ILLUSTRATED

ENCYCLOPEDIA

EARTH

ISBN 978 1 7418 3769 2

9 781901 323511

Quartz (below) is amineral made of silica

found in rocks.

Sandstone A sedimentary rock made upof grains of sand compressed and

cemented together.

Sedimentary rock A type of rock that isformed by compressing rock fragmentsover millions of years. These fragments,such as sand, gravel and mud, are formedwhen other rock types are worn away bywind and rain. They settle in layers in lakes,rivers and seas. As the layers build up, theparticles are compressed and cementedinto sedimentary rock.

Sill A flat sheet of igneous rock formed asmagma pushes through the Earth’s crustand squeezes between older horizontallayers of rock.

Obsidian A dark, glassy rock formed whenlava cools down so quickly that itsminerals do not have time to crystallize.

Ore A rock that contains a metal.

Rock cycle The process by which rockschange from one type to another. Rockson the surface of the Earth are constantlyworn away by wind and rain. Thefragments of rock left behind may bepressed together to make sedimentaryrock or may be baked and compressed toform metamorphic rock. Alternatively, themolten rock may rise, cool and solidify toform igneous rock.

Slate A dark grey metamorphic rock. Thecrystals that make it up are very flat, soslate easily splits into thin slabs. This makesit a useful material for tiling roofs.

Stratum A layer of rock. When a cliff face isexposed by erosion or faulting, differentstrata can often be seen in the rocks.

Weathering The breaking down of rocksdue to the weather. Water seeps into tinycracks in the ground and dissolves awaythe rock. In cold weather, ice forms in thecracks and expands, splitting off pieces ofrock. Tree roots can make the rock breakapart even faster.

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Rock is the hard substance thatmakes up the Earth’s crust. Rocks liebeneath both the soil and the

depths of the ocean. They are made up ofa solid mixture of minerals, consisting ofdifferent chemical substances. There aremany different kinds of rocks, but theycan all be divided into three main groups.These are: igneous rocks, sedimentaryrocks and metamorphic rocks.

Metamorphic rock A rock that haschanged due to intense heat and pressureunderground. Sedimentary rocks, igneousrocks and other metamorphic rocks mayall undergo this change. When a rock ischanged in this way, its mineralsrecrystallize into different shapes.

Marble A type of metamorphic rockformed when limestone is heated to hightemperatures. Marble has a smoothstructure and can be almost any colourfrom pink, to blue, to black.

Basalt A dark grey igneous rock. Theentire ocean crust and much of the Earth’ssolid surface is made of basalt.

Batholith A large mass of igneous rockformed when an upwelling of magmaspreads through rocks underground, butnever reaches the surface. It may berevealed later when erosion wears awaythe rocks above it.

Bedrock The solid rock that lies beneaththe soil.

! The oldest rocks on the Earth are about3800 million years old.

! Feldspar is the most commonlyoccurring mineral on the Earth.

! Diamond is the hardest naturalsubstance found on the Earth. It is a type ofgemstone that is usually clear andcolourless. Diamond forms when carbon,the element that makes up coal, issqueezed in theEarth’s mantle.

! The study ofrocks is calledpetrology. It comesfrom the Greekwords petra,meaning rock, andlogos, meaningknowledge.

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The Giant’s Causeway (above) is the result of lavacooling very slowly into large crystal shapes.

Sea

Sedimentslaid down

Risingmagma

Pressure from Earthmovements deepunderground formsmetamorphic rocks

Older, deeper layers hardeninto sedimentary rock

Glaciers, streamsand rivers carrysediments awayfrom the land

Magma coolsunderground toform intrusiveigneous rock

Volcanoes

Marble

1716

THE ROCK CYCLE

Mineral A naturally occurringsolid chemical substance.

Rocks are made up of variousdifferent types of minerals, such

as quartz, feldspar and mica.

Moh’s scale A scale used tomeasure how hard a mineral is,

where 1 is the softest and10 is the hardest.

Limestone

Mantle

Subductionzone

Crust

Volcano

1

2

3

4

Laccolith A dome of igneous rock that liesbetween layers of sedimentary rock. It isformed when magma pushes betweenthe sedimentary rock strata.

Limestone A type of sedimentary rock,often made up of the remains of tiny seacreatures that lived millions of years ago.It is made up of calcium carbonate, whichdissolves easily in water.

Certain conditions can make ordinary rocks changeinto gemstones. Diamonds form in the Earth’smantle (1). Rubies and sapphires occur in somemetamorphic rocks (2). Amethyst forms in coolingvolcanic rock (3). Jade (4) is produced in subductionzones ( 11), while topaz is found in granite (5).

5

Diamond ring

Granite

Crystal A substance with a regulararrangement of atoms (the basic buildingblocks of matter). Crystals form whenmolten rock cools and solidifies. The moreslowly the rock cools, the larger thecrystals will be. If the rock cools too quicklythe crystals may not have time to form.

