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23 DESERTSWhere deserts form

24 COASTLINESFeatures of a coastline • Fjords •Waves •How coral islands form

A T M O S P H E R E

26 ATMOSPHERELayers of the atmosphere • Reflection and absorption of the Sun’s rays

27 SEASONS AND CLIMATEEarth’s orbit • Winds • Climate regions

28 WEATHERWater cycle • Fronts • Dew and frost •Clouds

30 STORMSThunderstorms • Cyclones • Hurricanes • Tornadoes

32 INDEX

CONTENTS

E A R T H

4 PLANET EARTH

5 MAGNETIC EARTHCauses of Earth’s magnetism • Aurorae

6 INSIDE THE EARTHEarth’s crust, mantle and core

8 RESTLESS EARTHPlate tectonics • Seafloor spreading

10 OCEAN FLOORSeamounts, abyssal plain • Continental shelf

11 FOLDS AND FAULTSGreat Rift Valley

12 EARTHQUAKESWhy earthquakes happen • Shock waves

14 VOLCANOESVolcanic zones • Great volcanic eruptions

16 ROCKSIgneous, metamorphic and sedimentary rocks• Rock cycle

17 FOSSILS

L A N D S C A P E S

18 EROSIONWeathering • Action of water, glaciers and wind • The Grand Canyon

20 RIVERSFeatures of a typical river • Waterfalls

21 CAVESStalagmites and stalactites

22 GLACIERSFeatures of a glacier

C O N T E N T S

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JOB NO:E7-05129 TITLE:Earth150# DTP:95 PAGE:2

First published in 2009 by Orpheus Books Ltd., 6 Church Green, Witney, Oxfordshire OX28 4AW England

www.orpheusbooks.com

Copyright © 2009 Orpheus Books Ltd

Created and produced by Orpheus Books Ltd

Text Steve Parker

Consultants Susanna van Rose, writer and geologist

Illustrators Julian Baker, Alessandro Bartolozzi, Tim Hayward, GaryHincks, Steve Kirk, Lee Montgomery, Steve Noon, Nicki Palin,

Sebastian Quigley, Alessandro Rabatti, Claudia Saraceni, Peter DavidScott, Roger Stewart, Thomas Trojer, David Wright

All rights reserved. No part of this book may be reproduced, stored in aretrieval system, or transmitted in any form or by any means, electronic,mechanical, photocopying, recording or otherwise, without the prior

written permission of the copyright owner.

ISBN 978 1 905473 46 5

A CIP record for this book is available from the British Library.

Printed and bound in Singapore

Photograph on page 5: Michael Giannechini/Science Photo Library; on page 13: The Illustrated London News Picture Library

C O N T E N T S

2

The magnetic field stretches into space andprotects us from the Sun’s high-energyparticles. Some are attracted by themagnetic poles, however, and produce giantcurtains of glowing light in the night sky,known as aurorae (above).

E A R T H

5

MAGNETIC EARTH

THE EARTH has its own magnetism—an invisible field of magnetic force all

around us. Too weak to notice in daily life,the magnetic field affects iron-basedmaterials and other magnets. We can detectit using a magnetic compass. The compassneedle is a long, thin magnet that lines itselfup with Earth’s magnetism to point north-south. This helps us to read maps and findour way in remote places.

The Earth’s magnetic field is probablycreated by forces produced in the outercore, a layer of iron that lies some 2900kilometres below the surface (see page 6).Because of extreme pressure at this depth, itis incredibly hot—more than 4000˚C. Atthis temperature, the iron is liquid. Heatcurrents cause the liquid metal to swirlaround. The currents are themselves twistedby the spinning motion of the Earth intocorkscrew-like patterns, called “rollers”.These giant movements make electricitywhich, in turn, creates a magnetic field.

E A R T H

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Earth’s closest neighbour in space is the Moon. It is 3475kilometres across, about a quarter the width of Earth. Itsvolume is around one-thirtieth that of Earth. The rocks whichmake up the Moon are not as heavy or dense as Earth rocks,so the Moon weighs only one-eightieth as much as the Earth.

Earth’s magnetism extends into space as themagnetosphere. High-energy particles fromthe Sun, the solar wind, “blow” against oneside and make it teardrop-shaped.

Scientists believe that massive amounts of flowingheat energy inside the Earth, combined with theplanet’s daily spinning motion, make the semi-liquid rocks flow in spiral patterns. These generateelectricity, producing a magnetic field.

The Earth’s magnetic field is strongest attwo places, the North and South MagneticPoles, where it is directed straight downinto the ground. It is as though there was agiant bar magnet inside the planet.

PLANET EARTH

OUR PLANET EARTH is the fifthlargest of the eight planets which go

around, or orbit, our nearest star—the Sun.The Earth speeds through space at about 30kilometres every second, taking one year tocomplete one orbit. In addition the planetspins round like a top once every 24 hours.This makes the Sun appear to rise at dawn,pass across the sky and set at dusk, giving usday and night. The Earth is not quite aperfect ball or sphere shape. It is 12,756kilometres across its Equator (middle) and12,714 from Pole to Pole (top to bottom).The distance around the Equator is 40,075kilometres, and 40,008 from one Polearound to the other and back again.

Earth

Magnetosphere tail

Solar particles

Earth is “third rock from theSun”. It is the third planetout from our neareststar, and mademainly ofrockymaterial.

N

Lines ofmagneticforce

Earth

S

Outer core

Inner core

KEY1 Mercury2 Venus3 Earth4 Mars5 Jupiter6 Saturn7 Uranus8 Neptune9 Pluto

Sun

A s t e r o i d s

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Heat flows within the Earth, and slowly fromthe centre to the outside, by the movementsof the molten and “plastic” rocks inthe mantle. These flow in giantcircles, called convectioncurrents. Their motioncauses continentaldrift and seafloorspreading (seepage 8).

T H E C O R EAt the base of the mantle, there is a suddenchange. The material is no longer rock, butmetal—mainly iron plus small amounts ofnickel. In the outer core, the temperaturerises with depth to more than 3000°C nearthe boundary with the inner core. The ironof the outer core is liquid, and flows ingiant corkscrew-like currents or “rollers”.These probably produce the magnetic fieldof the Earth (see page 5). The temperaturerises still more at the inner core, to perhapsup to 7500°C at the centre of the planet.But the enormous pressure—several milliontimes that at the surface—means that theiron crystals are squashed into a solid ball.How do we know about the Earth’s

interior, if no-one has ever drilled deep intothe Earth? Evidence comes from the waythat shock waves from earthquakes passaround and through the Earth (see page 13),and from studying meteorites. Some earth-quake shock waves do not travel throughthe outer core, telling us that this part isliquid. We know the core must be made ofiron because we can compare it with thecomposition of iron meteorites, thought tobe the remnants of the core of an ancient,Earth-like planet which broke up long ago.

E A R T H

INSIDE THE EARTH

ON THE OUTSIDE, the Earth seemshard and solid. But if you could drill a

deep hole almost 6400 kilometres down tothe centre of the planet, you would noticemany changes as you descend. It becomeswarm, then hot. The average increase intemperature is about 3°C for every 100metres of depth. Soon it is so hot that therocks are not solid but melted or molten.You pass through the various layers of rockymaterial, from the hard crust on the outside,through the very thick mantle, to the liquidouter core. When you reach the inner coreat the centre there is no rock at all. Thecore is made of almost solid metal.

