more accurate moment Magnitude is related to the amount of magnitude scale（矩震級） , which can capture all seismic energy released at the focus. It is measured waves recorded from shaking. in Richter scale, which is recorded by a The scale has been computed so that the magnitude is roughly seismograph. The scale ranges from 1 to 9 equal to the Richter scale. or above. Nowadays, the USGS uses moment magnitude scale to report the magnitude of Figure 4 A seismograph earthquakes. Modified Mercalli I II –V VI –VII VIII – IX X–XI XII Intensity Scale Detected only by Causes all hanging Walls crack Chimneys and Many houses and Total destruction seismograph objects to move; and it becomes tree branches fall. bridges collapse. trees sway difficult to stand Houses collapse The ground opens up Richter scale Under 3.0 3.0 – 4.9 5.0 – 5.9 6.0 – 6.9 7.0 – 8.9 9 or above Average annual 300,000 55,000 800 120 18 1 occurrence worldwide Figure 5 Destruction associated with different readings on the Modified Mercalli Intensity Scale and the Richter scale Where and why earthquakes occur Each year, only about Earthquakes occur every day. Some are very powerful and destructive, but 20 major earthquakes occur worldwide. most of them are too mild to be felt. Figure 6 on p. 61 shows the distribution In general, most of the of earthquakes. They are most common around the Pacific Ocean (i.e. the energy released by earthquakes originates in a few, long, narrow Circum-Pacific Belt) and in a zone extending from northern India and belts that coincide with plate across to the Mediterranean Sea (i.e. the Alpine-Himalayan Belt). Both of boundaries, particularly the Circum-Pacific Belt. The Belt these zones are near to the plate boundaries. accounts for about 75% of the world’s seismic energy released. Although earthquakes are common at plate boundaries, they are also found in the interior of a plate, for example, on African Plate while some occur at hot spots, such as at the Hawaiian Islands. 1-60 intensity 強度 Modified Mercalli Intensity Scale 修訂麥加利震級 Richter scale 黎克特震級 seismograph 地震儀 hot spot 熱點  Earthquakes 1.4 About 80% of earthquakes occur in the Circum-Pacific Belt. N 0° Pacific Ocean Atlantic Indian Ocean Ocean 0 2,300 km Plate boundary Alpine-Himalayan Belt Source: NOAA Earthquake Circum-Pacific Belt Figure 6 Distribution of earthquakes 1 At plate boundaries In general, earthquakes are found at all types of plate boundaries. However, strong earthquakes occur more frequently at convergent and transform plate boundaries. At these plate boundaries, plates collide and slide past each other, both create great friction and stress. Energy is thus built up more easily and rapidly. c-c divergence 2 o-c collision o-o divergence 1 3 Transform fault 2 1 3 Refer to Figure 7 for the answer. Subduction On Figure 7, mark the plate boundaries with the following: 1 Divergent plate boundary Earthquake o-o divergence, c-c 2 Convergent plate boundary Direction of plate movement divergence and o-c 3 Transform plate boundary collision. Figure 7 Earthquakes at different plate boundaries 1-61  1 Earthquakes Earthquakes can occur at various depths in the earth’s crust. Most Refer to p. T-11 for more information earthquakes occur at shallow depths, especially those occurring at divergent about the characteristics of earthquakes at different types and transform plate boundaries. However, at convergent plate boundaries, of plate boundaries. shallow to deep earthquakes occur. In general, deep earthquakes tend to be less damaging than shallower ones. This is because seismic waves from deep earthquakes lose energy as they travel a long distance from focus to the surface. Convergent plate Oceanic Transform plate Convergent plate boundary crust boundary boundary Continental crust Continental crust Uppermost part Divergent plate Uppermost part of the mantle boundary of the mantle Asthenosphere How is the depth of earthquake foci related Depth of earthquake foci to plate movement? Shallow (300 km deep) Subduction of plates Figure 8 The depth of earthquake foci at different types of plate boundaries causes friction at a