Dyke A sheet of rock formed by magmarising vertically through the Earth’s crustand cutting across surrounding rock strata.

Erosion The transportation by water, windor ice of rock fragments that have beenbroken down by weathering.

Gemstone A type of mineral that is valuablebecause it is rare and beautiful. Diamonds,rubies and topaz are all types of gemstone.

Granite A hard, pink-grey igneous rock,made up of large, clearly-visible crystals.

Igneous rock A type of rock formed whenmagma ( 13) rises to the Earth’s surfaceand solidifies. Intrusive igneous rock, suchas granite, is formed when magmasolidifies beneath the ground. Extrusiveigneous rock, such as basalt, is formedwhen it solidifies above the ground.

! Covers 12 major subject areas of Earth science

! More than 250 keywords alphabetically listedand clearly explained

! Fact panels with extra information

! More than 70 detailed illustrations

! Comprehensive index

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EEAARRTTHHFirst published in 2012 by Orpheus Books Ltd., 6 Church Green, Witney, Oxfordshire, OX28 4AW, England

www.orpheusbooks.com

Copyright ©2012 Orpheus Books Ltd.

Created and produced by Nicholas Harris, Sarah Hartley, Katie Sexton, Ruth Symons and Erica Williams, Orpheus Books Ltd.

Text Ruth Symons

Illustrated by Julian Baker, Peter Dennis, Gary Hincks, Steve Noon, Nicki Palin

Consultant Professor Ian J. Fairchild, Director of Research, School of Geography, Earth and Environmental Sciences,

University of Birmingham

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 writtenpermission of the copyright owner.

ISBN 978 1 7418 3769 2

Printed and bound in Singapore

Photograph on page 9: Jan CurtisPhotograph of rock formation on page 22: Copyright ©2010 James M Phelps, Jr

Used under licence from Shutterstock.comPhotograph of Twelve Apostles on page 25: Copyright ©2010 CSLD

Used under licence from Shutterstock.com

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Each double page contains a brief introduction,explaining the general subject, followed by key

words arranged in alphabetical order. To look up aspecific word, turn to the index at the back of thisbook: this will tell you which page to go to. If you want tolearn more about a subject, take a look at the factfile, or followthe arrows to read related entries.

Cold desert An area with less than 25 cmof snow or rainfall a year, and with anaverage temperature of less than 10°C.For example, Antarctica is a cold desert,as it receives very little snow.

Desert pavement A hard, thin desertsurface of densely packed pebbles androcks. Desert pavements form when sandand small rocks are blown away, leavingbehind heavier lumps of rock.

Erg A flat, sandy area of desert.

Hamada A flat, rocky area of desert, withlittle or no sand covering.

Sand Tiny pieces of rock. Sand is formedwhen larger rocks are worn down by windand rain.

Sand dune A heap, or bank, of sand thathas been shaped by the wind. Sand dunesform where sand, blown by the wind, isstopped by an obstacle, such as a boulder,a plant or a change in the shape of theshape of the land.

Wadi A steep-sided gorge. It is formedwhen fast-flowing water from rarerainstorms wears away desert rocks.

Wind erosion The gradual wearing awayof the land caused by the wind. Winderosion gradually blows away soil andsand. Strong winds may hurl sand againstrocky surfaces, where it acts likesandpaper, wearing away the rock.

Plateau A flat, raised area of land.Plateaus form when the movement oftectonic plates ( 10) pushes up wholesections of flat rock. Buttes and mesas areboth types of plateau.

Playa A flat, dried-up lake, sometimescalled a salina. During a rare rainstorm, theplaya may briefly fill up with rainwater.

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Arid Very dry. Arid regions, such as deserts,have too little water to support plant life.

Barchan A crescent-shaped sand dune thatslowly shifts along with the wind.

Butte A desert hill with very steep sidesand a flat top. A butte is made up of hardrock that does not easily wear away. It islike a mesa but smaller.

Adesert is a dry area that receivesless than 25 centimetres of rain peryear. Most people think of deserts

as vast sandy regions, but only 20 per centof the world’s deserts are sandy. The restare bare rock, or covered with gravel.When rain does fall in occasional storms,there is no soil to soak it up so it quicklydries up. Some deserts are hot all yearround and some are always cold. In manydeserts, strong winds blast sand at therocks, carving out amazing shapes.

! Deserts and arid regions cover one-eighth of the world’s land area.

! The largest hot desert in the world is theSahara in Africa, which is over 5000 km wideand covers 9 million square km.

! Antarctica is the largest desert in theworld. It covers over 13 million square km. It receives very little snow each year—theequivalent of just 5 cm of rain.

! The continent with the largestproportion of desert—about one half itsarea—is Australia.

! The driest desert is the Atacama in Chile,South America, with an average of less than1 mm of rain per year.

! “Hot” deserts can be bitterly cold atnight. The Takla Makan Desert in China can be a scorching 40°C by day yet plunge totemperatures of - 40°C at night.