T H E C R U S TNo-one has bored a hole nearly this deep.The farthest we have drilled down is about15 kilometres, which is part way throughthe crust. The crust is thinner in proportionto the whole Earth than the skin on anapple. The crust itself is solid rock and variesin depth. Under the oceans it is about 5-10kilometres thick (with the ocean above) andmade mainly of basalt-type rocks. Underthe main land-masses or continents it is 35-70 kilometres thick and chiefly granite-typerocks. The taller the mountains above, thedeeper the crust below. The crust is not onesolid ball-shaped shell. It is cracked intolarge slowly-moving plates (see page 8).

T H E M A N T L EThe mantle also has two layers. Its outerpart is about 600 kilometres thick and madeof crystals of rock with molten or liquidrock between them. Its temperature is about2000°C and the molten rock, known asmagma, can flow like hot tarmac. It is undergreat pressure and sometimes bursts out ofholes or cracks at weak points in the surfaceof the crust, as the red-hot lava of volcaniceruptions. The pressure in the inner mantleis so great that the rock here is solid—butnot completely rigid. It is “plastic” and, verygradually, moves.

E A R T H

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The four main layers of the Earth (above) are the crust,mantle, outer core and inner core. At the base of the crust isa boundary called the Moho (Mohorovicic discontinuity). Thisseparates the crust from the mantle and the temperature hereis about 1500°C. The mantle is about 2900 kilometres thick.The next layer is the outer core which is 2200 kilometresthick. At the centre is the inner core, a solid ball of iron witha radius of about 2500 kilometres.

Some 4600 million years ago the Earth (along with the Sunand other planets) formed from clouds of gas and dust inspace. Some of this matter clumped together to form the earlyEarth, which warmed up and glowed red hot. Iron was theheaviest substance so it began to sink through the moltenmagma as droplets. These collected into drops, then largerblobs. Gradually they clumped at the centre of the youngplanet to form the inner core (left).

CRUST

CRUS

T

MANTLE

OUTERCORE

INNERCORE

Convectioncurrent

CORE

Iron droplet

Magma

Earth’ssurface MANTLE

OUTERCORE

Ocean floor

Continent



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S E A F LO O R S P R E A D I N GThe lithospheric plates fit together tightly.As they move, they rub and grind theiredges against each other. In some places theedges crash into each other and crumple,pushing up mountains. In other places, hotliquid rock wells up from deep below, intothe crack or boundary between the oceaniccrusts of two plates. The molten rock coolsand solidifies, adding to the edges of thetwo plates as they move apart. This processis called seafloor spreading and it makes thewhole ocean wider. The crack between theocean plates is called the mid-oceanic ridge.

E A R T H

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RESTLESS EARTH

THE ATLANTIC OCEAN gets widerby about the width of your thumb

every year, pushing North and SouthAmerica away from Europe and Africa. TheHimalayan mountains, already the highest inthe world, grow taller by about the lengthof your thumb every year. Many other partsof the Earth are moving and changingshape, too. This is because the Earth’s outerlayer is divided into enormous curvedpieces called lithospheric plates, which fittogether like a ball-shaped jigsaw. There aresix large plates and about 12-15 smallerones, and they are continually on the move.The theory of plate tectonics explainshow this happens.

Each plate consists of a piece of theEarth’s outer layer, the crust, plus a portionbelow it of a thin layer of outer mantle.Together the crust and thin slice of outermantle make up the layer known as thelithosphere. Its depth varies from 70-80kilometres below the oceans to 100-150kilometres where there are continents.Under the lithosphere is a slightly deeperpart of the mantle about 100 kilometresthick, called the aesthenosphere. This ispartly molten and allows the plates to slideabout over it. In fact the slow flowing ofthe mantle, due to the enormous heat andpressure within, pushes the plates and makesthem slide around the surface of the planet.As they do so, they carry the continentalland-masses like giant rafts.

E A R T H

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Plates meet at three types ofboundaries. At a mid-oceanicridge, new crust is made as theplates push apart. At a convergentboundary, oceanic crust is lost asa thinner oceanic plate is forceddown under a thicker continentalone (subduction), or is crumpledup into mountains whencontinental plates collide. Ata transform fault, theplates slide past eachother.

When an oceanic plate collideswith a continental plate, the edgeof the thinner, denser ocean plateslides beneath it in a subductionzone. The rocks of the ocean floorsink deeper and deeper into theEarth and melt. Some of thismolten rock rises through thecontinental crust, erupting at thesurface as volcanoes.

Continental shelf

Oceanic crust

See inset, above rightDeep-seatrench

Subductionzone

Direction ofheat flow

Ocean surface

Molten rock rises

Molten rock in upper mantle

Seamount

Volcano

Magma plume

Continental crust

These two global maps of the Earth (left) show howits surface is divided up into a number oflithospheric plates of varying shapes and sizes.

Abyssal plain

Mid-oceanicridge

Convectioncurrents inupper mantle(see page 7)

Mid-oceanic ridge

Convergent boundary(collision zone)

Transform fault

Solidifiedlava

Moltenrockrises



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E A R T H

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OCEAN FLOOR

ABOUT 71 per cent of our planet iscovered with water. The largest ocean is

the Pacific, which covers 166 million squarekilometres—almost the same area as allother seas and oceans added together. Thelandscape around us has tall mountains,wide plains, winding valleys and deepravines. Under the waves the seascape hasthe same features but on an even biggerscale. The highest mountain above sea levelis Everest in the Himalayas, at 8848 metres,but the tallest mountain from base to peakis Mauna Kea on the Pacific island ofHawaii, at 10,205 metres—with 6000metres below the waves. Everest woulddisappear into the deepest part of the ocean,the narrow Marianas Trench in the north-west Pacific near Japan, at 10,911 metresdeep. The average depth of the Pacific is3950 metres. Its wide abyssal plains coveralmost as much area as all the Earth’s land.

FOLDS AND FAULTS

SLIDING plates and drifting continents(see page 8) are responsible for some ofthe Earth’s major landscape features. As alarge slab or plate of the Earth’s surface issqueezed, the solid rock slowly wrinkles andcrumples. Its layers become wavy folds. Theland’s surface is pushed up as a series of hillsor even mountains. The wind, rain, sun, ice,snow and other forces of nature (see page18) may wear down the folds as fast as theypush up, keeping the surface low androunded. But if the folds rise more quicklythey form high, jagged peaks. The world’sgreat mountains, including the Himalayas inAsia, Andes in South America, Rockies inNorth America and Alps in Europe, are allfold mountains.In other places, rocks are stretched or

bent and they crack or split along weakpoints. These cracks are known as faults.They may be straight or zigzag and formnarrow slits or wide valleys. A block or stripof land sometimes slips down between twocracks to make a valley with steep slopes oneither side, called a rift valley. Rifting canalso make mountains, as the rocks on eitherside move in and squeeze the central blockupwards. Raised blocks are called horsts andthose which slip down are grabens.