great depth, therefore generating intermediate- to deep-focus earthquakes. Non-subduction 2 At hot spots plate movements cause shallow-focus earthquakes. Some earthquakes may be triggered by volcanic eruptions at hot spots, for example, in places near the Hawaiian Islands. More information about hot spots can be found on pp. 65–7. Human activities may also trigger some 3 At fault planes minor earthquakes. For example, water in large In some cases, earthquakes are generated from fault systems which are reservoirs puts pressure on affected by plate movements nearby. For example, in East Africa, the African rocks. This may trigger faulting and earthquakes. Besides, Plate is breaking under tensional force (p. 40). Stress accumulates along nuclear tests and mining may the fault planes. The breaking of rock and release of stored energy along also cause earthquakes. the faults cause earthquakes. pp. 62–3 Q1 & 3 The 2008 Sichuan earthquake in China was caused by the movement of fault Checkpoint in the Sichuan Pendi. Refer to pp. T-11–12 for more information. 1 At which of the following plate boundaries do strong earthquakes most frequently occur? A 2 only 1 Transform B 1 and 2 only 2 Divergent C 1 and 3 only 3 Convergent D 2 and 3 only  C pp. 60–2 1-62  Earthquakes 1.4 2 Figure 9 shows the occurrence of an earthquake. Which of the following shows the intensity of Earth’s surface the earthquake experienced at the four places 1 4 in descending order? 3 2 Epicentre A 1, 2, 3, 4 C 2, 3, 4, 1 B 2, 1, 3, 4 D 4, 3, 1, 2  B p. 59 Focus 3 Explain the occurrence of earthquakes in relation to convergent and transform plate Fault Seismic wave boundaries. (Hint: Refer to pp. 63–4 for guidance.) Relative movement of rocks  (6 marks) Figure 9 pp. 60–2 Guide to writing short essays Explain the occurrence of earthquakes in relation to convergent and transform plate boundaries. First of all, we need to identify the directive term in order to answer a short essay question. This is important because the term tells us how to answer the question. The directive term in the question above is ‘explain’, i.e. to give reasons. After knowing how to answer this type of question, we need to identify the key concepts which are to be included in the answer. In the above question, ‘occurrence of earthquakes in relation to convergent and transform plate boundaries’ is the key concept. This means students have to give reasons why earthquakes occur at boundaries where plates converge and slide past each other. To write a good essay, always include an introduction, main body and a concise conclusion. Introduction An earthquake is a kind of tectonic hazard. It is a sudden shaking of Briefly describe what an the land. It is caused by a sudden release of energy stored in the rocks earthquake is and its causes. Mention the types beneath the earth’s surface. Most of them are found at plate boundaries, of plate boundaries where particularly convergent and transform plate boundaries. most of the earthquakes are found Earthquakes are mainly confined into two belts– 1 the Circum- Pacific Belt and the Alpine-Himalayan Belt. Along these two belts, Writing tips Main body 1 Describe the global either ocean-ocean 2 (e.g. the Philippine Plate and the Pacific Describe and explain the distribution of Plate), ocean-continent (e.g. the Nazca Plate and the South earthquakes occurrence of earthquakes: at convergent plate American Plate) or continent-continent 2 (e.g. the Indo-Australian 2 To score high marks, boundaries Plate and the Eurasian Plate) collisions are found. provide examples as shown 1-63  Factors leading to urban 1 Volcanicand growth eruptions urbanization 3 At convergent plate boundaries where two plates collide, 3 Explain the formation great amounts of friction and stress are exerted on the rock. This of earthquakes at makes energy build up in the rock easily and rapidly. A fracture is convergent plate boundaries then formed in the rock. When the energy exceeds the strength of a Elaborate on the the rock, the rock suddenly breaks and displaces along the fracture. characteristics of the The energy released