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A flat Earth8.1

If you look from the top of a mountain towards adistant horizon the Earth looks flat. For manyyears, people thought that the Earth was flat.Some people believed that if you travelled farenough you would fall off the edge.

Many people collected information to showthat the Earth was round. Ships sailing out to seadisappear below the horizon, and the Earth’sshadow on the Moon during an eclipse is curved.Today we can see the whole Earth in images (pic-tures) from satellites.

In the 1400s traders from Europe travelled toChina by walking east. The most famous of these

was Marco Polo. Some traders thought that theyshould be able to sail west and reach China bysea. The first person to sail west in search ofChina was Christopher Columbus. After seventydays at sea Columbus reached America. Columbus never got to China, but his idea wascorrect. The world was round, and you could sailaround it. The first ships to sail around the worldand return to Europe were led by the explorerFerdinand Magellan. This journey took almostthree years.

Traders sailed to distant ports and returnedwith valuable products. In order to sail directly totheir destination they needed a good map, and away of measuring their position. Position can bemeasured with longitude and latitude. Longitudeis the east–west position around the Earth, andlatitude is the distance above and below the equa-tor. Latitude and longitude are shown by lines ona map. Position can be shown on a map. Butthere is a problem with maps: the Earth is roundand a map is flat. How can you flatten the Earthand still have size and direction accurate?

There are several ways of drawing maps. Theyall stretch part of the Earth while leaving otherparts at an accurate size. One way of reducing thedistortions (changes in shape) is by using gores.Gores are segments of the Earth’s surface, cut sothat they lay flat, but which can be joined to forma spherical Earth. A gore is not much use as a mapsince there are gaps between each of the gores.

The type of map most favoured is called theMercator Projection, or cylindrical projection. Itwas devised by Gerardus Mercator in 1552. Thismap shows the Earth as a rectangle, with thepolar regions exaggerated in size. (These regionshave been stretched in size to fill the gaps inbetween the gores.) This type of map is usedbecause the direction on the surface of the Earthis exactly the same as the direction on the map.The latitude and longitude lines on a Mercatormap cross each other at right angles, just like theydo on the Earth. This makes navigation easy.

A Mercator projection, and other types ofmaps, are shown in the illustrations on page 201.

An early view of the Earth

The curvature of the Earth can be seen clearly from space

THE RESTLESS EARTH 201

In recent times Peters’ projection has replacedMercator’s projection. It was created by ArnoPeters in 1974. Peters’ projection gives a betterrepresentation because areas of land are shown ina correct relation to each other; although shapesbecome distorted.

Various flat surface representations of the world

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T AIM: To make a model of a roundEarthThink about this experiment. Cut, vertically, anorange into halves, then quarters, then eighths.Peel the skin off each segment of the orange.Theskin can be pieced back together to make a hollow orange. Each segment of skin can be made to go flat.

If there had been a picture covering half of theorange skin, how could we have made it flat?Would this picture be distorted in the flatteningprocess? By cutting the orange into segments wecan flatten the skin on each segment more easilythan half an orange.The picture, even though it iscut into strips, would have fewer distortions.

A model of the Earth can be made in a similarway. Your teacher will give you a photocopy ofthe gores of the Earth. Cut them out and tapethem together to make a model of the Earth.

COPY AND COMPLETE

The type of ___ most favoured is called the ________ Projection, or ___________ projection. This map shows the_____ as a _________, with the _____ regions exaggerated in ____. This type of ___ is used because the_________ on the _______ of the Earth is _______ the same as the _________ on the ___. The ________ and_________ lines on a Mercator map cross each other at ______ ______, just like they do on the _____.

QUESTIONS

1 List the evidence to show that the Earth is not flat.

2 What are some problems with drawing a flat map of acurved Earth?

3 Most maps of the Earth are Mercator Projections.a Why are they given this name?b Why are they also called cylindrical projections?

c What is one disadvantage of these maps?d What is one advantage of these maps?

4 If you had a logo printed on the side of a softball,how could you get an accurate view of it drawn ontoa sheet of paper?

5 A Mercator Projection map of the Earth exaggeratesthe polar regions. In which sense are theyexaggerated? Is it north to south or east to west, or a combination of each? Explain.

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Continental drift8.2

As people explored the Earth and made maps,they noticed that some of the continents have thesame outlines. It was as if they were part of agiant jigsaw. Such ideas had been discussed forhundreds of years.

Alfred Wegener (pronounced Vague-nerr) col-lected evidence and proposed the theory of continental drift in 1915. He proposed that thecontinents were once joined together but hadmoved apart. When Wegener cut out maps of thecontinents and placed them side by side, manyfeatures lined up. Firstly their shapes were a closematch. There were eroded mountain ranges ondifferent continents, which matched up. So diddeposits of iron ore and other minerals. Patternsof scouring by glaciers were also similar.

There was other evidence. Ancient coral reefswere found in cold regions, where it was too coldfor coral to live. Fossils more than 150 millionyears old were similar on different continents,which implied that the continents were joined.Fossils younger than 150 million years were dif-ferent on different continents.

In his theory of continental drift, Wegener pro-posed that all continents had once been joinedtogether into one vast land mass that he calledPangaea (pan-jee-ah, which means ‘all Earth’.)About 250 million years ago Pangaea broke intotwo parts. Laurasia (Law-rasia) consisted of thenorthern continents of North America, Europeand Asia. Gondwana (Gond-wah-nah, sometimescalled Gondwanaland) consisted of the southerncontinents: South America, Africa, Antarctica,India and Australia.

Wegener believed that his idea was correct. Hedescribed his work in matching the continents aslike joining torn pieces of newspaper by matchingthe writing at their edges.