The true edge of a continent is not itscoastline. From here the sea bed extendsabout 50-100 kilometres, yet the water isless than 200 metres deep. This ledge, thecontinental shelf, is part of the continent. Atits edge, it plunges steeply about 2000-2500metres down the continental slope, thenfurther down the less steep continental rise,to the main ocean floor. This is the abyssalplain, lying at 4000-4500 metres deep.

The biggest gash in the Earth’s land is the Great Rift Valley.This series of rifts runs from the eastern Mediterranean south-east through the Dead Sea and Red Sea, then south acrossEast Africa through Lake Turkana. It divides around LakeVictoria to continue south to Lakes Tanganyika and Malawi.The valley system is some 5000 kilometres long and widensby up to 2 centimetres in places each year. In millions ofyears the Red Sea may become a broad ocean and seawatermay flood into the valley.

E A R T H

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A cutaway view reveals how the landscape is shaped bymassive forces that crumple and crack solid rock. A wideslab has slipped down between several cracks or faults toform a rift valley with a wide floor and steep sides. To eitherside of the rift valley the rocks are bent into folds, with somealmost tipped right over.

If all the ocean waters weredrained away you would seethe many features of the oceanfloor. The continental shelfextends from the land, thendips sharply to the abyssalplain. Seamounts areunderwater mountains. Peakshigh enough to break thewater’s surface form islands.New ocean floor is formingalong the mid-oceanic ridge byseafloor spreading. Old oceanfloor is disappearing into themantle along the deep trenchesof the subduction zones aroundthe Pacific Ocean.

Mid-AtlanticRidge

ARCTIC OCEAN

EastPacificRise

MarianasTrench

AleutianTrench

EmperorSeamounts

HawaiianIslands

Seamounts

NORTHAMERICA

SOUTHAMERICA

AFRICA

EUROPE

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ea

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Lake Tanganyika

GREAT RIFTVALLEY

GREAT RIFTVALLEY

GREAT RIFTVALLEY

Lake Malawi

INDIANOCEAN

A SI A

Fault Fold

Land slippedbetween faults

A F R I C A

Area of mapshown in red



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E A R T H

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EARTHQUAKES

THERE ARE about 6000 noticeableearthquakes each year. Scientific

devices called seismometers on continuous“quake watch” detect them all. Nine out often of these earthquakes are too small oroccur in very remote regions so they areonly of interest to scientists. Another 30-40cause small-scale problems, while 10-20cause major damage and make headlines.Every 5-10 years a massive quake results ingreat loss of life and devastates a wide area.An earthquake happens when the plates

which form the Earth’s outer layer (see page8) suddenly slip past each, snap or makesome other rapid movement, especiallyalong their edges or at cracks (faults).

The sudden jolt of a quake usually lastsnot more than a few minutes and may beover in just a few seconds. It spreads outfrom a place called the focus. A shallowfocus is down to 70 kilometres below thesurface, an intermediate one 70-300kilometres, and deep focus below 300kilometres. Juddering shock or seismicwaves spread out in all directions throughthe rocks. They reach the surfacefirst at the epicentre, directlyabove the focus, andare usuallystrongesthere.

The shock waves reach the Earth’s surfacearound the epicentre, spreading out likeripples on a pond. The immensely powerfulvibrations of a massive earthquake travelaround and through the whole planet,making it tremble and shake for up to 20minutes. Most earthquakes happen alongthe edges of the Earth’s huge plates,especially where these are actively moving(often resulting in volcanoes, too). High-riskregions are the “Pacific Rim”, around theshores of the Pacific Ocean, Southeast Asia(Philippines and Indonesia), from northernIndia west to southern Europe. Some largeearthquakes also happen away from theplate edges.

There are several types of shock waves. P(primary) waves travel quickly through theEarth, although its inner layers bend them. S(secondary) waves are slower and cannot gothrough the liquid outer core.Two scales measure earthquakes. The

Mercalli scale shows how much damage iscaused, from 1 (not felt) to 12 (totaldevastation), while the Richter scalemeasures the magnitude of the shock.

E A R T H

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Earthquakes under the sea set off under-water ripples. These reach land and rearup to form huge waves, called tsunamis.

Cities around the Pacific Rim suffer regularquakes. Tokyo was devastated in 1923.

Many earthquakes occur alongsubduction zones where onelithospheric plate with oceaniccrust grinds down below theedge of a continent. Themain slippage is deepunderground at apoint called thefocus.

Oceanfloor

Movement ofocean crust

Movement ofcontinental crust

Location of majorearthquakes (shownby red dots).

Trench

Seismicwave

S-wave

S-wave

P-wave

P-wave

Epicentre

INNERCORE

OUTERCORE

MANTLE

P-wave

PACIFICOCEAN

Focus

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V O LC A N I C Z O N E S Most volcanoes are situated along the edgesof the giant, jigsaw-like lithospheric plateswhich make up the Earth’s surface (see page8). The boundaries between plates havemany weak points. In particular, volcanoesform along subduction zones where oneplate slides down beneath another. As thelower plate melts back into the mantle itsgases and lighter molten rock “boil” andforce their way up through cracks withenormous pressure, causing eruptions.

The typical cone-shaped volcanoes onland may seem huge and powerful. But theymake up less than one-hundredth of all thevolcanic activity on Earth. Most magmaoozes to the surface deep under water,along the crack-like fissures of mid-oceanicridges (see page 8) or through smaller weak“holes” known as hot spots. If underwatervolcanoes build cones tall enough theyemerge at the surface as islands, such as theHawaiian Islands in the Pacific and theCanary Islands in the Atlantic.

E A R T H

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VOLCANOES

WHEN A VOLCANO erupts andhurls out its red-hot rock, this is one

of the most awesome events of nature. Ithappens at a hole, crack or weak point inthe solid rocks of the Earth’s crust. Meltedrock called magma from deep below forcesits way up under incredible temperature andpressure. As it emerges it is called lava.When it cools and hardens, it forms a typeof rock known as igneous rock (see page 16).Some lava is thin and runny. It oozes like

boiling syrup from the volcano and spreadsover a wide area. As it cools, it turns into a“shield” of solid rock known as basalt. Eachtime the volcano erupts it adds to theshield, in layers of lava up to 10 metresthick. Known as shield volcanoes, theeruptions are gentle.In explosive eruptions, lava is thick and

sticky. It moves slowly and hardens near thevolcano’s vent or crater. As this type ofvolcano erupts time after time, the lavabuilds up to form a tall, steep-sidedmountain known as a cone.

The temperature of lava erupting from avolcano can be more than 1000°C. It maytake months to harden. Volcanoes ejectother substances, too: gases and fumes richin sulphur. Some give out clouds of ash orcinders that fly high in the air. The cindersmay fall near the volcano and build up acinder cone. Some volcanoes have suchexplosive power that they blast out hugelumps of molten rock as big as houses.These volcanic bombs crash to the groundnearby. The ash is often blown away by thewind and may fall over a very wide area.

Santorini was anisland with a dormantvolcano (1). Suddenly the topblew up in a great explosion (2). Theeruption continued for days as sea water flooded the magmachamber (3). The whole island nearly disappeared in a finalblast (4). All that remains today is a small ring of islands (5).