from the breaking of rock transmits as seismic earthquakes generated waves, which move outward in all directions and shake the ground, at this type of plate boundary causing earthquakes. a Earthquakes generated along this type of plate boundary can be shallow to deep. Describe and explain Earthquakes also frequently occur at transform plate boundaries, 4 Describe and explain the occurrence of for example along the San Andreas Fault. the formation of earthquakes at transform earthquakes at plate boundaries 4 The formation of earthquakes at transform plate boundaries transform plate is similar to those formed at convergent plate boundaries. b The boundaries difference is in the direction of plate movement that creates friction b To score high marks, spot out the differences and stress on the rock. At a transform plate boundary, it is the plates between the that slide past each other to generate great friction and pressure, earthquakes generated which eventually lead to earthquakes. Compared with earthquakes at convergent and transform plate Conclusion generated at convergent plate boundaries, b earthquakes along boundaries Restate the occurrence transform plate boundaries are usually shallow. of earthquakes in relation to convergent Earthquakes are mostly found at convergent and transform and transform plate plate boundaries. This is because energy is built up easily and rapidly boundaries. Spot out there. However, the direction of plate movement that generates the the difference between the two earthquakes, and the depth of these earthquakes, are different. How do plate movements create volcanic eruptions? What volcanic eruptions are Volcanic eruptions occur when lava, gases (water vapour, carbon dioxide and sulphur dioxide) and pyroclastic materials are ejected or emitted from an opening in the earth’s surface. Figure 10 Eruption of Momotombo Volcano in Nicaragua p. 67 Q1 1-64 Nicaragua 尼加拉瓜  At convergent plate boundaries, the water released from Volcanic eruptions 1.4 the subducted plates causes the asthenosphere to melt at a lower temperature. Part of the asthenosphere melts into magma. The compressional force associated with the folding and Where and why volcanic eruptions occur subduction processes produces cracks, which may extend to the asthenosphere. Magma rises up 1 At convergent and divergent plate boundaries through the cracks and results in volcanic eruptions. More than 90% of volcanic eruptions are found at convergent and divergent At divergent plate boundaries, magma rises towards the plate boundaries. The line of active volcanoes which roughly surrounds the earth’s surface at some places. Pacific Ocean tends to produce the most violent eruptions. It is known as the The tensional force along the plate boundary forms fissures. Pacific Ring of Fire. Pressure in the asthenosphere reduces and rocks melt to form Refer to Figure 11. Can you explain the occurrence of volcanic eruptions magma. When magma rises through the fissures, volcanic at convergent and divergent plate boundaries? eruptions occur. Okhotsk Plate N Juan de Fuca Plate Eurasian Plate Yellowstone Caldera Pacific Ring of Fire North American Plate Caribbean Plate Arabian Plate Philippine African Plate Plate Hawaiian Islands Cocos Plate 0° South Pacific Plate American Plate Nazca Plate Indo-Australian Plate 0 2,300 km Antarctic Plate Scotia Plate Convergent plate boundary Transform plate boundary Active volcano Divergent plate boundary Plate boundary involving major and minor plates Figure 11 The distribution of active volcanoes Some hot spots are found on or near mid-oceanic Refer to p. T-12 for the ridges. Examples include Iceland (an on-ridge hot spot); comparison of the nature 2 Above a hot spot as well as Easter Island and the Galápagos Islands (near-ridge hot spots). Refer to Figure 14 on p. 67 for their locations. of the volcanic eruptions along the convergent and divergent Some volcanic areas such as the Hawaiian Islands and the Yellowstone in plate boundaries. the USA are located above a hot spot. A hot spot is a location where a Hot spots are local zones mantle plume rises