Zeugen A mushroom-shaped rock in thedesert. It is formed by wind erosion. Mostof the sand carried by the wind is foundabout a metre above the ground. Zeugensform when wind-blown sand blasts awaythe base of a boulder, leaving a narrow,top-heavy rock formation. Zeugens aresometimes called rock pedestals.

Rain shadow An area of land that receiveslittle rain because it lies on the far side of amountain range to the prevailing winds.Winds blow moist air over the mountains,where moisture condenses in the cold andfalls as rain or snow. By the time windreaches the far side of the mountains, ithas lost most of its moisture.

Salt flat A layer of salt crystals that formsin a dried-out lake. During a briefrainstorm, salts in the rocks of the desertdissolve in the warm rainwater. A pool ofwater may collect in an area of lowground. When it dries out, a layer of salt isleft behind. After each rainfall the layer ofsalt becomes thicker.

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MAP OF DESERT FEATURES

Naturalarch

Wadi

Salt flat

Zeugens

Oasis

Sand dunes

Natural arch

The Olgas, in western Australia, are the result ofextreme daily heating and cooling. These cause rocklayers to flake away, leaving smooth, rounded blocks.

A rock formation in the Valley of Fire park inAmerica (above). The park is named after itsdistinctive bright red sandstone. Monument Valley in America (below) has manyflat-topped buttes and mesas.

An oasis

2120

Mesa

Barchan dunes

Hoodoo

Hoodoo A strangely shaped column ofrock. Hoodoos are made up of layers ofsoft and hard rock, which the wind andsand erodes at different rates, producingbizarre shapes and colours, andsometimes a stripy appearance.

Hot desert An area with less than 25 cm ofsnow or rainfall a year, and with anaverage temperature of over 20°C.Hot deserts are cold at night because theydo not have any cloud cover to trap heat.

Mesa A desert mountain with a flat topand very steep sides. Mesas are formedwhen layers of soft rock are worn away bywind and water, revealing a block ofharder rock. Mesas take their name, theSpanish word for “tabletop”, from theirdistinctive, flat-topped appearance.

Natural arch A rock that is shaped like anarch. Natural arches form when the windforces open a crack in a rock face.

Oasis A lush area of water and trees in themiddle of a desert. An oasis forms wherean underground river comes to thesurface at a spring.

INTRODUCTIONThis explains thegeneral subjectand provides somebasic knowledge.

KEY WORDS AND ENTRIESKey words are arranged alphabeticallyacross each double page. Each entryprovides a short explanation of whatthe key word means.

PAGE NUMBERPage numbersare easy to findat the side ofthe page.

BOLD WORDSThese highlightuseful words thatdo not have theirown entry.

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ARROWS These arrows show you where to look up otherwords mentioned in the entry. For example, ( 26) tells you to go forward to page 26 and ( 6) tells you to turn back to page 6.

FACTFILEThe factfile provides extrainformation on the subject. Factsare presented in easy to readbullet points.

76

11 Once the planet’s surface had cooled, water began tocollect in low basins. These filled up to form the first oceans.

4 Fragments of dust spinning around the Sunbegan to clump together, becoming at firstsmall bands of rocks, then larger boulders,before growing into chunks severalkilometres across, known as planetesimals.

5 The planetesimals started to collidewith each other, building up tobecome the four rocky inner planets,Mercury, Venus, Earth and Mars.

9 In its early days, the Earth was abarren planet. Unprotected by anatmosphere it was continuallybombarded by meteorites. The Earth’s surface temperaturerose until the planet became aglobal sea of molten rock.

10 Eventually the surface cooled. Gases, trappedbeneath the newly solidified surface, burst through the

crust in volcanoes. They collected toform an atmosphere. Water vapour

rose to form huge clouds.

7 The Moon may have formed when alarge object collided with the newly-formed Earth.

Moon

Earth

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Planet Earth is one of eight planetsorbiting the Sun. The Sun is one ofbillions of stars in the Milky Way

Galaxy, itself one of billions of galaxies inthe Universe. The Universe came intoexistence 13.7 billion years ago. It was notuntil about 4500 million years ago thatthe Sun and its family of planets,including Earth, were formed. No one cansay for sure how the Earth was formed,but many scientists agree on a likelysequence of events, as illustrated here.

2 Something—perhaps a series ofshock waves emitted from anexploding star nearby—caused thecloud to clump together. A huge,swirling disc was set in motion.

1 The Solar System startedout as a cloud of gas anddust drifting in space.

3 Matter fell towards the centre and became hotterand denser than the edge of the disc. The core ofintense energy was the beginnings of our Sun.

6 Heavy particles ofiron and other metalssank to form the coresof the planets.

8 The impact “splashed” vastamounts of debris into space,surrounding the Earth. These particlescame together to form the Moon.

