unit 1 plate tectonics, earthquakes and voicanoesassets.cambridge.org/97805216/80592/excerpt/... ·...
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Unit 1Plate tectonics,earthquakes andvoIcanoes
Look at Figure 1.1 which is a photograph of Earth taken from asatellite in space. Most of the surface of planet Earth is coveredby sea, the rest is covered by continents. Which continent can yousee in the satellite photograph? Do you think this continent willbe in the same place in a million years time?In this unit you will discover that the continents and oceans formthe Earth’s crust which is constantly moving. This movement canresult in earthquakes and volcanoes.
This unit is divided into three sections:
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 moveMillions 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 layercalled the crust. But the crust is only the thin outer layer of theEarth as you can see if you study Figure 1.2 on page 2.
Figure 1.1The Earth from space A Earth’s crust on the move
B Earthquakes
C Volcanoes
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Figure 1.2The internal structure of the Earth
Copy and complete the table below using information fromFigure 1.2.
Scientists think that the surface of the Earth is made up ofdifferent pieces of crust and not one continuous crust. The piecesof crust are called tectonic plates. Most tectonic plates carryboth continents and oceans. You will find out more about tectonicplates in the next section.
The theory of continental drift and tectonic plates
In 1912, a German meteorologist called Alfred Wegener putforward the idea that the continents were once all joined in asingle 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 thetheory of continental drift. Other scientists of the time thoughtWegener was a fool to come up with such a crazy idea!
It was only when new evidence was found in the 1950s and1960s 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 twocontinents had once been joined. See if you can identify ourpresent continents in Pangaea in Figure 1.3.
Look at the map in Figure 1.4 which shows the main tectonicplates into which the Earth’s crust is divided today. The plates
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ACTIVITY 1
Layer number Name Density Thickness Description
1
2
3
4
GlossaryTectonic plates – large sections ofthe Earth’s crust
Direction of plate movement
Figure 1.3This is what Earth probablylooked like 200 million years ago
1. The crust – thin outer layer formed of solid rocks between 6 km and 90 kmthick. This layer is the least dense.
2. The mantle – about 2 900 km thick, more dense than crust. Rocks arein 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 bean extremely dense solid metalic ball.
HintThe Earth is continually changing.Although continents neared theirpresent locations about 50 millionyears ago, they are still driftingslowly over the planet’s surface.
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move a few centimetres every year. The edges of the plates arecalled plate boundaries. The arrows show the direction ofmovement of the plates.
Use the map in Figure 1.4 to find the answers to thesequestions.
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 SouthAmerica.
a Name the plates on either side of this plate boundary.
b Are the plates moving apart or towards each other at thisplate boundary?
3 Find the plate boundary which passes through the AtlanticOcean 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 thisplate boundary?
4 In which direction is Antarctica moving?
JAPAN
PHILIPPINES
NEWZEALAND
EUROPE
AFRICA
ASIA
OCEANIA
NORTHAMERICA
SOUTHAMERICA
Antarctic Plate
Indo-AustralianPlate
AfricanPlate
NazcaPlate
NorthAmerican
PlateEurasian Plate
PacificPlate
IranianPlate
ArabianPlate
Adriatic Turkish-Hellenic Plate
SouthAmerican
Plate
PacificPlate
Direction of plate movementPlate boundaries
MID
ATLA
NTIC
RIDGE
MID
ATLAN
TICRID
GE
Figure 1.4The different tectonic plates
ACTIVITY 2
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Moving plates
It is difficult to believe that the hard, rigid tectonic plates thatmake up the Earth’s crust are moving. But remember that, belowthe crust, the mantle has a plastic consistency and can thereforemove or flow. Scientists describe the upper layer of the mantle,immediately below the plates, as a “soft layer of slush”. The rigidtectonic plates can drift on this slush. The energy released by thismovement triggers off earthquakes and volcanoes, especially alongplate boundaries. Section B deals with earthquakes in more detail.
B Earthquakes
Plate movements cause cracks or faults in the crust. The faultsproduce great stresses leading to vibrations on the surface of theEarth that we call earthquakes. The damage caused by anearthquake can be devastating – thousands of people and animalscan be killed or injured and massive damage is done to buildingsand landscapes. Read about an earthquake that took place inTurkey on 17 August 1999 in the article below.
GlossaryEarthquake – vibrations on theEarth’s surface caused by stresses inthe crust
Istanbul: The pre-dawn quakewhich rocked much of Turkeyyesterday crushed apartmentbuildings and sent millions ofpeople fleeing in panic into thestreets.
Terrified earthquake survivorssought the safety of open groundyesterday as the enormity of whathad happened became clearer in thedevastated areas they left behind:the final toll was more than 15 500dead and almost 25 000 injured.