In spite of Wegener’s enthusiasm and the evi-dence he had collected, most people did notbelieve that continental drift was possible.Continents are thousands of kilometres acrossand they sit deep in the solid rock of the crust andmantle. Where would the energy come from tomove them?

To counter the idea of continental drift, peopleused the knowledge and theories of the time. Theidea of land bridges was popular. They suggestedthat strips of land, called land bridges, once joinedthe continents. These would allow animals tomove from continent to continent, and the seedsof plants to be blown or carried to new lands. Theland bridges, they said, were later drowned by rising sea levels or eroded away.

Land bridges have existed in the past. Australiawas once joined by land to Tasmania. This is whymany similar plants and animals live in Victoriaand Tasmania. With a drop in sea level therewould again be a land bridge between Australiaand New Guinea.

When you cut out the continents and jointhem together, they do fit, but the fit is not verygood. There are several reasons for this. Firstly,there has been a lot of erosion and deposition inthe 100–150 million years since the continentsseparated. This has changed their shape.

60°N

0°

30°NLaurasia

60°S

Pangaea

Gondwana30°Scontinental shelf

abyssal plain, lies underthe deep ocean

Pangaea was a large land mass composed of Laurasia and Gondwana

The continental shelf could link land masses if the sea level was lowered


Secondly, each continent is surrounded by a continental shelf. This is a part of the continent,nearly as high as the land, which is under the sea.Small changes in sea level could raise or flood

large areas of continental shelf. Torres Strait andBass Strait could become land bridges if the sealevel were to fall.

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T AIM: To make amodel ofGondwanaYour teacher will photocopy for you acopy of the continentcut-outs. Each continent has listed onit the information thatwas known toWegener. Cut out thecontinents and piecethem together, like ajigsaw, to makeGondwana. Make surethat all the evidencematches up.

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In his theory of ___________ drift, _______ proposed that all continents had once been ______ together into onevast ____ ____ that he called _______. About 250 million years ago Pangaea broke into ___ parts. Laurasiaconsisted of the ________ continents. Gondwana consisted of the ________ continents.To counter the idea of ___________ _____, people used the _________ and ________ of the time. They suggestedthat ____ bridges once ______ the __________.Each continent is surrounded by a ___________ _____. This is a ______ of the continent, nearly as high as the____, which is _____ the sea. Small changes in ___ _____ could raise or _____ large areas of continental shelf.

QUESTIONS

1 List the evidence known to Wegener for continentaldrift.

2 What were Gondwana, Pangaea and Laurasia?

3 Why was the theory of continental drift not acceptedin Wegener’s time?

4 What is a land bridge? How can land bridges explainthe distribution of animals on different continents?

5 What is a continental shelf?

6 When you cut out maps of the continents and try tojoin them together, they do not match up perfectly.Suggest three reasons for this. (Two reasons havebeen listed in this activity, one reason in theprevious activity.)

7 Antarctic explorers have found evidence of ancientglaciation, fossils of glossopteris and animals, androck formations that match up with Australia andSouth Africa. If this had been known to Wegener,how would it have affected his theory? Why?

Location of Glossopteris leaf fossils

Sea-floor spreading8.3

Wegener’s theory of continental drift was ignoredby most people. In spite of the evidence he collected, there was nothing known that couldmake continents move.

During the Second World War many newinstruments and technologies were developedand refined. Sonar and depth sounders, whichwere greatly improved during the Second WorldWar, enabled ships to detect submarines. Sonarcould also measure the depth of the oceanbeneath the ship.

Another new technology, called palaeo-magnetism, allowed people to measure the mag-netism preserved in the rocks. When a rock suchas basalt is molten, the magnetic minerals facetowards the Earth’s north pole. When the rocksolidifies the minerals are locked into this posi-tion. Palaeomagnetism provided more evidencefor continental drift, since the magnetic mineralsin the continents no longer pointed north.

The American Harry Hess was in the US Navyin the Second World War. He used a depthsounder to map the ocean floor in the Pacific. Theshape of the bottom of the oceans surprisedeveryone. Each ocean has a huge double-ridgedmountain range running along its length. Thereare also submerged mountains and volcanoes,which are named seamounts. Later people dis-covered deep trenches, active volcanoes andvents. Vents released minerals from inside the Earth.

In 1962, Harry Hess proposed the idea of sea-floor spreading. The huge mountain ranges downthe middle of each ocean were called mid-oceanridges. The idea of sea-floor spreading is that new

crust is being made in the mid-ocean ridges. Halfof the new crust goes to each side of the ridge.The growing ridge pushes the continents apart.The rock needed to make the new crust comesfrom the mantle.

In the 1970s, a specially built ship, GlomarChallenger, was used to drill into the sea floor atdifferent locations in the ocean. Some importantdiscoveries were made that supported sea-floorspreading:■ the basalt rock underlying the ocean is

youngest at the ridge, and gets older thefurther away it is from the ridge.

■ mid-ocean sediments are thinnest at the ridgeand are thicker further away from the ridge.

■ the magnetism in the rocks reverses regularly.This is known because the magnetic mineralsin the basalt face north, then south, inadjacent rock; then north again, then south,and so on.

Magnetism around the mid-ocean ridge isshown in a zebra diagram. Normal magnetism isshown as being black, and reversed magnetism iswhite. The zebra stripes are symmetrical each sideof, and parallel to, the mid-ocean ridge.

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mid-ocean ridgeocean sediments

hot rock from mantle

ocean level

1 2 3123

0

age in millions of years

ocean

marine sediments

basalt

mid-ocean ridge

New crust is formed in the mid-ocean ridges Scientists can determine the age of basalt and marine sediments.This helps to verify the idea of sea-floor spreading

A zebra diagram shows magnetism around the mid-ocean ridge


The age of the basalt, the marine sediments andpalaeomagnetism verify the idea of sea-floorspreading.