from between the core and the mantle (Figure 12 on of high heat flow and volcanism that occur far from p. 66). A mantle plume is a long, narrow column of upwelling hot mantle plate margins. They originate material. When the plume head touches the lithosphere, the heat facilitates deep in the mantle. the melting of rock into magma. Hot spot volcanoes are different from volcanoes at Magma pushes through cracks in the crust and erupts at the earth’s divergent plate margins. Lava comes from deep in the mantle. surface. This forms volcanoes, submarine volcanoes or volcanic islands. The magma in a plume rises due Visit https://digital.oupchina.com.hk/ssgeog3e_rma/ to decompression (a reduction in p1_65e_k.html or scan the QR code on the right for pressure) as hot material from the Pacific Ring of Fire 太平洋火環 1-65 a video clip showing the eruption of the Kilauea Volcano, lower mantle rises to the crust. which is produced by a hot spot beneath Hawaii.  1 Volcanic eruptions For a long time, hot spots are thought to Active be stationary. However, new volcano evidence shows that hot spot Hot spot can shift from one place to another. For example, it is found that the one forming Lithosphere the Hawaiian Islands has Head been moving southwards. Rising plume Tail tail Rising mantle plume Mantle Mantle Core Core Figure 12 Mantle plume rising upward from between the core and the mantle to form a hot spot The Hawaiian Islands are the most famous volcanic islands found over a hot spot within the Pacific Plate. Here, a mantle plume rises roughly in the centre of the Pacific Ocean and forms a hot spot there. Magma rises and erupts at the sea floor of the Pacific Ocean. After repeated eruptions, the Hot spot volcanism submarine volcano grows in height until it finally rises above the sea level to (swipe animation) form a volcanic island (Figure 13a). a b A new volcanic island or a Refer to submarine volcano forms at Magma rises and erupts at the Original volcano is carried the hot spot p. T-12 for more ocean floor, forming a submarine away from the hot spot information about the volcano or a volcanic island and becomes extinct Hawaiian Islands. Hot spot Hot spot Plate movement Pacific Plate Pacific Plate Mantle plume Mantle plume Asthenosphere Asthenosphere (Not drawn to scale) Kauai c A new volcanic island 3.8 – 5.6 Oahu (Hawaii) is formed 2.2–3.4 Molokai Erosion lowers Chain of volcanic 1.3–1.8 Maui old volcanoes. islands/submarine 0.8– 1.3 They move away volcanoes Hawaii with the plate 0.7 Maui Direction of plate movement Molokai Hot spot Plate Pacific Plate Pacific Figure 13 ere The formation of the nosph Asthe Numbers indicate the age of Asthenosphere Mantle plume Hawaiian Islands volcanic island (in millions of years) 1-66  Volcanic eruptions 1.4 As the Pacific Plate moves north-westwards, this volcanic island is carried away from the hot spot. The volcano on the island is being cut off from the source of magma. Volcanism ceases and the volcano becomes extinct. A There are different new submarine volcano and eventually a volcanic island is subsequently estimates about the number of hot spots over the formed above the hot spot (Figure 13b on p. 66). earth’s surface. Most geologists believe that there The cycle is repeated and results in a line of extinct submarine are a few tens of them. The volcanoes or volcanic islands. They lie to the north-west of the active hot spots at Hawaii, Yellowstone, Galápagos and volcano (Mauna Loa on the island of Hawaii), which is currently located Iceland are some of the most directly above the hot spot (Figure 13c on p. 66). active ones at present. The Hawaiian Islands Besides the Hawaiian Islands, the Yellowstone and the Easter Island are are the youngest examples of hot spots found beneath plates. Figure 14 below shows some of volcanic landforms formed over a hot spot. They are still the hot spots that have been active in the last 10 million years. growing today. Iceland N 80 Ma Emperor Yellowstone Seamounts Hawaiian Ridge 1° Canary 43 Ma 0 Ma 23 — 2 N Islands Hawaiian Islands Afar Galápagos 0° Islands Marquesas St Helena Reunion Easter 1° 23 — 2 S Island Ma: Million years ago 0 2,350 km Undersea mountain range Plate boundary Plate boundary involving major and minor plates Hot spot Figure 14 Some of the hot spots that have been active in the last 10 million years pp. 67–8 Q2 & 3 Refer to p. T-13 for more about volcanic eruptions in the Yellowstone. Checkpoint 1 Which of the following materials will be ejected 2 Volcanic eruptions occur during volcanic eruptions? 