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Cambridge University Press 978-0-521-68059-2 - NSSC Geography Module 1 Gerhard de Klerk Excerpt More information Unit 1 Plate tectonics, earthquakes and voIcanoes Look at Figure 1.1 which is a photograph of Earth taken from a satellite in space. Most of the surface of planet Earth is covered by sea, the rest is covered by continents. Which continent can you see in the satellite photograph? Do you think this continent will be in the same place in a million years time? In this unit you will discover that the continents and oceans form the Earth’s crust which is constantly moving. This movement can result in earthquakes and volcanoes. This unit is divided into three sections: Figure 1.1 The Earth from space A Earth’s crust on the move B Earthquakes C Volcanoes In this unit you will learn to: • recognise the Earth’s crust as the outer layer of planet Earth • realise that the Earth’s crust is made up of tectonic plates • identify tectonic plates on a world map • explain how earthquakes happen • investigate the Kobe earthquake of 1995 • explain how volcanoes happen • describe three types of volcanoes • describe the main types of intrusions and landforms • investigate the volcanic eruption of Mount Pinatubo in 1991 • identify zones of earthquake and volcanic activity on a world map • discuss the environmental and economic effects of earthquakes and volcanoes on people and landscapes • discuss strategies to manage the effects of earthquakes and volcanic eruptions. A Earth’s crust on the move Millions of years ago the Earth was a fiery ball of gas and liquid. As it cooled, the outer parts of the Earth formed a solid layer called the crust. But the crust is only the thin outer layer of the Earth as you can see if you study Figure 1.2 on page 2. © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-68059-2 - NSSC Geography Module 1 Gerhard de Klerk Excerpt More information 2 NSSC Geography 1. The crust – thin outer layer formed of solid rocks between 6 km and 90 km thick. This layer is the least dense. 2. The mantle – about 2 900 km thick, more dense than crust. Rocks are in a hot, thick liquid or molten state. The molten rock has a plastic consistency that allows it to move and flow. 3. The outer core – about 2 000 km thick, very dense but molten. 4. The inner core – 1 200 km thick, extremely hot. Thought to be an extremely dense solid metalic ball. Figure 1.2 The internal structure of the Earth ACTIVITY 1 Copy and complete the table below using information from Figure 1.2. Layer number Name Density Thickness Description 1 2 3 4 Glossary Tectonic plates – large sections of the Earth’s crust Hint The Earth is continually changing. Although continents neared their present locations about 50 million years ago, they are still drifting slowly over the planet’s surface. Direction of plate movement Figure 1.3 This is what Earth probably looked like 200 million years ago © Cambridge University Press Scientists think that the surface of the Earth is made up of different pieces of crust and not one continuous crust. The pieces of crust are called tectonic plates. Most tectonic plates carry both continents and oceans. You will find out more about tectonic plates in the next section. The theory of continental drift and tectonic plates In 1912, a German meteorologist called Alfred Wegener put forward the idea that the continents were once all joined in a single land mass which split up over a long period of time to form separate continents. He called the single land mass Pangaea (“all land” in Greek), and his idea became known as the theory of continental drift. Other scientists of the time thought Wegener was a fool to come up with such a crazy idea! It was only when new evidence was found in the 1950s and 1960s that Wegener was proved to be correct. For example, scientists found identical rock types and identical fossils in Africa and South America, which proved that these two continents had once been joined. See if you can identify our present continents in Pangaea in Figure 1.3. Look at the map in Figure 1.4 which shows the main tectonic plates into which the Earth’s crust is divided today. The plates www.cambridge.org Cambridge University Press 978-0-521-68059-2 - NSSC Geography Module 1 Gerhard de Klerk Excerpt More information Module 1 Unit 1 3 move a few centimetres every year. The edges of the plates are called plate boundaries. The arrows show the direction of movement of the plates. North American Plate NORTH AMERICA Eurasian Plate NT IC R AT LA MID Adriatic TurkishHellenic Plate AFRICA Pacific Plate SOUTH AMERICA South American Plate MID ATLANTIC RIDGE Nazca Plate Iranian Plate IDG E EUROPE ASIA JAPAN Pacific Plate Arabian Plate PHILIPPINES African Plate Indo-Australian Plate OCEANIA Antarctic Plate Direction of plate movement Plate boundaries NEW ZEALAND Figure 1.4 The different tectonic plates ACTIVITY 2 Use the map in Figure 1.4 to find the answers to these questions. 