E A R T H

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An active volcano regularly erupts lava,ash, fumes and other materials. In veryactive volcanoes, this happens almostcontinuously. In others, there areweeks or months between eruptions.When a volcano has not erupted formany years or centuries, but still mightin the future, it is dormant. When therehave been no eruptions for tens ofthousands of years the volcano isdescribed as extinct. Some volcanoesblast out gas and spurt red-hot lava ina spectacular eruption. Others eruptmore explosively, producing clouds ofash and gas. On land, most magmaoozes slowly to the surface over verylong time periods without explosivepower. It emerges from long cracks orfissures and spreads out to form lowmounds of volcanic rock.

Rainwater maytrickle downthrough cracksin rocks todeeper layerswhere it isheated bymagma. Itcomes blastingback out as afountain ofsteam, sprayand hot waterknown as ageyser.

GREAT VOLCANIC ERUPTIONS

• 1450 BC Santorini, GreeceBiggest explosion in ancienttimes• AD 79 Vesuvius, ItalyDescribed by Pliny theYounger. Pliny the Elder waskilled in the eruption• 1815 Tambora, IndonesiaKilled 90,000-plus people • 1883 Krakatau, JavaHeard 5000 kilometres away• 1980 Mt. St Helens, USA Filmed as it happened• 1991 Pinatubo, Philippines Affected the world’s weatherfor two years

All along the mid-oceanic ridge

(right), molten rockseeps into the ocean

floor from the mantlebelow in a series ofcontinually-eruptingvolcanoes. Hot, mineral-rich water and gas bubblefrom deep-sea hydrothermalvents that lie near these fissurevolcanoes.

This 5-stage sequenceshows the eruption ofSantorini’s volcano inthe Mediterranean Seain about 1450 BC.

Magma

Main vent

Crater

Hardened layersof lava and ashfrom previouseruptions

Crust

Lava, ash andgases

Side vent

(AB)

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The hard parts like bones and teeth areburied under sediment particles such as sandgrains on a beach, silt on a river bank ormud on a sea bed. Slowly the surroundingwater dissolves away the remains andreplaces them with rock minerals from thewater. Meanwhile the particles around themare also turning into rock. If undisturbed,the remains keep their original shape butthey are now solid rock—fossils.

17

ROCKS

ROCK is the hard material that makesup the Earth’s crust. Various

combinations of minerals make up hundredsof different types of rocks. For example, therock sandstone consists mainly of grains ofsand pressed and cemented together. Sand ismade mainly of the mineral quartz whichconsists of the chemical elements siliconand oxygen.All rocks can be divided into three main

groups depending on how they formed.Igneous rocks, such as granite and basalt,are formed when magma, molten rock frombeneath the Earth’s crust, rises, cools andsolidifies. Sedimentary rocks, such assandstone and mudstone, are made fromsand, gravel, mud and other fragments ofrock that result from erosion (see page 18).These settle in layers in lakes, rivers andseas. As more layers settle on top of eachother, the particles are compressed andcemented into sedimentary rock. The third

FOSSILS

FOSSILS ARE remains of once-livingthings preserved in rock. Most living

things are eaten or die and their soft partsrot away leaving no trace. But sometimeshard body parts remain, like the shells,bones, teeth, horns and claws of animals andthe bark, cones and seeds of plants. Theseare the parts most likely to form fossils.Trace fossils are not actual body parts butsigns and traces of living things such as eggshells, footprints and droppings.

group, metamorphic rocks, such asmarble and slate, are formed when rocks aresubjected to such great pressure and heatthat their mineral composition is altered.Rocks are constantly being changed.

Weathering attacks all kinds of surfacerocks. The eroded fragments form newsedimentary rocks which may sink into theEarth, melt, then later cool to becomeigneous rocks. Alternatively, they may becooked and crushed deep in the crust,forming metamorphic rocks. This changefrom one type to another is called the rockcycle (below).

Fossils take many thousands or millions ofyears to form and are found only insedimentary rocks (see opposite). Then, as partof great earth movements and the wearing-away forces of erosion, the sediments andtheir fossils may be exposed at or near thesurface. Experts called palaeontologistssearch for fossils, dig them from the ground,study their shapes and structures, andcompare them with similar body parts ofliving things today. This shows the kinds ofdinosaurs, mammoths and other animals andplants that lived millions of years ago.

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Most magma risesslowly through thecrust and turns intosolid rock under-ground. There, likejelly in a mould, ittakes on the shape ofits surroundings,resulting in ledges,columns, domes andother shapes.

Fossils form insedimentary rocks. Theyare destroyed if the rock isheated or squashed too muchso they do not occur in igneousor metamorphic rocks. Somesediments contain layer upon layer offossils, like this shelly limestone. Itformed after thousands of ammonites diedand their shells piled up on the sea bed.

The sticky sap or resin that oozesfrom trees and other plants mayfossilize as a hard yellowsubstance, amber. This sometimescontains insects and other smallanimals that were trapped in it,preserved in amazing detail.

An ammonite, a prehistoric cousin of the octopus and squid,lived in a coiled shell and floated or swam in the sea. When itdied its soft, fleshy parts soon rotted away or were eaten (1).Sandy sediments slowly covered the hard shell on the seabed (2). Both shell and sand gradually turned into rock whichwas lifted and tilted by massive earth movements (3). Erosionuncovered the fossil shell at the surface (4).

This is the fossil skeleton of an ichthyosaur (right),a marine reptile from the Age of Dinosaurs. A tracefossil of a dinosaur footprint is shown (below right).

Volcanic lavacools at surfaceto form extrusiveigneous rock

Forces oferosion wearrocks intosediments

Magma coolsdeep undergroundto form intrusiveigneous rock

Streams andrivers wash awaysediments

Older, deeperlayers harden intosedimentary rock

Pressure deep inmountains formsmetamorphic rocks

Desert

Mountains

Sea

Volcanoes

Sediments laiddown

Risingmagma



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T H E G R A N D C A N YO NThe Grand Canyon is a great gorge thattwists across the dry, rocky region ofArizona, USA for 350 kilometres. It formedover the past six million years as earthmovements pushed up the land by morethan 1200 metres. The fast-flowingColorado River has steadily cut into theland to maintain its downward flow to thesea. The result is a step-sided canyon, onaverage 16 kilometres wide and 1600metres deep in places. The region’s desertclimate (see page 23) means the softer, upperrock layers have not been washed away.The river is especially powerful in spring

when melting snows in the distant Rockiessend floodwaters down the canyon. Theysweep along boulders that chip away at theriver’s bed and banks. As the river cutsdeeper, it reveals ancient layers of rock andthe fossils they contain, almost like chaptersin the Earth’s prehistory. The lowest layersof rocks are 1700 million years old. Theywere themselves once mountains toweringthousands of metres above sea level.

L A N D S C A P E S

19

EROSION

OVER THE MILLIONS of years ofgeological time, mountain ranges have

formed—then disappeared. Continentaldrift, faulting and other earth movementshave created these mountains. What hashappened to them? Most have been worndown by the slow processes of weatheringand erosion. Changes in temperature, rain and frost all

break down the rocks in a process calledweathering. Rocks heat up and expand inthe hot sun, then cool and contract at night.The temperature changes crack the rocksurface and small pieces flake off. Rainwaterseeps into crevices in rocks and, as it freezes,it expands with great force and splits offpieces. This is known as frost-wedging.Erosion is the removal of fragments of

rock by the action of running water, glaciersor wind. Rivers, especially if they are fast-flowing or in flood, can carry away piecesof rock. Waves crashing on to cliffs,sometimes hurling pebbles or boulders atthe rock, are also powerful forces of erosion.