1 at hot spots. 1 Lava 2 at mid-oceanic ridges. 2 Ashes 3 at transform faults. 3 Pyroclastic materials A 1 and 2 only C 2 and 3 only A 1 and 2 only C 2 and 3 only B 1 and 3 only D 1, 2 and 3  A B 1 and 3 only D 1, 2 and 3  D pp. 65–7 p. 64 1-67  Factors leading to urban 1 Tsunamis growth and urbanization 3 Figure 15 shows the landforms formed on the Which of the following descriptions about sea floor. Figure 15 is/are correct? Plate 1 Landforms W, X, Y and Z are formed over Ocean a hot spot. W X 2 The age of Landform W is younger than that Y of Landform Z. As Z 3 Landform Z is active. the no sp A 1 only he re B 1 and 2 only C 1 and 3 only Direction of plate movement D 2 and 3 only  C Figure 15 pp. 65–7 How do plate movements create tsunamis? Tsunamis are different from ordinary sea waves. While sea waves are driven by the wind, tsunamis are the result of a sudden movements in the ocean floor. Hong Kong had been attacked by tsunamis What tsunamis are four times in the last century. ‘Tsunami’ is a Japanese word which describes a series of big sea waves. However, they were all insignificant. The waves were It is caused by the sudden vertical movement of the ocean floor. below 1 metre in height. Tsunamis can travel at a speed of 700 kilometres per hour or more. They are not easy to notice in open seas because they are only about 1 metre Tsunamis (with high. But when they reach the shallow seas at the coast, they slow down and English subtitles) (video) can tower up into huge waves over 30 metres high. Short wavelength Long wavelength Wave height: 5m Wave height: 1m Sea level Deep Ocean (Not drawn to scale) Figure 16 Changes in wavelength and wave height of Figure 17 The forecasted wave heights of the tsunami tsunami waves as they approach the shore that struck Japan in 2011 were up to 10 metres 1-68 Extended reading An animation showing how the Tonga submarine earthquake Tsunamis 1.4 Tsunamis caused by generated tsunami waves is available at https://digital.oupchina. non-submarine landslides com.hk/ssgeog3e_rma/p1_69e_m.html or the QR code on the right. and volcanic eruptions Where and why tsunamis occur More than 75% of tsunamis are triggered by submarine Figure 1 on p. 57 shows the distribution of tsunami events. Tsunamis can be earthquakes. So, tsunamis are often called seismic waves. generated at different types of plate boundaries. However, they are more However, not all earthquakes actively generated at convergent plate boundaries than the others. This generate tsunamis. Earthquakes measuring below 7 on the Richter is because the causes of vertical movement of the ocean floor are most scale seldom produce tsunamis. active at convergent plate boundaries (Figure 18). Sometimes, The South Asian Tsunami in 2004 was caused by a submarine earthquake measuring 9.1 on the tsunamis can be Richter scale. It is estimated that the seabed overlying the fault, where the earthquake occurred, has resulted from two causes been uplifted by several metres. combined. Earthquakes can trigger the collapse of thick Shallow focus sediment accumulated near associated with Sea the steep trench wall. This vertical displacement Lithosphere 3 Submarine causes submarine of rock landslide 2 landslides, which further Submarine volcanic eruption displace bodies of seawater. Body of water displaced 1 Submarine Earthquake occurs earthquake here Former seabed Asthenosphere (Not drawn to scale) 1 Submarine earthquakes 2 S  ubmarine 3 Submarine landslides Destructive tsunamis are usually caused by strong volcanic In cases where large-scale submarine earthquakes, which are associated eruptions submarine landslides occur in with vertical displacement of ocean floor