1 Name the plates that contain the following continents: a Africa b Australia c Japan d India. 2 Find the plate boundary along the west coast of South America. a Name the plates on either side of this plate boundary. b Are the plates moving apart or towards each other at this plate boundary? 3 Find the plate boundary which passes through the Atlantic Ocean from north to south. a Name the plates on either side of this plate boundary. b Are the plates moving apart or towards each other at this plate boundary? 4 In which direction is Antarctica moving? © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-68059-2 - NSSC Geography Module 1 Gerhard de Klerk Excerpt More information 4 NSSC Geography Moving plates It is difficult to believe that the hard, rigid tectonic plates that make up the Earth’s crust are moving. But remember that, below the crust, the mantle has a plastic consistency and can therefore move or flow. Scientists describe the upper layer of the mantle, immediately below the plates, as a “soft layer of slush”. The rigid tectonic plates can drift on this slush. The energy released by this movement triggers off earthquakes and volcanoes, especially along plate boundaries. Section B deals with earthquakes in more detail. B Glossary Earthquake – vibrations on the Earth’s surface caused by stresses in the crust Earthquakes Plate movements cause cracks or faults in the crust. The faults produce great stresses leading to vibrations on the surface of the Earth that we call earthquakes. The damage caused by an earthquake can be devastating – thousands of people and animals can be killed or injured and massive damage is done to buildings and landscapes. Read about an earthquake that took place in Turkey on 17 August 1999 in the article below. Desperate families claw at rubble Istanbul: The pre-dawn quake which rocked much of Turkey yesterday crushed apartment buildings and sent millions of people fleeing in panic into the streets. Terrified earthquake survivors sought the safety of open ground yesterday as the enormity of what had happened became clearer in the devastated areas they left behind: the final toll was more than 15 500 dead and almost 25 000 injured. Layer by layer, from thousands of sites around western Turkey, hundreds of bodies were removed, many in the pyjamas and nightgowns they were wearing when the immense quake struck over about 45 seconds in the cool hours before dawn. Some bodies were shrouded in blankets and sheets from their beds. As many as 10 000 people could still be trapped in rubble around Golcuk, a naval base about 130 km south east of Istanbul near the quake’s epicentre. There was severe damage in some areas of Izmit, some 90 km © Cambridge University Press east of Istanbul near the epicentre of the quake. Fahrettin Duman, working alongside a mechanical digger, struggled to pull fallen masonry from a apartment block near his own house in Izmit. “There are around eight people trapped in the staircase near the entrance, but even with this machine we can't reach them. There’s no sign of life,” he said. In Izmit, medical workers shattered pharmacy windows to get supplies for hospitals swamped by injured people. More than 100 sailors were dead or missing from a nearby barracks. Local authorities were overwhelmed by the task of clearing debris. “Hundreds of buildings collapsed. We need everything – hospitals, kitchens, tents and ambulances,” said Nihat Ozgol, governor of the nearby town of Yalova. In the Avcilar district on the impoverished fringe of Istanbul, a boy cried out from the cracked slabs and splintered plaster that was once his home, “Help me, I’m still alive!” As rescuers – some digging with bare hands – carted away corpses and listened for signs of life, they heard: “I’m here”. A worker touched his hand. “Oh thank you,” sobbed six-year-old Memet Arsuglou. Soon he was free and hugging his mother. The edges of Istanbul, such as the Avcilar district, revealed the hazards of hastily built accommodation for rural migrants to the city. Shoddy apartment blocks and makeshift dwellings fell to pieces or collapsed floor upon floor. Many nations, including the USA, Germany, Switzerland, Iran and Greece offered rescue teams and equipment. The quake measured 7,4 on the Richter scale according to the National Earthquake Information Centre in Golden, Colorado, US. Since the rescue operation, officials are concerned about epidemics, water shortages and power cuts. (adapted from articles in Cape Times, 18 and 19 August 1999) www.cambridge.org Cambridge University Press 978-0-521-68059-2 - NSSC Geography Module 1 Gerhard de Klerk Excerpt More information Module 1 Unit 1 5 TURKEY IZMIT Figure 1.5 Earthquake damage in Izmit Figure 1.6 Turkey and the