Magnetic field The region surrounding amagnet, an object which has two polesand a force of attraction between them.The Earth has its own magnetic field,which stretches into space and protects itfrom the Sun’s high-energy particles.

Magnetic poles The two points at eitherend of a magnet, where its force is at itsstrongest. The Earth’s magnetic poles arelocated close to the geographical northand south poles. These are the two pointswhere the Earth’s axis meets the surfaceof the Earth.

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The Earth is a spinning ball of rockand metal. It is one of eight planetsthat orbit, or circle, our nearest

star, the Sun. Its surface is made up ofoceans and landmasses called continents.A layer of air called the atmosphere ( 28)

surrounds the whole planet. The Earth’souter layer, the crust, is a thin, rockyshell. Beneath the crust lies the mantle, athick layer made of hot, dense rock. At the very centre of the Earth is its core,a ball of metal, liquid on the outside, solid on the inside.

Asthenosphere A soft layer in the Earth’supper mantle. It lies just beneath thelithosphere. It is partly molten and allowsthe lithosphere to slide about over it.

Aurora The display of coloured lights seenin the night skies close to the Earth’smagnetic poles. It occurs when high-energy particles from the Sun are trappedby the Earth’s magnetic field.

Axis An imaginary straight line that runsNorth to South through the centre of the Earth. The Earth rotates, or spins, on its axis.

The aurora borealis, or Northern Lights, light upthe skies of the Northern hemisphere ( 28).

Mantle The rocky layer of the Earth that lies between the crust and the core. It ismade up of the upper and lower mantle.

Moho (Mohorovi!i" discontinuity) Theboundary between the Earth’s crust andmantle. It is recognized because of the wayit affects the movement of seismic wavestravelling through the Earth ( 11).

Outer core The outer layer of the Earth’score. It is 2200 km thick and made upmostly of molten iron. Temperatures hererise to more than 4000°C. The liquid outercore spins around the solid inner core asthe Earth rotates. This movement, knownas rollers, is probably the cause of the

Earth’s magnetic field.

Rollers The twisting movement of liquidiron inside the Earth’s outer core.Convection currents in the core causeliquid metal to swirl around. The currentsare twisted by the spinning motion of theEarth’s rotation and form giant corkscrew-like patterns called rollers. Thesemovements make electricity, whichcreates a magnetic field around the Earth.

Upper mantle The upper 600 km of theEarth’s mantle, made of crystals of solidrock with liquid rock between thecrystals in some places. The molten rock, calledmagma, is under greatpressure and can collect toburst out of holes orcracks in the crust asvolcanoes ( 12).

The Moon orbits the Earthonce every 27 days.

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THE EARTH’SINTERNALLAYERS

Sun

EarthMoon

Earth’s orbit

Outercore

Mantle

Inner core Outer

core Mantle

Crust

Earth

High

-ene

rgy p

artic

les

from

the S

unThe Earth orbits the Sunonce every 365 days.

Magnetosphere tail

CONVECTION CURRENTSIN THE EARTH

Continent One of the seven largelandmasses on the Earth. They also includethe continental shelf ( 22), an area thatextends beyond the seashore.

Convection current The movement of heatthrough liquids and gases. Heated frombelow, a liquid or gas will expand, becomeless dense, and rise. Away from the sourceof heat, it will cool down and sink.Convection currents in the Earth’s mantleare responsible for continental drift ( 10).The orbits of the Earth and Moon

Magnetosphere The region around theEarth in which its magnetic field exerts aforce. The magnetosphere stretchesthousands of kilometres into space andprotects us from the Sun’s high-energyparticles, known as the solar wind. Thesolar wind “blows” the magnetosphereinto a teardrop shape.

THE MAGNETOSPHERE

98

The Earth spins onceevery 24 hours.

Continent

Ocean floorCrust

Lines ofmagnetic force

N

S

THE EARTH’S MAGNETIC FIELD

DDiiaammeetteerr:: 12,756 kmDDaayy:: 23 hours 56 minutesAAvveerraaggee ddiissttaannccee ffrroomm tthhee SSuunn:: 149.7 million kmSSuurrffaaccee tteemmppeerraattuurree:: -70°C to +55°CAAttmmoosspphheerree:: nitrogen, oxygen, water vapour

! The Earth is about 4.5 billion years old.

! The Earth is not a true sphere, but is aslightly squashed shape called an oblatespheroid. The distance around the Earth’sequator is slightly greater than the distancearound the Earth from pole to pole. This iscaused by the spinning motion of the Earthpushing its mass out around the equator.

! The deepest anyone has ever drilleddown into the Earth is 15 km.

Core The innermost part of the Earth,mostly made of iron with a little nickel. It isdivided into the outer and inner core.

Crust The thin, rocky outer layer of theEarth. There are two main types of crust:continental crust and oceanic crust.Continental crust is between 35 and 70 kmthick. Oceanic crust is only 5 to 10 km thick.