L A N D S C A P E S

18

Weathering wears awayhigher areas of land fastest(above). Ice, wind andwater carry the rockyfragments and deposit themin lower regions such asplains, rivers and lakes,where they are known assediments.

The world’s mostspectacular example oferosion is the Grand Canyonin Arizona, USA (below).The soft rock layers woreaway more easily than thehard layers, which todaystand out as near-verticalcliff faces.

The Colorado River once flowed across desert (above, 1), butas the land rose (2) it cut a deeper and deeper valley (3).



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A typical river starts as a spring gushingout of the ground, as a melting glacier or asrain-water collecting in small brooks andstreams. The river’s upper waters usuallyflow fast and steep. The swift current washesaway soil or mud so the bed is stony andthe banks are bare. Gradually the slope easesand the river flows more slowly in itsmiddle reaches, widening as smaller rivers,called tributaries, add to it. A slower currentresults in the river shedding its load ofsediment on its bed or its banks, sometimesproducing braids. As the river’s coursebecomes flatter it flows in huge curvescalled meanders across plains, but stillfollows a downward route. Finally it entersthe sea at the river mouth or estuary. Thetiny fragments of rock and soil it carriessettle as sand and mud banks. It may divideinto many channels, forming a delta.

L A N D S C A P E S

21

RIVERS

RIVERS ARE natural channels thatcarry rain, melted ice and snow

downhill from mountains and uplands tolowlands, lakes and seas. They support muchwildlife in their waters and along theirbanks. The world’s longest rivers are theNile in Africa and the Amazon in SouthAmerica, both about 6600 kilometres long.But the Amazon is so wide and fast-flowing, it carries more water than the Nileplus the next five longest rivers combinedtogether. The Amazon gathers water fromseven million square kilometres of land, anarea larger than Western Europe.

CAVES

CAVES are underground holes in therock. Some caves open up when the

ground splits, as in an earthquake. Some areeroded by waves hurling stones and pebblesat a cliff. But most are made in limestonerocks by a chemical process. Rainwaternaturally contains tiny amounts of acid. Ittrickles into cracks and reacts with therock’s lime substances to dissolve themaway. Over thousands of years small cracksare widened into huge caves.

Rivers have had great effects on ourhistory. Early towns and cities grew upalong them because they provided transportroutes by boat, food such as fish, and watersupplies for drinking, cooking and raisingfarm crops and animals.

Rivers shape the land as they flow overrocks of varying hardness, widening anddeepening their valleys by erosion. Thefaster they flow the greater their erosivepower, and the larger the rocks and amountof sediment they can transport. In a limestone landscape, a stream

disappears into a swallow hole, or when thestream no longer flows, a dry pot hole.Underground is a cave system of manychambers, some with underground lakes,and linked by upright shafts and horizontalgalleries. As water drips from the ceiling,dissolved minerals in it gradually harden toform icicle-like shapes of rock calledstalactites hanging down. Stalagmites growup from the floor where water containingdissolved minerals drips from the ceiling.

L A N D S C A P E S

20

This river (right)begins high in themountains as the snout of amelting glacier. After receiving atributary it flows over a waterfall andthrough a gorge, then forms a network ofsmaller channels, called braids. On the lowlandplains it follows loop-like meanders sometimesedged by raised banks of sediment, known aslevees. At its mouth it divides into manychannels. These trickle through a muddy delta.

Inside this cave, a stalactite above hasmerged with a stalagmite below to form acolumn or pillar of rock. Stalactites andstalagmites can reach 30 metres in length.

The Guilin Hills, southern China, are theremains of an ancient limestone land-scape, weathered away by rainwater.

A waterfall forms where ariver cascades down a cliff,or where its bed changesfrom hard to softer rock. Theriver wears down the softerrock more quickly so a “lip”of hard rock forms above it.The world’s highest waterfallsare Salto Angel (Angel Falls,right) on the Carraro River inVenezuela, South America.The total height is 979 metreswith the tallest single drop at807 metres. The waterbecomes a mist beforereaching the bottom.

Glacier

Waterfall

Gorge

Meanders

Levees

Delta

Sea

Braids



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11 1

2

3

4

5

6

7

8

23

DESERTS

ADESERT is an area with very lowrainfall, usually with less than 25

centimetres of rain (or snow) yearly. It maybe hot all year round, as in the Sahara ofAfrica, or always cold as in Greenland orAntarctica. The Gobi desert of Central Asiais hot in summer and cold in winter. “Hot”deserts may be bitterly cold at night. TheTakla Makan Desert, China, may be ascorching 40°C by day yet plunge to minus40°C at night.

The Mojave Desert in North America,and the Patagonian Desert of SouthAmerica have formed because moist windshave dropped their water as rain or snowover nearby mountains. The lands beyondlie in what are known as rain shadows.Deserts like the Gobi form in the middle ofhuge land-masses far from the main sourceof moisture, the sea. Most deserts, includingthe Atacama, Sahara, Kalahari and Namibdeserts, lie in a band to the north and southof the Equator. These are the result ofglobal patterns of winds and rainfall.

L A N D S C A P E S

23

GLACIERS

AGLACIER is a moving mass of ice.Some glaciers snake down mountain

valleys, while others such as the ice sheetsof Greenland or Antarctica are so huge andthick they almost totally cover the land.Although it is solid, ice can flow downslopes and around bends—although muchmore slowly than a river, often less than ametre per day. Glaciers occur in very coldregions, high in mountains and in the farnorth and south polar regions. Ice coversabout 15 million square kilometres, nearlyone-tenth of the Earth’s land surface. Thelargest glacier in the world is Antarctica’sLambert Glacier which is usually more than500 kilometres long.

A glacier is fed by snow. Over manyyears, the snow piles up at the head of ahigh valley and compacts into ice. It collectsin a cirque, a bowl-shaped feature. Beingthick and heavy, the ice moves under thepressure of its own weight, flowingdownhill as a glacier. The ice carries piecesof rock loosened by frost weathering andscrapes against the valley sides. It carries thisloose rock along in long bands called lateralmoraines, or underneath the glacier assubglacial moraine. As two glaciers mergetheir lateral moraines combine into a medialmoraine. Where the ice runs over a steeperslope, it develops cracks known as crevasses.Lower down, the glacier melts at its snout,leaving a pile of the rocks it has carried as aterminal moraine, and a meltwater stream.

Deserts and arid regions cover one-eighthof the world’s land area. The driest desert isthe Atacama in Chile, South America, withan average of less than one millimetre ofrain yearly. In some places it has not rainedfor centuries. The largest desert is the Saharain Africa, over 5000 kilometres wide andcovering 9 million square kilometres. Thecontinent with the largest proportion ofdesert—about one half its area—is Australia.

Most people think of deserts as vast sandyregions, but only about 20 per cent of theworld’s deserts are sandy. The rest are barerock, or covered with gravel. The erosivepower of the wind and rainstorms sculptsthe desert landscape. Storms hurls sand atrocks, producing shapes like arches (above)or mushrooms and other strange landscapes.