Scientists now have much better techniques forstudying the Earth and the oceans that cover it.

Satellites in particular have improved our knowledge and understanding of the oceans, bymeasuring water temperature, wave height, current speed and direction, and ocean depth.

Sea floor topography map. The yellow areas indicate mid-ocean ridges

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Each ocean has a huge ______-______ mountain range running along its length. There are also submerged_________ and _________. Later people discovered deep ________, active _________ and _____.In 1962 Harry Hess proposed the idea of ___-_____ _________. The idea of sea-floor spreading is that ___ _____ isbeing made in the ___-_____ ______. The _______ ridge pushes the __________ apart. The ____ needed to makethe ___ crust comes from the ______.In the 1970s, a specially built ____ was used to _____ into the sea _____ at different _________ in the _____. Theage of the ______, the ______ _________ and _______________ verify the ____ of sea-floor spreading.

QUESTIONS

1 Why did Wegener not know about sea-floorspreading?

2 Which new technologies made the discovery andverification of sea-floor spreading possible?

3 Use the drawing of normal magnetism and reversedmagnetism to locate the mid-ocean ridge. Explainyour reasons.

4 The word palaeomagnetism is made of two parts. a What are they?b What is the meaning of each?

5 List, in point form, the evidence for sea-floorspreading.

6 Explain how the evidence listed in question 4supports the theory of sea-floor spreading.

7 Describe how sea-floor spreading supports Wegener’stheory of continental drift.

A B C D E F G H

normalmagnetism

reversedmagnetism

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Plate tectonics8.4

Plate tectonics theory explains continental driftand sea-floor spreading. Tectonic means building.This theory shows how movements in the crusthave shaped the Earth that we know. Plate tec-tonics explains many other observations, including:■ deep sea trenches■ the locations of active volcanoes■ the locations of most earthquakes■ the formation of mountain ranges■ why the Earth’s tallest mountains are made of

rocks which formed under the sea.Plate tectonic theory pictures the Earth covered

with rigid slabs of rock called plates. These platesmove across the Earth’s surface. At the edges ofthe plates are mid-ocean ridges, deep-sea trenches and fault zones. Crust is made at themid-ocean ridges and an equal amount is consumed at the trenches. Rates of movement ofthe plates vary from 2 cm up to 10 cm a year fordifferent plates.

Studies of the sea floor show that the Earth isbroken into 13 major plates, with some smallerplates. The major plates are named after the continent that sits on them. (In July 1995, a newtype of plate boundary was recognised that sepa-rates the Australian plate from the Indian plate.)

New crust is made where plates are movingaway from each other. These plates are said to bediverging. The separation happens at mid-oceanridges. It is only in Iceland that the ridge is abovesea level, and people can watch it change. Innorth Iceland there was no change for 100 years.Then between 1975 and 1981 volcanic activitywidened an 80 km length of the ridge by 5 m. Assmall sections of the ridge are widened, transformfaults are made. The stop–start movement alongthe transform faults causes earthquakes.

Plates that are moving together are called col-liding or converging plates. Sometimes one plateis pushed under the other plate making a subduc-tion zone.

PACIFICPLATE

JUAN DE FUCAPLATE

NORTH AMERICANPLATEEURASIAN

PLATEEURASIAN

PLATE

AUSTRALIANPLATE

AUSTRALIANPLATE

SOUTH AMERICANPLATE

NAZCAPLATE

SCOTIA PLATE

EQUATOR

INDIANPLATE

ANTARCTICPLATE

AFRICANPLATE

CARIBBEANPLATE

COCOSPLATE

ARABIANPLATE

PHILIPPINEPLATE

new crust moves in opposite directions

A transform fault

Tectonic plates of the Earth


Crust is being destroyed at subduction zones.One plate is being subducted, pushed under, theother plate. Usually the thin ocean plate is pushedunder the thick continental plate. As one plate ispushed deeper into the mantle it melts and themolten less-dense rock rises to the surface in aline of volcanoes. A deep-sea trench forms whereone plate is subducted under the other.

Sometimes plates collide and produce foldmountains, rather than volcanic mountains.When the Indian plate crashed into the Asianplate, the marine sediments between them weresquashed, crinkled and pushed upwards. These

are the Himalayan Mountains, the highest in theworld. The many earthquakes in this regionremind us that India is still moving north andpushing the Himalayas higher and higher.Tourists can buy fossils of tropical shells from themountains high above the snow line.

The coast of California illustrates the third typeof plate boundary. Here two plates slide past eachother. This is explained further in section 8.5.

sediments are washed into thesea between the continents

the sediments are compressed andpushed upwards to form mountainsocean plate

molten rockfrom plate

line of volcanoes

continental plate

lithosphere(crust andtop ofmantle)

asthenosphere

A subduction zone

Plates collide to form fold mountains

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Plate tectonics ______ explains ___________ _____ and ___-_____ spreading._____ ________ theory pictures the _____ covered with _____ _____ of rock called ______. These plates ____across the Earth’s _______. At the _____ of the ______ are mid-_____ ______, deep-___ ________ and _____zones. Crust is ____ at the ___-_____ ridges and an _____ amount is ________ at the ________.