Inner core The very innermost part of theEarth. The inner core is a solid ball of themetal iron, about 2500 km in diameter. It isunder so much pressure that it is solid evenat temperatures of 5430°C.

Lithosphere A layer of the Earth made upof its crust, plus a thin layer of the uppermantle. It is split into large pieces calledlithospheric, or tectonic, plates ( 11). Theseslide about on top of the asthenosphere.

Lower mantle The inner layer of the Earth’s mantle. It is about 2300 km thick.The pressure exerted by the layers aboveprevents the rock from melting.

Focus The point beneath the Earth’ssurface where rocks break or shift,triggering an earthquake.

Aftershock A small earthquake thatimmediately follows a larger one.Aftershocks occur as rocks in the Earth’scrust re-adjust after the main shock.

Continental drift The movement of thecontinents ( 8) around the globe. Thecontinents are attached to the tectonicplates, and are carried by their movements.

Fold A bend in the rocks of the Earth’scrust. Folds occur when intense pressure isexerted on rocks by movements in thecrust. Folds vary in size from small wrinklesto large mountains.

Mercalli Scale A scale used to measure theamount of damage caused by anearthquake. The scale ranges from 1, anearthquake so small it can barely be felt, to12, an earthquake that causes totaldevastation and changes the landscape.

Richter Scale A scale used to measure thestrength, or magnitude, of an earthquake.Earthquakes with a magnitude of less than2 cannot be felt. Earthquakes with amagnitude of 7 or more can cause hugeamounts of damage and loss of life.

The Earth’s outer layer, thelithosphere, is divided intoenormous irregular slabs, called

tectonic plates. These fit together like aball-shaped jigsaw. The plates “float” ontop of the liquid asthenosphere ( 8).Convection currents ( 8) in the mantlepush the plates and makes them move. In some places, the plates push againsteach other and crumple, formingmountains. In other places, they pull awayfrom one another and molten rock wellsup between them. Sometimes plates locktogether for a time, then suddenly joltapart, causing earthquakes.

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! There are about 500,000 earthquakesevery year. About 100,000 of these can befelt without detection equipment. Everyfew years a massive earthquake results ingreat loss of life.

! Major earthquakes only cause large lossof life in towns and cities. The suddenviolent shaking of the ground may collapsebuildings and bridges, and burst pipes.Broken cables can lead to fires. In some places affected by earthquakes,houses are now built to resist shaking.

! The strongest earthquake ever recordedwas the Great ChileanEarthquake of22nd May 1960. Ithad a magnitudeof 9.5 on the RichterScale.

Rift A place where the Earth’s crust isbeing pulled apart by the movement oftectonic plates. Rifting causes rocks at theEarth’s surface to crack, creating faults. Ablock of land may slip down betweenfaults to form a rift valley ( 15).

Seismic waves Waves of pressure thatmove through the Earth. They are causedby earthquakes. Primary waves travelthrough all layers of the Earth, squeezingand stretching rock. Secondary wavesshake rock from side to side. They areslower than primary waves and cannottravel through the Earth’s liquid outercore ( 9).

When an oceanic plate collides with acontinental plate, the thinner, denser oceanplate slides beneath the continental plate ina subduction zone. The rocks of the ocean

floor sink deeper and deeper into the Earthand melt. Some of this molten rock risesthrough cracks in the continental crust,erupting at the surface as volcanoes.

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A damaged city after a strong earthquake

A tsunami rears up as it enters shallow water.

These two maps of the Earth show how its surfaceis divided up into tectonic plates.

Continental crust

Seamount

Magmaplume

VolcanoSubductionzone

Direction ofheat flow

Oceanic crust

Moltenrock rises

Convection currents

Mid-oceanic ridge

Abyssal plain

Deep-seatrench

Continentalshelf

Transformfault

Divergent boundary

Convergentboundary

Convergent boundary A boundarybetween two tectonic plates that arepushing against each other. If one of theplates is oceanic, its edge may be forceddown under a heavier continental plate. Ifboth plates are continental, they willcollide and crumple up into mountains.

Divergent boundary A boundary betweentectonic plates that are moving away fromeach other. Molten rock wells up to fill thegap between the plates. Most divergentboundaries are between oceanic platesand are called mid-oceanic ridges ( 23).

Earthquake A shaking or trembling of theground, caused by the sudden movementof rocks in the Earth’s crust. Mostearthquakes happen along the edges oftectonic plates, when plates collide or slidepast each other. The two plates locktogether before the pressure becomes toomuch and the rocks suddenly snap apart.

Epicentre The point on the Earth’s surfacedirectly above the focus of an earthquake.

Fault A crack in the Earth’s crust, alongwhich there is movement of one siderelative to the other. Faults form whenrocks are stretched or bent and crackalong weak points.

10 11

Transformfault

Some earthquakes occur alongsubduction zones where one tectonicplate with oceanic crust slides downbelow the edge of a continent.