Layer by layer, from thousands ofsites around western Turkey,hundreds of bodies were removed,many in the pyjamas andnightgowns they were wearing whenthe immense quake struck overabout 45 seconds in the cool hoursbefore dawn. Some bodies wereshrouded in blankets and sheetsfrom their beds.
As many as 10 000 people couldstill be trapped in rubble aroundGolcuk, a naval base about 130 kmsouth east of Istanbul near thequake’s epicentre.
There was severe damage insome areas of Izmit, some 90 km
east of Istanbul near the epicentreof the quake.
Fahrettin Duman, workingalongside a mechanical digger,struggled to pull fallen masonryfrom a apartment block near hisown house in Izmit. “There arearound eight people trapped in thestaircase near the entrance, buteven with this machine we can'treach them. There’s no sign of life,”he said. In Izmit, medical workersshattered pharmacy windows to getsupplies for hospitals swamped byinjured people. More than 100sailors were dead or missing from a nearby barracks.
Local authorities wereoverwhelmed by the task of clearingdebris. “Hundreds of buildingscollapsed. We need everything – hospitals, kitchens, tents andambulances,” said Nihat Ozgol,governor of the nearby town of Yalova.
In the Avcilar district on theimpoverished fringe of Istanbul, aboy cried out from the cracked slabsand splintered plaster that was oncehis home, “Help me, I’m still alive!”
As rescuers – some digging withbare hands – carted away corpsesand listened for signs of life, theyheard: “I’m here”.
A worker touched his hand. “Ohthank you,” sobbed six-year-oldMemet Arsuglou. Soon he was freeand hugging his mother.
The edges of Istanbul, such as theAvcilar district, revealed thehazards of hastily builtaccommodation for rural migrants tothe city. Shoddy apartment blocksand makeshift dwellings fell topieces or collapsed floor upon floor.
Many nations, including the USA, Germany, Switzerland, Iranand Greece offered rescue teams and equipment.
The quake measured 7,4 on the Richter scale according to the National EarthquakeInformation Centre in Golden,Colorado, US. Since the rescueoperation, officials are concernedabout epidemics, water shortagesand power cuts.
(adapted from articles in Cape Times, 18 and 19 August 1999)
Desperate families claw at rubble
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The structure of an earthquake
The point in the Earth’s crust where an earthquake begins iscalled the focus. Earthquake foci are usually found along plateboundaries, between 70 and 700 km below the Earth’s surface.The point on the Earth’s surface immediately above the focus iscalled the epicentre.
The earthquake is caused by seismic waves, called P and S waves, which pass through the Earth’s crust from the focus tothe surface. When they reach the surface these waves become L waves which cause the most damage and this damage isconcentrated at the epicentre.
Find these structural features in Figure 1.7
Figure 1.5Earthquake damage in Izmit
Figure 1.6Turkey and the affected areas
GlossaryFocus (plural: foci) – the origin ofthe earthquake in the Earth’s crustEpicentre – the point on theEarth’s surface immediately abovethe focus
epicentre
L waves
Earth‘scrust
focus
P and S waves
Figure 1.7Structure of an earthquake
TURKEY
IZMIT
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Tsunamis are huge sea waves caused by earthquakes in the seafloor. Following the earthquake, water spreads out in alldirections travelling at speeds up to 800 km/h. If a tsunamireaches a coastline it can present as a wall of water between10 and 30 metres high which can flood whole towns and cities anddrown thousands of people.
The Richter scale
The Richter scale measures the size of the seismic waves andtheir 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 amagnitude of 3. The following examples give you some idea of thedamage caused by earthquakes measured at numbers on the Richter scale:
Answer the following questions which are based on the article on page 4. You will need to refer to the background informationon 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.
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 maycollapse. Some damage to houses.
7–7,9 Major earthquakes. Huge damage to buildings,roads, railways, powerlines.
Over 8 Total destruction.
ACTIVITY 3
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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 werereferring to?
8 a Turkey has suffered many earthquakes in the past. Inview 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 earthquakezones of the world.
JAPAN
PHILIPPINES
NEWZEALAND
ITALY
GREECE
TURKEYEUROPE
AFRICA
ASIA
OCEANIA
NORTHAMERICA
SOUTHAMERICA
Equator
Earthquake activityKEY
HIMALAYA
Figure 1.8The main earthquake zones of the world
The distribution of earthquakes
Earthquakes occur in zones or belts. The most important earthquakebelt is around the Pacific Ocean. It follows the west coasts of the Northand South Americas, the Aleutian Islands, Japan, the Philippines andNew Zealand. From Indonesia, a belt stretches westward throughthe Himalaya Mountains in India to the Mediterranean Sea, alongTurkey, Greece and Italy. Another belt stretches down East Africa andis known as the East African Rift Valley.