L A N D S C A P E S

22

Wind blows sand into crescent dunes, orbarchans, with “horns” pointing downwind.They slowly crawl along with the wind.

The Olgas, Australia (left), are theresult of “onion-skin” weathering.Daily heating and cooling flakerock layers away. Hard rock hasresisted erosion to form the flat-topped buttes and mesas ofMonument Valley, Utah (below).

KEY1 Cirques2 Glacier3 Lateral moraine4 Crevasses5 Medial moraine6 Snout7 Terminal moraine8 Meltwater streams



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L A N D S C A P E S

25

Some islands are partsof continents separated from the

mainland by shallow seas. Other islands are thetips of volcanoes (see page 14). In warm, shallow, tropicalwaters, billions of coral skeletons build up to form massivecoral reefs around island coastlines (below). A volcano mayeventually sink back into the ocean floor. Coral animals muststay near the light, so they build the reef taller, forming acircular barrier reef enclosing a lagoon containing thedisappearing island. The volcano’s tip may sink out of sightleaving a ring of coral islands—an atoll.

COASTLINES

THE COAST is a continuing battlebetween land and sea. Sometimes the

sea “loses” as shingle, sand or mud piles upand the land grows. In other places, the sea“wins” as waves, currents and tides batterand break up the coast. Even hard rocks likegranite are gradually worn away, especiallyduring storms when high winds whip uphuge waves powerful enough to smashpebbles and boulders against the shore.The shape and features of a coastline

depend on its rocks, winds and currents.Very hard rocks erode slowly and stand outas high headlands. Waves blow at the shorewith their greatest force in the direction ofthe main or prevailing winds in the region.Cliffs are sculpted both by the waves and byrockfalls and landslides. When wavesundercut soft rocks, these collapse on to theshore and break up into tiny fragments.Wide beaches may eventually form,protecting the cliffs behind them from thefull erosive force of the sea. A fast currentmay scour away broken rock particles fromone part of the shore. As it slows down, itdeposits them further along the coast as amudflat, sand bar or shingle spit.

W AV E SAs wind moves across the surface of theocean, the water turns over and over incircles, forming waves. The wave itselfmoves along, but the water in a wave doesnot. It spins around in the same place andmakes the water below turn, too. Theheight and power of a wave depend on thestrength of the wind and the expanse ofwater it has blown across. In mid-ocean,large waves, called swell, can develop.As a wave approaches the shallower

seashore (see illustration above), the lower partdrags on the sea bed, while the upper parttravels on until, eventually, it topples over, or“breaks”, on the shore. Waves, particularly during a storm, can be

powerful forces of erosion (see page 18).They cut away cliffs at the bottom, causingthem to collapse. A headland may beattacked on both sides, becoming narrowerand narrower. Joints and other weak areasare enlarged into sea caves. If caves form oneither side of a headland, they may join upas a tunnel, later becoming a natural arch. Ifthis collapses, a stack will result. This, too,eventually crumbles away over time.

L A N D S C A P E S

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Along this coast (above) a hard rocky outcrop has become anisland, cut off from the mainland by a flooded valley. Belowthe estuary, an arm of land is still linked to the mainland as apeninsula. A third outcrop has been wave-worn into anisolated pillar or stack, an arch and steep cliffs. Currentscarry shingle from the bay’s beach, slow down and drop it toform a pointed spit and sand bar. Nearby, river mud isdeposited to form a marshland and delta. A coral reef forms

around a volcanicisland.

Island sinks butbarrier reefgrows upwards.

Island disappearsto leave coral

atoll.

In mountainous regions, valleys have been gouged out byglaciers (see page 22). They have a characteristic U-shape.In some parts of the world, most notably Norway and NewZealand, U-shaped valley near the coast have been“drowned” by rising sea levels. These deep inlets, known asfjords (left), have very steep sides. Some fjords snake inlandfor many kilometres.

Saltmarsh

Island Estuary

PeninsulaCliffs

BeachArchStack

Spit

Bar

Delta



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TROPOSPHERE

A T M O S P H E R E

27

ATMOSPHERE

THE AIR we breathe is part of a thickblanket of air wrapped around the

Earth, known as the atmosphere. This air isa mixture of gases, mainly nitrogen (four-fifths) and oxygen (one-fifth). It gets thinneror less dense with height and fades awaycompletely about 800 kilometres above theground, where the atmosphere ends and thenothingness of space begins.The atmosphere has layers which rise and

fall in temperature as the air gets thinner.The troposphere extends to nine kilometreshigh over the poles and 16 above theEquator. It is only one-seventieth of theatmosphere’s total volume yet it containsfour-fifths of all the air. Its temperature fallsto minus 55°C, which marks the start of thestratosphere. The temperature rises here to10°C at about 50 kilometres high, wherethe mesosphere begins. It then plunges to alow of minus 75°C at 80 kilometres, beforerising again in the thermosphere.

IN TROPICAL REGIONS of the Earth(around the middle or Equator) it is hotall year round. Farther north thetemperature varies through the year. Itbecomes warmer in spring, hot in summer,cool in autumn and cold in winter. Thesetime periods are called the seasons. Theyhappen because of the way the Earth goesaround or orbits the Sun.The Earth’s orbit is not anexact circle around theSun, but an oval-likeellipse. Also, the Earthspins each day around animaginary line or axisgoing through the Northand South Poles, but thisaxis is not at right anglesto the orbit. It is tilted atan angle of 23.5°. Thecombination of tilted axis and ellipticalorbit produce the yearly cycle of seasons innorthern and southern regions.

SEASONS ANDCLIMATE

The atmosphere not only provides uswith oxygen to breathe. It shields us fromthe Sun’s harmful rays. Some of these arereflected by various layers such as thestratosphere and the clouds (above). Otherrays have their energy absorbed and spreadout through the atmosphere. The gas ozoneoccurs thinly in the stratosphere and absorbsmost of the Sun’s dangerous ultraviolet rays.

Weather varies from day to day aroundthe world (see page 28). Over a longerperiod, especially many years, each regionhas a regular pattern of rain, wind,temperature and other weather features.This long-term pattern of weather is calledclimate. It is due to the way the Earth orbitsthe Sun (top) and the way that oceancurrents and wind patterns (above) carry theSun’s warmth and rain-laden clouds aroundthe globe.

A T M O S P H E R E

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At the middle of the year (1) Earth’s top or northern half leanstowards the Sun. The Sun is nearer and higher in the sky forlonger each day so the north has summer. The southern halfleans farther away from the Sun and days are shorter so it iswinter. As Earth continues its orbit the tilt becomes sidewaysto the Sun (2), giving autumn in the north and spring in thesouth. At the year’s end the southern half leans nearer theSun and has summer while the north has winter (3), followedby southern autumn and spring in the north (4).