affected areas The structure of an earthquake Glossary Focus (plural: foci) – the origin of the earthquake in the Earth’s crust Epicentre – the point on the Earth’s surface immediately above the focus The point in the Earth’s crust where an earthquake begins is called the focus. Earthquake foci are usually found along plate boundaries, between 70 and 700 km below the Earth’s surface. The point on the Earth’s surface immediately above the focus is called the epicentre. The earthquake is caused by seismic waves, called P and S waves, which pass through the Earth’s crust from the focus to the surface. When they reach the surface these waves become L waves which cause the most damage and this damage is concentrated at the epicentre. Find these structural features in Figure 1.7 epicentre L waves Earth‘s crust P and S waves focus Figure 1.7 Structure of an earthquake © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-68059-2 - NSSC Geography Module 1 Gerhard de Klerk Excerpt More information 6 NSSC Geography Tsunamis are huge sea waves caused by earthquakes in the sea floor. Following the earthquake, water spreads out in all directions travelling at speeds up to 800 km/h. If a tsunami reaches a coastline it can present as a wall of water between 10 and 30 metres high which can flood whole towns and cities and drown thousands of people. The Richter scale The Richter scale measures the size of the seismic waves and their speed of travel to classify the magnitude of the earthquake. The scale extends from 1 to 9. Each step in the scale represents a magnitude ten times greater than the previous step. For example, a magnitude of 4 is ten times greater than a magnitude of 3. The following examples give you some idea of the damage caused by earthquakes measured at numbers on the Richter scale: Less than 2,6 We do not usually feel this. 2,5–5,4 We can feel this, but very little damage is done. 5,5–6 Cars and houses shake. Windows crack. 6,1–6,6 Old buildings or poorly built buildings may collapse. Some damage to houses. 7–7,9 Major earthquakes. Huge damage to buildings, roads, railways, powerlines. Over 8 Total destruction. ACTIVITY 3 Answer the following questions which are based on the article on page 4. You will need to refer to the background information on earthquakes as well. 1 a Give the date and time when the Turkish earthquake occurred. b How long did the earthquake last? c Do you think the time at which the earthquake occurred could have led to a higher death toll? Explain your answer. 2 a Where was the epicentre of the earthquake? b Why was the damage worst at, or near, the epicentre? 3 a What caused most of the deaths and injuries in this earthquake? b How could there have been less damage and fewer deaths? Give evidence from the article. 4 a What magnitude was this earthquake? b Describe the scale used to measure the earthquake. © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-68059-2 - NSSC Geography Module 1 Gerhard de Klerk Excerpt More information Module 1 Unit 1 7 5 Find Turkey on the map in Figure 1.6. Is Turkey situated on or near a plate boundary? If so, name the plates on the other sides of the boundary. 6 Why would the earthquake cause water shortages and power cuts? 7 a What is an epidemic? b What types of epidemic do you think the officials were referring to? 8 a Turkey has suffered many earthquakes in the past. In view of this do you think the authorities were prepared for this earthquake? Give evidence from the case study to support your answer. b Suggest ways in which Turkey could avoid such heavy loss of life and damage in future earthquakes. Earthquake zones Look at the map in Figure 1.8 which shows the main earthquake zones of the world. ASIA NORTH AMERICA ITALY EUROPE TURKEY HIMALAYA JAPAN GREECE AFRICA PHILIPPINES Equator SOUTH AMERICA OCEANIA NEW ZEALAND KEY Earthquake activity Figure 1.8 The main earthquake zones of the world The distribution of earthquakes Earthquakes occur in zones or belts. The most important earthquake belt is around the Pacific Ocean. It follows the west coasts of the North and South Americas, the Aleutian Islands, Japan, the Philippines and New Zealand. From Indonesia, a belt stretches westward through the Himalaya Mountains in India to the Mediterranean Sea, along Turkey, Greece and Italy. Another belt stretches down East Africa and is known as the East African Rift Valley. © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-68059-2 - NSSC Geography Module 1 Gerhard de Klerk Excerpt More information 8 NSSC Geography ACTIVITY 4 Compare the map in Figure 1.8 with the map of tectonic plates in Figure 1.4 and answer the questions. 1 Describe any similarities you notice between the plate boundaries and the earthquake zones. 2 Explain these similarities. 