Japan

PacificOceanVolcanoes

Movement ofcontinental crust Movement of

continental crust

Subductionzone

Focus

Seismic wave

Seismometer An instrument used tomeasure the strength of an earthquake.

Subduction The process by which onetectonic plate slides beneath another. Theplace where this occurs is called asubduction zone. A thinner, denser oceanicplate will slide beneath a continental platewhen the two collide.

Tectonic plates The large slabs into whichthe Earth’s surface is divided. They aresometimes called lithospheric plates. Eachplate consists of a piece of the Earth’scrust, plus a portion of upper mantle,which together form the lithosphere ( 8).

Transform fault A boundary betweentectonic plates that are sliding past each other. The San Andreas Fault, whichstretches 750 miles along the west coast ofNorth America, is an example of atransform fault.

Tsunami A massive wave caused by anearthquake deep under the seabed. In theopen ocean, tsunami waves are barelynoticeable, but as they near the shorethey grow much bigger—up to 30 m high.The wall of water surges ashore with anunstoppable force, quickly floodingcoastal areas.

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Aa Lava that carries lumps of solid rockcalled clinker.

Ash Lava that has been blown to powderby the force of an eruption. Ash cloudscan reach up to 40 km into the sky.

Caldera A large, deep hole formed when avolcano collapses in on itself after aviolent eruption.

Crater A circular, funnel-shaped openingat the summit of a volcano.

Dormant volcano A volcano that hasstopped erupting, but which may burstinto life again in the future. Mount St.Helens in the USA was a dormant

volcano that came back to lifespectacularly in 1980.

Magma chamber An underground pool ofmagma. Over time, pressure in thechamber builds up, forcing magma topush through cracks in the Earth’s crust.Many volcanoes are situated above amagma chamber.

Magma Hot, molten rock beneath theEarth’s surface. Magma is formed in theEarth’s mantle. It is composed mostly ofsilicates, with gases contained in bubbles.

Pahoehoe Very runny lava that flowsbeneath a hardening, rocky crust. Oncooling, it has a smooth, billowy textureand rope-like appearance.

Pillow lava Runny lava that erupts underthe sea and hardens into round, pillow-shaped lumps of rock.

Pumice A volcanic glass “froth” formedfrom cooling, gassy lava. It containsnumerous bubbles.

Pyroclastic flow The extremely hotmixture of volcanic rock and gases thatsweeps down a volcano’s slopes at morethan 300 km/h, destroying everything inits path. The residents of Pompeii werewiped out by a pyroclastic flow when MtVesuvius erupted in AD 79.

Ring of Fire An area surrounding thePacific Ocean where large numbers ofearthquakes and volcanic eruptions occur. There are about 452 volcanoes in the Ring of Fire.

Shield volcano A large volcano withgently sloping sides. Shield volcanoesbuild up over time from repeated lavaflows. They erupt thin, runny lava.

Silicates Types of mineral ( 17) thatcontain silicon and oxygen. Silicates makeup more than 90% of the rocks in theEarth’s crust. Thin, runny magmas consistof different types of silicates to those thatmake up thick, viscous magmas.

Stratovolcano A tall, steep, cone-shapedvolcano. Stratovolcanoes are formed fromlayers of volcanic lava, built up by multipleeruptions over hundreds of thousands ofyears. They erupt ash, pumice and thick,viscuous lava. Stratovolcanoes are themost common type of volcano.

Tuff A type of rock made of volcanic ashwelded together. It was used by theRomans to make many buildings andbridges, and by the natives of Easter Islandto make their famous statues.

! The eruption in Tambora, Indonesia in1815 killed about 92,000 people. The hugevolume of ash in the atmosphere cooled theworld climate for more than a year. It wascalled “the year without a summer”.

! The world “volcano” comes from theItalian island of Vulcano. Centuries ago,people believed that it was the chimney ofthe forge of Vulcan, the Roman god of life.

! There are at least 1500 active volcanoesabove sea level around the world. Indonesiahas the most: 86 have erupted in its history.

! One in 10 people live within “dangerrange” of a volcano.

! Volcanic ash forms very fertile soil, soplants grow quickly after an eruption.

! More than 80% of the Earth’s surface is volcanic in origin. The ocean floor was formed entirely by lava from volcanic eruptions.

! In 1943 a Mexican farmer watched aneruption in his field. He came back the nextmorning to find a volcanic cone 10 m high. Iterupted repeatedly: after one year, the conehad grown to 336 m.

Avolcano is an opening, or vent, inthe Earth’s crust through whichmagma, molten rock, erupts.

The word volcano is usually used todescribe a cone-shaped mountain with acentral vent and a crater at the summit,but this is not always the case. Many volcanoes have gently spreadingslopes. Others are simply cracks in theocean bed. Most volcanoes are situatedalong the edges of the giant plates thatmake up the Earth’s surface. More thanhalf of the world’s active volcanoes abovesea level encircle the Pacific Ocean toform the so-called “Ring of Fire”.