QUESTIONS

1 What does ‘plate tectonics’ mean?

2 Explain transform faults and subduction zones.

3 What is the difference between fold mountains andvolcanic mountains?

4 What evidence is there that the Earth is really madeof moving plates?

5 The diagram above shows the plates of the Earth andtheir directions of movement. Copy the diagram intoyour notebook and answer the following questions.a What makes the mantle convection cells operate?b What is happening at the mid-ocean ridge?c What is meant by continental crust and oceanic crust?d What happens to the oceanic crust when it is

pushed under the continental crust?

e On your diagram, show where the plates areconverging and diverging.

f Where is the oceanic crust when it is pushed underthe continental crust?

g Give an example in the world where you would finda mid-ocean ridge.

h Give an example in the world where you would finda subduction zone.

continentalcrust

continentalcrust

ocean

mid-oceanridge mantle

volcano oceaniccrust

Plate tectonics is a continuous process. It is hap-pening even now. The Australian Plate is movingnorth at 4 cm each year. It is being pushed by themid-ocean ridge in the Southern Ocean betweenAustralia and Antarctica.

The Pacific Plate is moving north relative to theNorth American Plate. This grinding action causes the earthquakes that are a well known feature of the San Andreas Fault. The photographshows the San Andreas Fault where it crossesland.

Modern continents are breaking apart. TheGreat Rift Valley in east Africa is the beginning ofthe split of Africa into two new continents. In theRed Sea and the Gulf of Aden, that separation ishappening. It shows clearly on maps and in satellite photographs.

There must be something driving plate tecton-ics. What moves the continents across the surfaceof the Earth?

The plates are made of solid rock. But they‘float’ on the mantle. The mantle rocks are semi-molten, and are said to be plastic. They can movelike plasticine. They move slowly with time.

The Earth has a rigid surface layer called thelithosphere. It includes the crust and the top ofthe mantle. It is thicker under the continents and thinner under the oceans. Under the lithosphere is the asthenosphere. This layer is made of partially melted rock and it moves slowly withtime. The rest of the mantle is called the mesosphere, and is more solid.

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What drives plate tectonics?8.5

The San Andreas fault

plate tectonicview of Earth

conventionalview of Earth

lithosphererigid and firm

asthenospheresoft and pliable

crust

mantle(extends furtherinto Earth)

Conventional and plate tectonic views of Earth

The Red Sea and the Gulf of Aden

Great Rift Valley in east Africa


Uneven heating inside the Earth causes con-vection currents in the asthenosphere.Convection currents carry the heat from itssource to cooler places. Convection currents canbe seen in soup being heated on a hot plate. Thefroth is pushed to the edge of the saucepan by the current.

Like the soup, the asthenosphere has con-vection currents. The moving rock in theasthenosphere drags the bottom of the litho-sphere across the surface of the Earth. This movesthe plates.

There are several theories about the source ofthe heat inside the Earth. It is thought it mayhave come from: ■ heat still left in the Earth; ■ heat due to pressure or squashing; and ■ decay of radioactive atoms.

Some geologists believe that the heat producedshould be uniform, and that convection currents(caused by uneven heating) do not exist. Theypropose that the plates slide ‘downhill’ by theforce of gravity. The section of plate at the mid-ocean ridge is higher than the section of plate atthe subduction zone. So the plate should slidefrom the mid-ocean ridge downwards to the sub-duction zone. In addition, the rock at the mid-ocean ridge is hot and not as dense as the cooler,denser rock near the subduction zone. The heavyrock at the subduction zone tends to sink anddrag the rest of the plate behind it.

These are just two of the ideas used to explain platemovement. Convection currents remain the mostpopular theory at the moment. New discoveries inthe future may change currently accepted theories.

Heat is carried awayin convection currents.Froth collects at edgeof saucepan.

lithosphere

asthenosphere

cool zonein mantle

cool zonein mantle

hot zonein mantle

Convection currents in a saucepan

Zones of heat inside the Earth

COPY AND COMPLETE

The Earth has a rigid _______ layer called the ___________. Under the lithosphere is the _____________. This layeris made of _________ melted rock and it _____ slowly.The asthenosphere has __________ ________. The ______ rock in the _____________ drags the bottom of the___________ across the _______ of the Earth. This moves the ______.

QUESTIONS

1 What is the most popular theory to explain whatmoves the tectonic plates? Include details in yourexplanation.

2 Explain how convection currents form in a saucepanof soup being heated by a single heat source.

3 Explain two other theories for plate movement.

4 What is meant by ‘plastic’ rocks?

5 Why do geologists not accept one theory as beingtrue?

6 Is the analogy of soup to asthenosphere a valid one?Why or why not?

7 How might current theories change in the future, asnew discoveries are made?

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How theories develop8.6

60°N

0°

30°N

60°S

30°S

225 million years before present

(a)

60°N

0°

30°N

60°S

30°S

(b)


60°N

0°

30°N

60°S

30°S

(c)


60°N

0°

30°N

60°S

30°S

(d)


60°N

0°

30°N

60°S

30°S

(e)

Present

Pangaea

Gondwanaland

Laurasia

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QUESTIONS

1 Why do people believe that the theory of platetectonics is the best available?

2 What would happen if someone discovered somethingabout the Earth that could not be explained by theplate tectonic theory?

3 Why are theories always being tested and checked?

4 Plate technology theory is a development of thecontinental drift theory.a Explain the hypothesis and observations that led

to these theories.

b What would happen if observations were madethat conflicted with these theories?

5 The break-up of Gondwana occurred in a series ofstages. The continents we know broke away atdifferent times.a Use an atlas to name each continent. On the maps

above, each is shown in a different colour.b Use the diagrams to identify the order in which

the continents separated.c What evidence might have been used to construct

these maps? What further evidence would you lookfor to confirm or refute this sequence?

Our knowledge of the Earth, gathered by geologists andpalaeontologists, helps people understand and explain thefeatures of the Earth.

The theory of plate tectonics can explain the continents,oceans, magnetism, fossils, earthquakes and volcanoes,and much more. It is the result of the work of Wegener,Hess and countless other scientists who added informationor checked what had been discovered. Note that science isa team activity; each person adds to the information foundby someone before them.