3 Look at the table of major earthquakes. Year Place Estimated deaths 1960 Morocco 1976 China 1978 Iran 15 000 1985 Mexico 10 000 1995 Japan 12 000 500 000 5 400 Table 1 a Find each place on the maps in Figures 1.4 and 1.8. For each earthquake, name the plates on either side of the plate boundary. b List the factors which could have led to the different numbers of deaths in each of these earthquakes. 4 Study the map (Figure 1.9) below and answer the questions. Equator KEY Earthquake belts Figure 1.9 Earthquake belts a Describe the distribution of earthquake belts. b Briefly explain why earthquakes are more likely to occur in these areas. © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-68059-2 - NSSC Geography Module 1 Gerhard de Klerk Excerpt More information Module 1 Unit 1 9 Case study: The Kobe earthquake The congested old centre of the city was one big bonfire yesterday morning. Thousands of homes were shown to be at risk not just of collapse, but of catching alight. All big Japanese cities have thousands of wooden buildings which are natural firetraps. Some are old, others were built in the last 20 to 30 years. The rescue services found themselves unable to cope with the sheer size of the operation. One group of people were imprisoned in their homes, trapped and waiting for rescuers for more than 14 hours after the earthquake. The only light in the centre of Kobe yesterday came not from electricity but from 129 separate fires. Where fires died out it was because there was nothing left to burn. JAPAN Kobe. Wall of fire 300 – 500m wide. 77 000 residents evacuated to temporary rescue centres. 05:45 local time. An earthquake centred under the Island of Awajishima measures 7.2 on the Richter scale, killing 4 500 and injuring 6 334. It was the worst earthquake in Japan since 1946 when 3 769 people were killed in central Japan. Ashiya Kobe Kyoto Osaka Island of Awajishima e scal m 30 k Sea of Japan Kyoto Kobe Tokyo Nearly 10 000 houses, buildings and other structures were destroyed or partially damaged Osaka Pacific Ocean Awajishima Japan Source: British: Geological Survey Trench Thrust zone Ocean floor Eurasian plate CHINA PHILIPPINES Pacific plate Figure 1.10 Earthquake damage in Japan ACTIVITY 5 Figure 1.10 Eurasian plate Earthquake Pacific damage in Japan plate Upper mantle 1 INDONESIA Philippines plate PAPUA NEW GUINEA Figure 1.10 contains information about a major earthquake that affected an area in Japan, especially the city of Kobe, in January 1995. a b c d e What is an earthquake? What is a plate? Using Figure 1.10, explain why plate movements off the east coast of Japan sometimes cause earthquakes. Give three reasons why this earthquake caused such a great loss of life and property. What can be done to reduce the effects of earthquakes in areas where they are likely to occur? In Section C you will examine another natural event which is caused by plate movements: volcanoes. © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-68059-2 - NSSC Geography Module 1 Gerhard de Klerk Excerpt More information 10 NSSC Geography C Glossary Magma – molten rock from the mantle within the Earth’s crust Lava – molten rock which erupts onto the Earth’s surface Volcano – a landform built up of lava, ash and cinders ejected from a vent in the surface of the Earth Volcanoes When a crack or fault appears in the Earth’s crust, molten rock called magma, forces its way from the mantle up the fault and erupts on the surface of the Earth to form a volcano. When the magma reaches the surface of the Earth it is called lava. Besides lava, some volcanoes eject steam, ash, dust and poisonous gases, such as sulphur. Where are the world’s active volcanoes? Volcanic eruptions are most common along plate boundaries where movement and cracking occurs. Look at Figure 1.11 which is a map showing the world’s active volcanoes. ICELAND Mount St Helens 1980 NORTH AMERICA Surtsey 1963 Montserrat 1995 Mauna Loa 1984 Nevada del Ruiz 1985 Cotopaxi 1904 Vesuvius EUROPE AD 79 Unzen 1792 1991 ASIA Pinatubo 1991 Etna 1669 1998 AFRICA Kamchatka Peninsula JAPAN PHILIPPINES Equator Krakatoa 1883 Tambora 1815 SOUTH AMERICA OCEANIA KEY Volcano activity Volcano belts Historic volcanoes Tristan da Cunha 1961 Ruapehu 1995 NEW ZEALAND Figure 1.11 The world’s active volcanoes The distribution of volcanoes Three-quarters of the Earth’s 550 historically active volcanoes lie along a belt called the “Ring of Fire”, which runs along the edge of the Pacific Ocean. The zone stretches from Antarctica through New Zealand, the Philippines, Japan, the Kamchatka Peninsula in Russia, the Aleutian Islands, Alaska and along the west coast of Northern South America. From Indonesia a branch stretches westward to Italy, the Azores and the West Indies. The mid-Atlantic ridge, stretching from Iceland in the north to the island Tristan da Cunha in the south, is also characterised by a number of volcanoes. © Cambridge University Press www.cambridge.org