Hot spot A weakness in the Earth’s crustwhere magma is liable to force its waythrough. Over time, solidified lava builds upand may eventually break the surface ofthe ocean. The Hawaiian Islands, forexample, are the summits of volcanoesrising from the floor of the Pacific Ocean.

A close-up ofash shows thatit is made ofpieces ofpumice.

The HawaiianIslands are a seriesof volcanoescreated as anoceanic plate driftsover a hot spot.

Direction of platemovement

Hawaiian Islands

Rising magma

Undersea volcanoes on the mid-oceanic ridge

A geyser erupts

Rock churches carved out of soft, volcanic rock inCappadocia in southern Turkey. Fast-moving, runny lava

flows are characteristic ofvolcanoes that eruptfrequently andcomparatively gently.

Lava flows

Clouds ofdust and ashCrater

VentLayers of dustand ash frompreviouseruptions

Secondaryvent

Magmachamber

Eruption As pressure in the magmabeneath the Earth’s crust builds up, itforces its way up through cracks anderupts at the surface as lava. If themagma is quite runny, the gas containedin it can escape easily. The lava oozes outof the opening and flows down the sidesof the volcano. If the magma is thick andsticky, the gas cannot escape, but buildsup until it explodes. Huge clouds of gasesand ash fly high up into the air.

Extinct volcano A volcano that has noterupted for tens of thousands of years.

Fissure A large crack or opening in theEarth’s crust. Lava may emerge through a fissure without an explosive eruption.Undersea volcanoes are fissures in the seabed.

Geyser A natural fountain of hot water orsteam that periodically ejects from anopening in the Earth’s crust. Geysers eruptwhen underground water comes intoclose contact with hot, volcanic rocks.

Lava Magma that has erupted at theEarth’s surface through volcanoes. It mayflow as hot, molten rock for a while, butthen quickly cools and solidifies. Lavasvary from thick to runny, according to theamount of gas contained within them andthe type of silicates they are made of.

Vent An opening in the Earth’s surfacethrough which lava and gases seep out.

Viscosity A measure of the fluidity ofmagma and lava (and other liquids). Thick lava is more viscous than thin, runny lava.

CROSS-SECTIONTHROUGH A

VOLCANO

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Ice shelf A large sheet of ice that extendsout from the land to float on the sea.

Iceberg A large lump of ice floating in thesea. About 90% of it lies underwater. Arctic icebergs break away from glaciers.Antarctic icebergs break away from iceshelves and are often flat-topped.

Arête A narrow ridge between glacialvalleys. It is formed when two glacierscarve out adjacent valleys.

Bergschrund A deep gap formed when aglacier pulls away from the solid,immobile wall of ice behind it.

Cirque A semicircular basin in amountainside, from which compactedareas of snow spill out to form a glacier.

Crevasse A deep crack in a glacier. It isformed when ice splits as it moves overuneven rocks or turns a corner.

Dome mountain A mountain that isformed by molten rock pushing the rockabove it into a large lump.

Drumlin A smooth, tear-shaped hill,formed when glaciers move over soft clayor gravel that has been deposited thereby previous glaciers.

! Ice covers about 15 million square km,nearly one-tenth of the Earth’s land surface.

! The largest glacier in the world isAntarctica’s Lambert Glacier, which is usually more than 500 km long.

! The word iceberg comes from theGerman word eisberg, which means “ice mountain”.

! The highest mountain in the world is Mt Everest on the Tibet-Nepal border. Its summit is 8848 m above sea level. Everestis part of the Himalayan range of mountains.

! The highest mountain in the Solar Systemis Olympus Mons, on the planet Mars.

Moraine The soil and rock carried anddeposited by glaciers. Lateral moraine isthe debris carried at the sides of a glacier.When two glaciers join, their lateralmoraines form a medial moraine in themiddle of the combined glacier. A terminalmoraine is the ridge deposited at the end,or snout, of a glacier when it melts.

Nunatak A mountain summit that pokesthrough the top of a glacier or ice cap.

Post-glacial rebound The rising up of landthat has previously been pressed down bythe weight of ice. For example, much ofScandinavia, which was covered byglaciers almost 20,000 years ago, is stillrising by about 1 cm per year.

Randkluft A deep crack formed where aglacier pulls away from the wall of rockbehind it.

Rift valley A valley with steep slopes oneither side. Rift valleys (also known asgrabens) are created when a strip of landslips down between two faults ( 10).

Roche moutonnée A smooth, tear-shapedmound of bedrock ( 16) formed by glacial erosion.

Serac A large block of glacial ice that forms between two crevasses. Seracs areoften unstable, and prone to collapsing. For this reason, they pose a great risk to mountaineers.

Snout The lowest point, or end, of aglacier, sometimes called its terminus.