All scientific theories develop in a similar manner.Someone proposes a hypothesis to explain some observa-tions or ideas that they have. Data is collected to supportthe hypothesis. Trials or experiments may be conducted.Data is collected and analysed. The theory is modified andimproved by many people working on the same problem.New technology and inventions are made, and these areapplied to the hypothesis. When all the evidence and testssupport the hypothesis it is then called a theory.

Plate tectonics is currently the best theory available toexplain the shape and structures on the Earth. If observ-ations are made that conflict with this theory, then it willbe modified or discarded in favour of a theory that bettermatches the known facts and observations.

The plate tectonic theory explains the recent history ofthe Earth. Its major aspects have been explained in thisbook.

The series of drawings in the column opposite show howthe location of the continents has changed. Evidence fromsea floor spreading, palaeomagnetism, fossils, radioactivedating and other sources have helped in preparing thesemaps.

The location of continents has changed over millions of years


Hot spotsAnother way of checking to see if a theory‘works’, is to apply it to new situations.

Most volcanoes are found at plate boundaries. But some volcanoes occur inthe middle of the plates. They are thoughtto occur due to mantle plumes, or ‘hotspots’, in the asthenosphere. A mantleplume is a region of greater than normalheat. Heat applied at one spot could melt ahole in the plate above it. This would produce a volcano. If the plate is movingand the hot spot is still, then there would bea long thin volcano or a line of volcanoes.The Pacific tectonic plate is moving over ahot spot in the mantle, which melts thecrust and results in the formation of volcanoes. The volcanoes of the HawaiianIslands are an example of this activity.

The map shows volcanic sites in easternAustralia and their age. These are thoughtto have been caused by a hot spot in theasthenosphere. This information can beused to support or refute (show to bewrong) the idea of hot spots and platetectonics.

?

Hillsborough33.2 million years

Nebo31.0 million years

Peak Range31.0 million years

Glass Houses25.8 million years

Main Range24.2 million years

Tweed (Mt Warning)22.7 million years

Ebor18.7 million years

Comboyne16.3 million years

Springsure27.3 million years

Nandewar18.3 million years

Canobolas12.0 million years

Western Victoria3.5 million years

Note: Ages are of the approximateoldest rocks found associatedwith each volcano

Current hot spot location

1000 km

Warrumbungle15.0 million years

Volcanic sites in eastern Australia

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QUESTIONS

Study the map and answer these questions. You will need aruler and calculator.

1 What is the trend in age and location of volcanicfeatures in eastern Australia?

2 In which direction has the Australian plate beenmoving?

3 Use the scale to confirm that the distance between thePeak Range and the Warrumbungle Mountains is 1000km. Convert this distance into centimetres. (Multiplyby 1000 to convert kilometres into metres, thenmultiply by 100 to convert metres into centimetres.)

4 Subtract the age of the Peak Range volcanic rocks fromthe age of the Warrumbungle rocks.

5 Calculate the speed of the plate over the hot spot thatcaused these volcanic events. Speed is distance dividedby time. Calculations taken in other places put the speed of the Australian Plate at 4 cm per year. Are these results more or less similar? Does thissupport plate tectonic theory? (We have made someassumptions that may or may not be true. For example,we assumed that the hot spot was not moving.)

EXTENSION6 Has the direction of motion of the Australian plate

changed in the past 30 million years?

7 Calculate the speed of the plate from 20 to 30 millionyears ago, and from 12 to 25 million years ago. Hasthe speed of the plate changed?

8 Estimate where a volcano might erupt at the presenttime.

HOW THEORIES DEVELOP 9 Rearrange these steps in developing a scientific theory:

■ trials or experiments test the hypothesis

■ hypothesis proposed to explain observations or ideas

■ new technology and inventions applied to confirmthe theory

■ idea and explanation becomes a theory

Science is a team activity. Use the theory of platetectonics to show how:a people cooperate and work in teams to make new

discoveries b people use evidence and information that was

collected by previous generations or earlierresearchers.

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Earthquake!8.7

Around the world there are many earthquakesevery day. Most are small and not noticed. A feware very destructive. What causes earthquakes?

People have plotted on maps of the Earth thelocations of earthquakes. One such map is shownabove.

Most earthquakes occur on or near plateboundaries. This is where the plates are movingpast each other. The plates are made of hard solidrock. As the plates move past each other the edgerocks will catch on each other. The plates keep onmoving and the rocks become strained. They con-tain a lot of elastic potential energy. When therock reaches its breaking point all that energy isreleased. The breaking of rocks and the release ofenergy is the earthquake. The energy travels as awave motion that shakes the Earth as it passes.The shaking of the Earth due to waves causesdamage far away. Waves that travel through theEarth are called seismic waves.

The place on the Earth directly above where anearthquake occurred is called the epicentre(‘eppy-centre’). This is drawn on a map andreported in news bulletins. More interesting togeologists is the focus. The focus is the exact point

where an earthquake occurred. The focus may bemany kilometres underground.

The waves that travel out from an earthquakecause the Earth to shake. The shaking, due to theenergy in the waves, causes damage to buildings.

Earthquake locations around the world

epicentre

focus

surface of Earth

inside Earth

frame moves

wire flexes

this heavy weightdoes not move

pen

base

bedrockEarth moves

horizontal motion

base moves rotating drum

The focus of an earthquake

A seismograph


After the shock of the initial earthquake theremay be a series of aftershocks. These smallerearthquakes occur as the rocks surrounding theinitial breaks move and settle into their new posi-tions. Aftershocks often occur for a few days afterthe main shock.