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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.
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.
a Describe the distribution of earthquake belts.
b Briefly explain why earthquakes are more likely to occur in these areas.
ACTIVITY 4
Year Place Estimated deaths
1960
1976
1978
1985
1995
Morocco
China
Iran
Mexico
Japan
12 000
500 000
15 000
10 000
5 400
Table 1
Equator
Earthquake beltsKEY
Figure 1.9Earthquake belts
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Figure 1.10Earthquake damage in Japan
Case study: The Kobe earthquake
1 Figure 1.10 contains information about a major earthquake that affected an area in Japan, especially the city of Kobe, in January 1995.
a What is an earthquake?b What is a plate?c Using Figure 1.10, explain why plate movements off
the east coast of Japan sometimes cause earthquakes.d Give three reasons why this earthquake caused such
a great loss of life and property.e What can be done to reduce the effects of earthquakes
in areas where they are likely to occur?
JAPAN
Kobe. Wall of fire 300 – 500m wide. 77 000residents evacuated totemporary rescuecentres.
05:45 local time. An earthquake centred under the Island of Awajishimameasures 7.2 on the Richter scale, killing 4 500and injuring 6 334. It was the worst earthquake in Japan since 1946 when3 769 people were killed incentral Japan.
Kyoto
Kobe
Island of Awajishima
scale
30 km
Ashiya
Osaka
Kyoto
Tokyo
Osaka
Kobe
Nearly 10 000houses, buildingsand other structures weredestroyed or partially damaged
Sea of Japan
Pacific Ocean
Source: British: Geological Survey
Trench
Oceanfloor
Eurasianplate
Pacific plate
Upper mantle
Thrust zoneEurasianplate
Philippines plate
Pacific plate
PHILIPPINES
CHINA
INDONESIA
PAPUA NEW GUINEA
AwajishimaJapan
In Section C you will examine another natural event which iscaused by plate movements: volcanoes.
Figure 1.10Earthquake damage in Japan
The congested old centre of the city was one big bonfireyesterday morning. Thousands ofhomes were shown to be at risknot just of collapse, but of catching alight.
All big Japanese cities havethousands of wooden buildingswhich are natural firetraps. Someare old, others were built in thelast 20 to 30 years.
The rescue services foundthemselves unable to cope withthe sheer size of the operation.One group of people were imprisoned in their homes,trapped and waiting for rescuersfor more than 14 hours after the earthquake.
The only light in the centre ofKobe yesterday came not fromelectricity but from 129 separatefires. Where fires died out it wasbecause there was nothing left to burn.
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C Volcanoes
When a crack or fault appears in the Earth’s crust, molten rockcalled magma, forces its way from the mantle up the fault anderupts on the surface of the Earth to form a volcano. When themagma reaches the surface of the Earth it is called lava. Besideslava, 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 boundarieswhere movement and cracking occurs.
Look at Figure 1.11 which is a map showing the world’s active volcanoes.
The distribution of volcanoes
Three-quarters of the Earth’s 550 historically active volcanoes liealong a belt called the “Ring of Fire”, which runs along the edge ofthe Pacific Ocean. The zone stretches from Antarctica throughNew Zealand, the Philippines, Japan, the Kamchatka Peninsulain Russia, the Aleutian Islands, Alaska and along the west coastof Northern South America. From Indonesia a branch stretcheswestward to Italy, the Azores and the West Indies. The mid-Atlantic ridge, stretching from Iceland in the north to theisland Tristan da Cunha in the south, is also characterised by anumber of volcanoes.
Figure 1.11The world’s active volcanoes
JAPAN
PHILIPPINES
NEWZEALAND
ICELAND
EUROPE
AFRICA
ASIA
OCEANIA
NORTHAMERICA
SOUTHAMERICA
Tristan da Cunha1961
Etna16691998
VesuviusAD 79
Montserrat1995
Cotopaxi1904
Mauna Loa1984
Mount St Helens1980
Nevada del Ruiz1985
Surtsey1963
Unzen17921991
Pinatubo1991
Tambora1815
Ruapehu1995
Krakatoa1883
Volcano activityVolcano beltsHistoric volcanoes
KEY
Equator
KamchatkaPeninsula
GlossaryMagma – molten rock from themantle within the Earth’s crust Lava – molten rock which eruptsonto the Earth’s surface Volcano – a landform built up oflava, ash and cinders ejected from avent in the surface of the Earth
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