The map below shows the world’s main climate regions. TheSun is nearest and highest in the sky over the tropical regionson either side of the Equator. Also it shines directly downthrough the atmosphere here rather than at a low, slantingangle, so the atmosphere absorbs and scatters less of itsheat. This is why tropical regions are hot all year. If dry windsblow over a tropical region they cause a tropical desertclimate. At the top and bottom of the Earth are polar regionswhere the Sun is farther away and lower in the sky, so theseplaces are much colder. Between the topics and poles aretemperate lands which have warm summer and cool winters.The atmosphere’s

layers have varyingtemperatures. Thetroposphere at thebottom contains most ofthe clouds, winds andweather. The stratospherehas higher clouds and jetaircraft. The mesosphereabove becomes much colder,then the thermosphere warms toover 1400°C due to the Sun’sfierce heat hitting the very thinair. Meteors burn up as shootingstars and aurorae (northern andsouthern lights) glow. Starting at

about 500 kilometres theoutermost exosphere fades

into space.

Space shuttle

Aurorae(see page 5)

Militaryintelligencesatellite

Concorde

700km

600km

500km

400km

300km

200km

100km

50km

Meteors(shooting stars)

TROPOSPHERE

Tropic ofCancer

Winds result from unevenheating of different parts ofthe world. In the tropics,the surface is hot. Thisheats the air above it,which rises. Cooler airfrom the north and southblows in to replace it.These are called tradewinds. Their direction isaffected by the Earth’s spin.

Westerlies

Westerlies

Northeast trade winds

Southeast trade winds

Equator

Tropic of Cancer

Tropic of Capricorn

Atmosphere

Reflectedby clouds

Reflected byground

Absorbed byclouds

SUNLI G

HT

Tropical

DesertTemperate

Cool temperate

PolarMountain

1

4

Sun

2

3

22nd December

23rdSeptember

21st June

21stMarch

Earth’s axis

Equator

Equator

Tropic of Capricorn

STRATOSPHERE

M ES OSP H ER E

E X O S P H E R E( o u t e r a t m o s p h e r e )

T H E R M O S P H E R E



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3

4

2

1

C LO U D SA cloud is a vast gathering of billions oftiny water droplets, ice crystals or a mixtureof both (below). They are so small and lightthat they float in the air. Clouds form atvarious heights above the ground and havedifferent shapes (right). The names describethese shapes using a combination ofmeanings. For example, cirrus is wispy orfeathery, stratus is flat and blanket-like andcumulus is puffy and fluffy. Clouds form atground level, too. We call them mist if theyare thin and fog if they are thicker.It is possible to forecast the weather from

observing the different types of clouds.Highest at 10 kilometres and above arecirrus, made of tiny ice crystals. Thin andwispy, they signal fine, dry, settledconditions. Cirrocumulus are small, regular-shaped clouds that look almost like fishscales and make a so-called “mackerel sky”.Altostratus and altocumulus form atmedium heights and often mean that rain ison the way. Cumulus are the “cotton wool”clouds of a summer’s day. Stratus are lowclouds that cover the whole sky like a flat,pale grey sheet. Nimbostratus are evenlower and usually bring heavy rain or snow.The biggest and most impressive cloud is

the cumulonimbus. It towers 5000 metresor more, with a fluffy white top and flat,grey “anvil” base. It usually brings fiercestorms with thunder and lightning.

A T M O S P H E R E

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WEATHER

WEATHER is the conditions andchanges that take place in the lower

atmosphere, up to about 20 kilometres high.It includes temperatures by day and night,wind speed and direction, cloud type andcover, rain, hail, snow, frost, ice, droughts andstorms. Weather can change by the minuteor from day to day. The study of theweather is called meteorology.

W AT E R C YC L EA vital part of weather and climate is thewater cycle (above). On our planet, water isnot produced or destroyed—the same watergoes round and round in an endless cycle.In rivers, lakes and seas it is warmed by theSun’s heat. This evaporates or turns it intoan invisible gas, water vapour. The warmwater vapour rises high into the atmospherewhere it is colder, so the vapour cools andcondenses or turns back into liquid water. Itforms tiny droplets or ice crystals floating asclouds. These merge, become bigger and fallas rain or snow. The rain and melted snowflow into rivers, lakes and seas—and theendless cycle continues.

F R O N T SThe driving force for our weather is theSun. By day and night, winter and summer,it warms different parts of the Earth’ssurface by different amounts. It evaporateswater into the atmosphere to form cloudsand also makes some regions of air warmerthan others. Warm air rises and cooler airflows along to take its place, producingwinds. When warm air (above, 1) flows upand over colder, heavier air, the moisture init condenses, causing clouds to form andrain to fall. This is a warm front, shown onweather maps as a line with semicircles (2).When cold air (3) pushes against warm airalong a cold front (4), it forms a low wedge,bringing a narrow band of heavy rain andthen cooler, fresher, showery conditions.

D E W A N D F R O S TWater vapour is in the air around us,although we cannot see it. Sometimes itturns into liquid water or ice, which ofcourse we can see. When the Sun goesdown, the land cools faster than the air.Water vapour in the warmer air thattouches the cooler land condenses. Thiscovers everything with drops of waterknown as dew. If surface temperatures arebelow freezing, the water vapour turns intoa layer of sparkling ice crystals called frost.

A T M O S P H E R E

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Cirrus

Cirrostratus

Cirrocumulus

Altostratus

Altocumulus

Cumulus

Stratocumulus

Stratus

Waterdroplet

Icecrystal

Nimbostratus

Cumulonimbus

Clouds

Evaporation

Evaporation

Rain orsnow

Water flowsin rivers orthrough theground

Rain orsnow

Sea

Dew

Frost

9000metres

8000metres

7000metres

6000metres

5000metres

4000metres

3000metres

2000metres

1000metres

0metres



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+ +

– – –

+++

T O R N A D O E SThe fastest winds occur in the smalleststorms—tornadoes. A tornado usually formsat the rear of a thundercloud as the windsswirl at 400 kilometres per hour or more. Atwisting column of air grows from thecloud like an upside-down funnel. Its base isonly 20-100 metres across. But the windsare so powerful that animals, people, carsand even houses are plucked up into theclouds and then thrown outwards. As themain storm moves at 40-80 kilometres perhour the base may “skip” along the ground,touching down here and there to do mostdamage. Most tornadoes occur in theMidwest US states, east of the Andes inSouth America and in eastern India.

A T M O S P H E R E

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STORMS

THERE ARE many different kinds andsizes of storms. Most involve severe,

violent weather with regions of high windsand heavy rain, and perhaps a suddenchange in temperature. Some have thunderand lightning. They move across sea andland and may cause great damage and lossof life. Powerful winds blow down buildingsand bridges and toss cars and trucks aboutlike toys. Heavy rain or snow causes floods,mudslides or avalanches.Most storms begin as the Sun heats an

area of land or sea and causes warm air torise rapidly. Storms vary greatly in size andduration. A small tornado or “twister” mayhave a base just a few metres across and begone in half an hour. A typical thunder-storm is 5-10 kilometres wide and lasts for afew hours. A large hurricane may be morethan 2000 kilometres across and rage on fortwo or three weeks.

There are about 50,000 thunderstormsaround the world every day. Each second,100 bolts of lightning flash through cloudsor down to the ground. A typical bolt hasan electrical force of 100 million volts ormore, and lasts one-fifth of a second.Thunderstorms often become more severeover cities because warm, moist air risesfrom the buildings and adds to their power.

H U R R I C A N E SHurricanes begin as warm, moist air isheated by the fierce Sun and rises high intothe atmosphere, usually over the westernparts of the tropical Atlantic and PacificOceans. The rising air sucks in more air,which begins to swirl around in a spiral.