The shaking is detected using a seismograph.The chart it records is called a seismogram (‘size-mo-gram’). At seismological recording stationsthree seismographs are set up at right angles toeach other. This allows them to record all themotion of the Earth.

Earthquakes cause a lot of damage to buildingsand other structures. They also cause landslides

on land and under the sea. Undersea landslidescan push away large amounts of water. This surgeof water is called a tsunami (‘sue-narm-ee’), or atidal wave. Tidal wave is a bad name as tsunamishave nothing to do with tides. Tsunamis are barely noticeable in the ocean, but they rise up atthe coast and can wash inland for many kilometres.

The strength of an earthquake is measured indifferent ways. The Mercalli Scale measures theeffect of an earthquake. The Richter Scale mea-sures the power of an earthquake.

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T AIM: To make a seismographHang a kilogram or similar mass from a clamp heldon a retort stand.Tape a felt pen to the side ofthe mass, and tape some paper onto the base ofthe retort stand.Adjust the pen orstring so that thepen writes on thepaper. Put theapparatus, a simpleseismograph, on astool and leave ituntil it stopsswinging.

Bump the stool gently.The retort stand movesbut the mass stays still. But it looks like the mass isswinging.The pen leaves lines on the paper.The

direction of theline shows thedirection ofmovement of thestool.

Set the massswinging withabout a 5 cmswing. Drag asheet of paperunder the penwhile it is swinging. (The movement of the penand paper should be at right angles to each other.)What is the shape of the line left on the paper?Explain its shape.

bosshead and clamp

kilogram mass withfelt pen taped toits side

sheet of paperto recordseismogram

kilogram massand pen swingingside to side

pull paper atright angles tothe swinging mass

A model seismograph

Observing the seismogram

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Most earthquakes occur on or near _____ __________. The plates keep on ______ and the rocks become ________.The ________ of rocks and the _______ of ______ is the __________. The ______ travels as a ____ motion. Wavesthat travel _______ the Earth are called _______ waves.The _____ on the Earth directly _____ where an __________ occurred is called the _________. The _____ is the_____ point where an earthquake ________.The _____ that travel out from an __________ cause the Earth to _____. The shaking is ________ using a___________. The _____ it records is called a __________.Undersea __________ can ____ ____ large amounts of _____. This _____ of _____ is called a _______.

QUESTIONS

1 What is the meaning of the following words: seismicwave, epicentre, focus and tsunami?

2 Why do earthquakes damage buildings? Why canearthquakes damage buildings hundreds of kilometresaway?

3 What is the difference between seismographs andseismograms?

4 An earthquake releases large amounts of energy. Where does the elastic potential energy come from?

5 Earthquakes can be explained by the ‘grip and slip’theory. Explain this idea.

6 Why are seismographs set up in a group of threealigned in three different directions? What are thesedirections?

7 What are aftershocks? Are they as severe as the initialearthquake?

214

Finding the epicentre8.8

When an earthquake strikes, the ground shakes.A record of the shaking is recorded on a seismo-gram. By looking at different seismograms, geologists can determine where the epicentre waslocated.

The most common seismogram is drawn on acylinder which rotates once each hour. The penrecords progressively lower on the paper. Whenthe recording is finished it is cut and opened out.The 24 lines each represent one hour of the day.The bumps at the top and bottom are for check-ing the seismograph.

When the reflected waves and aftershocks aretaken away, the earthquake waves look like this.

All seismograms have this shape. The fastestwaves to leave the earthquake reach the seismo-graph first. These are the primary waves, or Pwaves. These push and pull the Earth as theytravel through it. Their speed in the Earth isknown.

The second waves to arrive are the secondarywaves, or S waves. These are the shear waves thatmove the ground up and down as they pass

through it. These waves travel slower than P waves, and their speed is also known.

The last waves to arrive are the long waves, orL waves. These waves travel around the surface ofthe Earth. They are a complex mixture of wavetypes. These are the waves that cause most of thedamage in earthquakes.

Actual seismograms are more complicated thanseparate P, S and L waves. P and S waves can travel directly to the seismograph and arrivequickly. These waves can also be reflected off layers in the Earth and arrive later. (They havetravelled further.) An actual seismogram is a combination of all these waves arriving at different times.

If the exact time of an earthquake is known, itsdistance (but not direction) can be found by mea-suring how long it takes the P and S waves toarrive. If the time of the earthquake is notknown, then the difference in arrival time of theP and S waves can be used to find the distance.The distance from three recording stations mustbe known to give an accurate position of the epicentre and time of the earthquake.

Australia is not near any plate boundaries butstill has earthquakes. These are called intra-plateearthquakes and are difficult to predict. A map ofthe epicentres of earthquakes is shown on the following page.

P S L

time

A seismogram

Earth moves up and down as S waves pass

Earthquake waves

Earth moves backwards and forwards as the P waves pass

A seismograph


Tennant Creek 7.21988

Alice Springs

Darwin

Newcastle 5.51989

Brisbane

SydneyPicton 5.5 1973

Hobart

Warrnambool 5.51903

Beachport 6.61897

Perth Meckering 6.91968

Cadoux 6.2 1979

CanberraMelbourne

Adelaide5.7 1954

Meeberrie 7.01941

magnitude < 5.0magnitude 5.0–5.9magnitude > 5.9

Bundaberg 6.21918

The magnitude of earthquakes in Australia

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The fastest waves to leave the __________ reach the ___________ first. These are the _______ waves, or _ waves.The ______ waves to arrive are the _________ waves, or S waves.The last waves to arrive are the ____ waves, or _ waves. These waves travel around the _______ of the _____. Theyare a _______ mixture of wave _____. These are the _____ that cause most of the ______ in ___________.The distance from _____ recording stations must be known to give an ________ position of the _________ and timeof the __________.