and Very violent eruptions coastal areas, tsunamis may seawater (see Figure 19). of submarine volcanoes result, because any large-scale Those submarine earthquakes with no vertical may also displace water collapse of solid coastal materials displacement of the ocean floor do not cause tsunamis and cause tsunamis under water can displace water Figure 18 The causes of vertical displacement of seawater which result in tsunamis Ice caps melt under global warming. When a Submarine earthquake with vertical displacement of rocks occurs on the seabed. large pieces of ice at their Water is displaced and appears as a huge bulge on the sea surface edges collapse into the sea, a 3 Water bulges Coast large body of water may also 4 Water withdraws before the first wave be displaced suddenly. 2 The seabed is Tsunamis can be generated. lifted up Seabed 1 Vertical movement occurs during an earthquake under the sea b After the earthquake, gravity collapses the bulge to start a succession of waves, or tsunami. The tsunami waves move away in all directions 1D  escribe the Tsunami is heading Coast movement of sea towards the coast waves shown in the 1 diagrams. 2 2 2W  here will a ship turn Seabed over, at the open sea or near the coast? Direction of water movement Why? Figure 19 How a tsunami is generated during an earthquake p. 70 Q1 1 When an earthquake strikes, the water is displaced and appears as a bulge on the sea surface. After the earthquake, the bulge collapses and sea waves spread and move away in all directions. 2 The ship will turn over near the coast. This is because at the open sea, the height of waves are low (about 1 m high), 1-69 but they can tower up into huge waves (over 30 m high) as they reach the shallow seas near the coast.  1 Tsunamis The wave height is much lower at the affected places (less than 1.7 m) than from where the tsunami was generated (12 m). [Note to teachers: The tsunami struck at high tide at Galápagos Islands and the western coast of South America. Therefore, the effects of tsunami were stronger than those occurred in Hawaii.] Refer to Figure 20. Over 95% of all tsunamis occurred around the Pacific Ocean, which 1 How long did it is almost entirely encircled by unstable convergent plate boundaries. For take for the Tohoku example, the South Asian tsunami in 2004, which occurred in the Indian tsunami of 2011 to reach the Hawaiian Ocean, was triggered off at the convergent plate boundary between the Indo- Islands? About 7 to 8 Australian Plate and the Eurasian Plate. hours 2 What do you notice about the predicted Once a tsunami is generated, the islands in the Pacific Ocean are often wave height found affected, because tsunamis travel in all directions. They hit these islands at different affected places? first before they arrive at the opposite coast. The Hawaiian Islands in the Pacific Ocean were hit by tsunamis more than 40 times in the last 200 years. Figure 20 shows the route of the N 2011 Tohoku tsunami, and the time it took to travel to other places in the Vancouver Sendai, Japan Island, Pacific Ocean (turn to pp. 90–1 for the Canada 12 m 1m details about this tsunami). Hong Kong, 1° China 3 23 — 2 N In contrast, tsunamis generated 0.2 m East coast of at divergent plate boundaries, such the Philippines 6 as the plate boundaries in the Indian Hawaii, 0.5 m the USA 0° 1m Galápagos Ocean and the Atlantic Ocean, are 9 Islands, less frequent and not as serious. Ecuador Peru 1.7 m 1.5 m p. 70 Q2 12 15 1° 23 — 18 2 S 0 2,000 km 21 Tsunami travelled from here Hawaii, the USA Location and predicted 3 Number of hours the tsunami 1m wave height Figure 20 The route of the 2011 Tohoku hit the land after the earthquake tsunami and the time it took to travel to struck other places in the Pacific Ocean Checkpoint 1 Submarine earthquakes and volcanic eruptions 2 Which of the following statements about can generate tsunamis because they can tsunamis are correct? 1 displace a large body of ocean water 1 They are generated whenever there is an directly. earthquake. 2 trigger large-scale submarine landslides. 