Many storms are parts of cyclones. Theseare regions or centres of low air pressurearound which winds blow. Because of theway the Earth spins around, the winds blowanticlockwise in the northern hemisphereand clockwise in the southern hemisphere.Near the Equator, winds blowing around atropical cyclone may increase in speed andtighten into a spiral to become a hurricane(known as a typhoon in the Pacific Ocean).

The moisture in the rising air condensesinto clouds and begins to fall as heavy rain.Quickly the hurricane balloons in size andthe swirling winds reach 250 kilometres perhour. The spirals of deep cumulonimbusclouds unleash massive downpours, bringingup to half a year’s average rainfall in a fewhours. The hurricane moves along at 25-50kilometres per hour as warmed air rises andswirls most powerfully near its centre. Mostspills out at the top and is flung to theedges where it sinks. A small portion driftsdown at the centre or “eye” of the storm.This is usually 25-50 kilometres across and,amazingly, it is calm with light winds andclear skies.

A T M O S P H E R E

30

Hurricanes may cause extensive damage. The winds roll cars,tip over planes, blow off roofs and uproot trees. They alsowhip up great waves that pound the coast.

In 1970 a tropicalcyclone (hurricane ortyphoon) whipped uphuge waves thatsurged over the low-lying mouth of theRiver Ganges inBangladesh. Up tohalf a million peoplelost their lives.

After hot, dry weather someparts of the land become verywarm. As cool, moist air blowsover them it is heated, risesfast, and cools. Its moistureturns into water droplets or icecrystals which form toweringcumulonimbus clouds. Thedroplets and crystals swirl upand down inside the cloud. Asthey bump together, theybecome charged with staticelectricity. This builds up untilit is suddenly released as agreat spark of lightning. Theheat of the flash makes the airaround it expand so fast itmakes a boom of thunder.

OutwardflowingwindsUpcurrents

Whole hurricanemoves along

Air sinks intoeye

Air spiralsupwardsaround eye

Warm moist airflows into stormregion

Eyeis

calm

A tornado causes anarrow but severetrail of damage 300kilometres or morein length.



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A Babyssal plain 9, 10aesthenosphere 8air 26, 28, 30-31Aleutian Trench 10Alps 11altocumulus 29altostratus 29Amazon, River 20amber 17ammonites 16Andes 11Angel Falls 20Antarctica 22, 23arches 23, 24, 25Atacama Desert 23Atlantic Ocean 8, 10atmosphere 26, 27, 28, 30layers of 26temperature of 26

atoll 25aurorae 5, 26basalt 6, 14, 16beaches 24buttes 23

CCanary Islands 15Cancer, Tropic of 27canyon 19Capricorn, Tropic of 27caves 21, 25cirque 22cliffs 18, 20, 24, 25climate 27climatic change 42clouds 26, 27, 28-29coastlines 24-25Colorado River 19condensation 28, 31continental drift 7, 8,

11, 18continental rise 10continental shelf 8, 10continents 6-7, 8-9, 10, 11convection currents (heat

currents) 5, 7, 9convergent boundary 9coral reefs 25core 5, 6-7, 13craters 14crevasses 22crust 6, 8-9, 12, 14, 16crystals 7currents 21, 24, 27cyclones 30

D Edelta 20-21, 24desert climate 19, 23, 27 deserts 19, 23Earth, atmosphere of 26formation of 6

magnetosphere 5mantle 6-7, 8-9, 13, 15marble 16Marianas Trench 10Mercalli scale 13mesas 23mesosphere 26metamorphic rocks 16, 17meteorites 7meteorology 28meteors 26mid-oceanic ridge 9, 10,

15minerals 16, 17, 21Mohorovicic discontinuity

(Moho) 6Mojave Desert 23Monument Valley 23Moon 4moraines 22mountain climate 23mountains 16, 18-19, 20,

22

N ONamib Desert 23New Zealand 25nickel 7Nile, River 20nimbostratus 29nitrogen 26Norway 25ocean floor 7, 9, 10, 12,

25oceans 6, 10Olgas, mountains 23oxygen 16, 26ozone 26

PP-waves 13Pacific Ocean 10Pacific Rim 13particles, high-energy solar

5Patagonian Desert 23peninsula 24Pinatubo, Mount 15planets 4plate tectonics 8-9plates, continental 9oceanic 9tectonic (lithospheric) 6, 8-9, 11, 12-13, 15,

polar climate 27

Rrainfall 20, 23, 27, 28-29,

30-31Red Sea 11Richter scale 13rivers 18-19, 20-21, 28rocks 4, 6-7, 9, 11, 12, 14,

16, 17, 18-19, 20-21, 22, 23, 24

Rocky Mountains 11, 19rotation 5

history of 19interior of 5, 6-7landscape features of 11, 18-25

orbit of 27rotation of 27, 30shape of 4surface temperature of 27, 28, 30

temperature of interior 5, 6-7, 8

tilted axis of 27earth movements 11,

12-13, 17, 18-19earthquakes 7, 12-13, 21East Pacific Rise 10Emperor Seamounts 10epicentre 12-13erosion 16, 17, 18-19, 20,

21, 23estuary 21, 24evaporation 28Everest, Mount 10exosphere 26

F Gfaults 11, 12fjords 25floods 18, 30folds 11fossils 17, 19fronts, weather 28geological history 19geysers 15glaciers 18, 20-21, 22, 25Gobi 23gorge 19Grand Canyon 18-19granite 6, 16, 24Great Rift Valley 11

H Iheat currents see

convection currentsHimalayas 8, 10, 11hurricanes 30-31ice 18, 20, 22, 28-29, 30igneous rocks 14, 16, 17iron 5, 6-7islands 24-25

K LKalahari Desert 23lagoon 25Lambert Glacier 22lava 7, 14, 16levees 20-21lightning 30limestone 16, 17, 20, 21lithosphere 8lithospheric plates see

tectonic plates

Mmagma 6-7, 14, 15, 16magnetic field 5, 7magnetic poles 5magnetism 5

SS-waves 13Sahara desert 23St. Helens, Mount 15salt marsh 24sand dunes 23seasons 27sedimentary rocks 16, 17sediments 16, 18, 20-21seismic (shock) waves

12-13snout (of glacier) 20, 22snow 22, 23, 28-29, 30solar wind 5springs 21stalactites 21stalagmites 21storms 30-31stratosphere 26subduction zone 8-9, 10,

12, 15Sun 4, 26-27, 28, 30

TTakla Makan Desert 23temperate climate 27thermosphere 26thunderstorms 29, 30-31tides 24tornadoes 30-31trench, deep-sea 8, 10, 12tributaries 20-21tropical climate 27troposphere 26tsunamis 13typhoon 30

U Vultraviolet rays 26valleys 11, 20, 22, 25Vesuvius 15volcanoes 7, 8-9, 13,

14-15, 16, 25active 14extinct 14fissure 15shield 14

Wwater cycle 28water vapour 28waterfalls 20wave erosion 18, 21, 24-25waves 24-25, 30-31weather 26, 27, 28-29,

30-31winds 23, 24-25, 27, 28,

30-31 trade 27

wind erosion 18, 23

I N D E X

32

INDEXPage numbers in bold

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