QUESTIONS

1 What are the three types of earthquake waves? Whichtype is quickest? Which type causes the mostdamage?

2 Draw a table showing the characteristics of P, S andL waves. Use the following headings: Name of wave,How it shakes the Earth, Order of arrival atseismograph.

3 Use your knowledge from the experiment to explainhow the time and location of an earthquake can bedetermined from seismograms.

4 Why does an actual seismogram show manyvibrations when there are only three types ofearthquake waves?

5 Why are there seismographs located in many parts ofAustralia?

6 At the seismograph recording station, there is alwaysmore than one seismograph. They may be orientatedeast-west, north-south or up-down. Why areseismographs located at different angles to eachother?

216 THE RESTLESS EARTHE

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of an earthquakeDrawn below are the seismograms of an

imaginary earthquake that occurred in New SouthWales.The location of the earthquake is notknown, so the difference in arrival times of the Pand S waves has to be used to find the distance.

The time of arrival of the P and S waves isshown on the seismograms.Time is recorded inhours, minutes and seconds. Follow theinstructions to find where and when theearthquake occurred. Write the results neatly inyour notebook.Your teacher will give you aphotocopy of the map from the back of this book.

P S

L

10.26.3210.26.45

Canberra

Melbourne

P S

L waves not shown

P

L waves not shown

Scale: 1mm = 1 second

10.27.15

Sydney

10.26.44

S

10.27.12

10.28.13

To find the epicentre and time:■ Read from the seismograms the difference in

arrival times of the P and S waves. ForCanberra the time is 10.26.32 for the P wavesand 10.26.45 for the S waves.This difference is13 seconds. Repeat for Sydney and Melbourne.

■ Use the graph on page 217 to find the distanceof the earthquake from the recording stations.For Canberra this is 100 km. Repeat for Sydney and Melbourne.

■ From the scale on the map, read the distancefrom the city. For Canberra this is 7 mm.Repeat for Sydney and Melbourne.

■ Set a compass to this distance. Put the point ofthe compass over Canberra and draw a circlearound Canberra on the map.

■ Repeat this for each of the two other recordingstations. Where the three circles intersect is theepicentre of the earthquake.

■ Assume that the P waves travel at 8 kilometresper second. At what time did the earthquakeoccur? Check with all three recording stationsto obtain an average.

■ Draw a seismogram for this earthquake, whichwould have been recorded in Adelaide.Consider the height of the P and S waves onthe seismogram, and the time of arrival of eachof them. Do not include L waves.


Difference

in travel

times of

P and S

waves

(seconds)

807570656055504540353025201510 5

1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0

Distance from earthquake (km)

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Graph the depth of earthquakes against the distance west of the Tonga Trench from the information given in the following table. Markeach earthquake focus with a clear dot on your

graph. Do not join the dots. How can you interpret the information in terms of the plate tectonics theory?

Depth of earthquakes in the Tonga Arc RegionDistance west of Depth of focus Distance west of Depth of focusTonga Trench of earthquake Tonga Trench of earthquake(km) (km) (km) (km)

580 680 545 620200 160 150 135505 580 140 80285 260 270 195375 300 480 590385 375 415 360425 380 550 640555 670 45 5485 610 465 540500 520 30 5470 490 85 100440 420 465 500285 170 490 595540 610 530 615490 510 470 585485 470 235 205185 80 35 4030 30 215 160

105 15 405 50550 East 15 10 East 20

AIM: To investigate the focus of earthquakes that occur west of the Tonga Trench

To the centre of the Earth8.9

In the book and film ‘Journey to the Centre of theEarth’, explorers climbed down a volcano andexplored the forests and caverns inside the Earth.This book, written by Jules Verne, was publishedin 1864. At that time many people believed thatthere were worlds inside the Earth.

When the seismograph was invented in 1880people had a way of studying the inside of theEarth. The P and S types of earthquake wavestravel differently in the Earth: P waves go

through the Earth, while S waves are stopped bythe liquid outer core. The speed of the wavesgives clues about the density of the layers. Thesefindings, and others, suggest that the core is madeof iron and nickel, and the mantle is made of themineral olivine (a magnesium silicate).

Convection currents in the asthenospherecause continental drift. The Earth’s magnetic fieldis caused by movements in its liquid outer core.

Better seismological techniques have led to fur-ther discoveries. The lithosphere and astheno-sphere can be ‘seen’ on sensitive seismograms.The structure, depth and composition of theupper mantle changes in ways that no one under-stands.

In the 1970s, small submarines were developedwhich could explore the depths of the oceans.Some were controlled from the surface by cables,and others carried a crew of people. They couldgo down to a depth of over 3000 metres. Thesesubmarines are used to look at shipwrecks andthe mid-ocean ridge. At these depths the ocean istotally dark, the water temperature is 2°C, andthe pressure is huge. The pressure is due to theweight of three kilometres of water pushingdown.

Key:

innercore outer

coremantle

noS-wave

received

focus of earthquake

crust

103°

103°

S earthquake waveRay

giant tube wormscontain bacteriathat provide food

plumes of hot waterrich in sulfides; poisonous

to most animals, but a richfood source for bacteria

black water, likesmoke, is emittedfrom the vents

giant clams filterbacteria and small

animals from seawatersmall animals eat the bacteria that gather around the vent. These are eaten by carnivorous fish

hydrothermal vents,made from mineraldeposits

S waves do not travel through the Earth

Hydrothermal vent community

218


Explorations of the mid-ocean ridge began in1977. Unexpected discoveries were made. Super-heated water, at 350°C, was coming out of vents.Seawater seeped into the ridge, was heated bylava, and then squirted out. The hot water con-tained dissolved minerals from the lava. The min-erals were precipitated (deposited) when the hotwater reached the cold ocean water. Large blackchimneys, called hydrothermal vents, separatedthe hot water from the seawater. Black watercame out like smoke, so the vents were also calledblack smokers. The minerals were sulfide salts ofmetals such as copper, manganese, zinc and iron.