2 They have low wave height in open seas. 3 produce cracks on the ocean floor and 3 They can travel in all directions. cause the water to sink. A 1 and 2 only C 2 and 3 only A 1 and 2 only C 2 and 3 only B 1 and 3 only D 1, 2 and 3  C B 1 and 3 only D 1, 2 and 3  A pp. 68–70 p. 69 1-70  1.4 Tackling DSE questions 2015 HKDSE Paper 1, Section B Question 1 Due to copyright issues, please refer to the printed textbooks for the Tackling DSE questions. 1-71  1 Due to copyright issues, please refer to the printed textbooks for the Tackling DSE questions. 1-72  1.4 Due to copyright issues, please refer to the printed textbooks for the Tackling DSE questions. 1-73  1 Unit summary Refer to the table below. Answer the following questions to show the relationship between the distribution patterns of tectonic hazards and plate boundaries. 1 Tick the correct boxes in rows 2 to 4 to show the plate boundaries where the tectonic hazards are mostly found. 2 Suggest reasons why there are more tectonic hazards at certain plate boundaries. Fill in the fifth row. 3 Suggest reasons why some tectonic hazards occur away from plate boundaries. Fill in the last row. Plate boundary Earthquake Volcanic eruption Tsunami Divergent ✓ ✓ Convergent ✓ ✓ ✓ Transform ✓ Main reason Friction and At  convergent plate The following three stress boundaries: Magma causes of tsunamis are formed at the most active there: cause energy to build up subduction zone rises in the rock layers through cracks and submarine earthquakes reaches the earth’s surface submarine volcanic At divergent plate eruptions boundaries: Tensional force forms cracks. submarine landslides Magma wells up through these cracks to the earth’s surface Reason for Volcanic eruptions at Some volcanic Tsunami travel in all distribution hot spots eruptions occur at directions. away from plate trigger earthquakes hot spots boundaries Friction where mantle plumes — accumulates at the rise from between the fault planes core and the mantle in rift valleys 1-74 1.5 What destruction do tectonic hazards cause? How can we deal with them? Refer to p. T-13 for the This activity provides an opportunity for students to interpret satellite images in teaching note of this unit. studying the impact of hazards. For more able students, they can be motivated to 1.6 learn and present the destruction caused by tectonic hazards through project work. Indonesia badly hit by a tectonic hazard The province of Aceh in Indonesia was the area India N hardest hit by the 2004 South Asian tsunami. The Nicobar Islands disaster killed 200,000 people and displaced over Andaman Sea 6°N 500,000. Most of the villages along the shore were Banda Aceh destroyed. Malaysia For more practices, Aceh teachers may use the Kuala Lumpur before-and-after satellite images N showing different places affected Hong by the Tohoku tsunami in 2011. Kong Images can be found at https:// Indian Ocean digital.oupchina.com.hk/ Indonesia ssgeog3e_rma/p1_75e_y.html or 0 150 km the QR code below. 0° Sumatra I nd on esia Epicentre of the 2004 earthquake Figure 1 The location of Aceh, Indonesia a b Figure 2 Scenes of Banda Aceh before and after the hazard 1-75  1 1 Refer to Figure 2 on p. 75. What tectonic hazard struck Aceh? Tsunami 2 Figure 3 is a sketch of the coastline shown in Figure 2a on p. 75. With reference to Figure 2b on the 4 same page, identify and match the damage caused by the tectonic hazard listed below with labels 1–5 3 shown in Figure 3. a Fish ponds destroyed 5 1 b Bridge collapsed 1 c Roads inundated 4 d Low-lying areas inundated 3 e Vegetation washed away 2 Earthquakes and 3 a Name the other two tectonic hazards. volcanic eruptions 5 b For each of the tectonic hazard mentioned above, conduct an Internet search on the hazard which has occurred within the past year. Record the 2 information in the following sheet. Figure 3 A sketch of Banda Aceh before the hazard Table 1 Record sheet for the information search Date: Location: Magnitude (if applicable): Cause(s) (e.g. Is it lying on a plate boundary? What type of plate How frequent does this event occur in boundary is it? Is it lying on a fault? What type of fault is it?): t