Crowded around these mineral-rich hot watervents was an entire community of living things.Bacteria obtained their energy for life from thechemical called hydrogen sulfide. It is dissolved inthe heated water. Bacteria are the producerorganisms in a community that contains tube-worms, mussels, crabs and crustaceans. The bac-teria are eaten by small animals, which are, inturn, eaten by fish. In some communities small,white, blind sharks are the top predators.

Each animal is specially adapted to live in thecold, the dark, and the extreme pressure at thispart of the ocean. They live nowhere else. Thesecommunities contain 30 cm long clams and bright

red tubeworms 3.7 m long. Bacteria live in thewalls of tubeworms in an example of mutualism.The bacteria provide food and the tubewormsprovide a place to live. Some crustaceans are parasites.

Different vents in different oceans have variedcommunities living beside them. Clams and tube-worms are characteristic of eastern Pacific Oceanveil communities. If the vent stops producing sulfide rich water, the community will die. Someanimals relocate to new vents.

As this part of the crust moves away from themid-ocean ridge, the life-giving heat and chemi-cals slow and stop. The vents and their communi-ties die. New vents start up closer to the ridge andsoon a new community is living there. The oldvents are an important source of metals, such ascopper, manganese, iron, silver, and gold.Already, mining engineers are prospecting theocean floor and devising ways of mining thesemetals.

Many rich mining sites on land have beenformed in this way. The deposits at Broken Hillare ancient hydrothermal vents, originallyformed 1600 million years ago, and changed byheat and pressure.

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When the ___________ was invented, people had a way of ________ the ______ of the Earth. The _ and _ types of__________ waves ______ differently in the _____: P waves go _______ the Earth, while S waves are _______ bythe ______ outer core.Explorations of the ___-_____ _____ began in 1977. Unexpected ___________ were made. Superheated _____, at_____, was coming out of _____.Crowded around these _______-____ hot water vents were an entire _________ of ______ things.

QUESTIONS

1 What did Jules Verne and other nineteenth centurywriters believe about the interior of the Earth?

2 What evidence is there that the Earth is solid andmade of layers?

3 Why did people wait until 1977 before exploring themid-ocean ridge?

4 Why is the pressure at the mid-ocean ridge so great?

5 a Plant life is said to be photosynthetic. How doplants obtain their energy for life processes?

b Bacteria living on the mid-ocean ridge are said tobe chemosynthetic. How do they obtain theirenergy for life processes?

6 Unusual communities of living things have beenfound living at the mid-ocean ridge. Answer thesequestions about them.

a What are the producer organisms?b What is their energy source?c Why do algae not grow near the warm water of

the mid-ocean ridge?d What special features of this environment are

these animals adapted to?

7 Should these ‘chimneys’ be mined? Assume they areinactive without any life. Give one reason for andone reason against.

8 Water at 350°C is said to be superheated. Why doesthis water not boil at 100°C like water in a beaker?

220

Metamorphic rocks8.10

The rock marble is used to decorate buildings andmake statues. It is pale coloured, can be polishedto a high shine, and contains beautiful patterns.Marble is an example of a metamorphic rock.Metamorphic rocks have been changed by heatand/or pressure. A single rock can give rise tomany types of metamorphic rock, depending onthe conditions.

Metamorphism takes place when an igneous orsedimentary rock is changed by being heated orsubjected to a large pressure. The changes happenwhile the rock is deep underground. One of themost common causes of metamorphism is thecollision of tectonic plates. This force can bendand reshape rocks into mountains, and can alsobury rocks at a great depth.

The heat and pressure causes some minerals tobreak down and reform producing a rock with anew appearance and texture. If the heat and pres-sure are slight then the changes are minor. This iscalled low-grade metamorphism. If the heat andpressure are so great that the rock almost melts,this is called high-grade metamorphism.

Metamorphism that acts over a large area, andchanges rocks by pressure and heat, is calledregional metamorphism.

Shale, a sedimentary rock, can be changed byregional metamorphism. At 250°C and 2000atmospheres pressure, shale is slowly trans-formed into slate. Slate splits easily into thin

sheets. It is used as floor tiles. If slate is heated to400°C it is transformed into phyllite. Phyllite haslarger crystals than slate, and it feels more grainy.If phyllite is heated even further, then schistforms. Schist is a sparkling flaky rock common insome mountains. If schist is heated almost to itsmelting point the rock gneiss (pronounced ‘nice’)is formed. All the minerals have been rearranged,reformed and recrystallised. Gneiss looks likegranite but with its crystals in layers. The greatestpressure in all these metamorphic rocks is at rightangles to the layers of crystals.

Another type of metamorphism happens withheat alone. Around the outside of molten rocks,such as the batholiths under volcanoes, the rockscan be melted. The rocks must be in contact with,or touching, the heat source. This is called contactmetamorphism. Marble is formed from limestoneby the action of heat. The layers of coral andshells melt and are recrystallised into interlockingcrystals of calcite. This gives the marble a grainytexture and makes it hard. Impurities in the lime-stone give coloured streaks in the marble. No twosamples of marble are ever the same.

When sandstone is metamorphosed by heatthe grains of sand may melt and join together.The very hard rock that forms is called quartzite.

Some gems are metamorphic. Jade is made inthis way. Other gems such as rubies and sapphiresmay be metamorphic or igneous in origin.

A slate floor Phyllite Gneiss

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