Amountain is a high peak of land. A glacier is a moving mass of icethat is formed when layers of

snow pile up and compress. The ice thenstarts to move under its own weight.Some glaciers snake down mountainvalleys, while others are so huge andthick they almost totally cover the land.

MMOOUUNNTTAAIINNSS&& GGLLAACCIIEERRSS

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Summit The highest point of a hill or mountain.

Tributary glacier A small glacier that leadsinto a larger one.

Valley A low area of land betweenmountains or hills. Glacial valleys arecarved out by glacial erosion. River valleysare shaped by river erosion ( 18).

Ice cap A large glacier that envelops anarea of land. The tops of mountains maybreach the surface of the ice.

Ice sheet An ice cap that covers wholecontinents for a long period of time.Today the only ice sheets are in Greenlandand Antarctica.

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A fjord winds through the mountains of Norway.

An iceberg

Meltwater stream The water that comesfrom a melted glacier. Glacial meltwatermay run under the glacier to form lakes infront of its snout.

Moulin A vertical hole through a glacier,carved out by meltwater trickling downthrough cracks in the surface of the ice.

Glacial ice

CrustMantle movement

CrustMantle movement

Esker A narrow, winding ridge of gravel,deposited by meltwater streams runningbeneath a glacier.

Fjord A deep U-shaped valley that hasbeen gouged out by a glacier and“drowned” by rising sea levels. There aremany fjords on the coast of Norway andNew Zealand.

Fold mountain A mountain that is formedwhen tectonic plates ( 10) collide,squeezing layers of rock up into wrinkles.

Glacial erosion The gradual wearing downof the land by the movement of glaciers.Rocks and pebbles carried on the bottomand sides of a glacier scrape along theground, acting like sandpaper.

Hanging valley A small glacial valley thatjoins a larger valley. The base of thesmaller valley is much higher up than that of the main valley and so “hangs”over its side. Hanging valleys are formedby small, tributary glaciers flowing intolarger glaciers.

Horst A block of land that is pushedupwards between two faults ( 10) so thatit is higher than the land around it. Horstsare sometimes called block mountains.

The peaks of the Rockies, a mountain range in North America (below)

Cirque

Moraines from twoglaciers meet together.

Glacier

Snout

Terminalmoraine Meltwater

streams

POST-GLACIALREBOUND

A GLACIER

EskerDrumlins

Medialmoraine

Lateralmoraine

Roche moutonée

Quartz (below) is amineral made of silica

found in rocks.

Sandstone A sedimentary rock made upof grains of sand compressed and

cemented together.

Sedimentary rock A type of rock that isformed by compressing rock fragmentsover millions of years. These fragments,such as sand, gravel and mud, are formedwhen other rock types are worn away bywind and rain. They settle in layers in lakes,rivers and seas. As the layers build up, theparticles are compressed and cementedinto sedimentary rock.

Sill A flat sheet of igneous rock formed asmagma pushes through the Earth’s crustand squeezes between older horizontallayers of rock.

Obsidian A dark, glassy rock formed whenlava cools down so quickly that itsminerals do not have time to crystallize.

Ore A rock that contains a metal.

Rock cycle The process by which rockschange from one type to another. Rockson the surface of the Earth are constantlyworn away by wind and rain. Thefragments of rock left behind may bepressed together to make sedimentaryrock or may be baked and compressed toform metamorphic rock. Alternatively, themolten rock may rise, cool and solidify toform igneous rock.

Slate A dark grey metamorphic rock. Thecrystals that make it up are very flat, soslate easily splits into thin slabs. This makesit a useful material for tiling roofs.

Stratum A layer of rock. When a cliff face isexposed by erosion or faulting, differentstrata can often be seen in the rocks.

Weathering The breaking down of rocksdue to the weather. Water seeps into tinycracks in the ground and dissolves awaythe rock. In cold weather, ice forms in thecracks and expands, splitting off pieces ofrock. Tree roots can make the rock breakapart even faster.

RROOCCKKSS

Rock is the hard substance thatmakes up the Earth’s crust. Rocks liebeneath both the soil and the

depths of the ocean. They are made up ofa solid mixture of minerals, consisting ofdifferent chemical substances. There aremany different kinds of rocks, but theycan all be divided into three main groups.These are: igneous rocks, sedimentaryrocks and metamorphic rocks.

Metamorphic rock A rock that haschanged due to intense heat and pressureunderground. Sedimentary rocks, igneousrocks and other metamorphic rocks mayall undergo this change. When a rock ischanged in this way, its mineralsrecrystallize into different shapes.

Marble A type of metamorphic rockformed when limestone is heated to hightemperatures. Marble has a smoothstructure and can be almost any colourfrom pink, to blue, to black.

Basalt A dark grey igneous rock. Theentire ocean crust and much of the Earth’ssolid surface is made of basalt.

Batholith A large mass of igneous rockformed when an upwelling of magmaspreads through rocks underground, butnever reaches the surface. It may berevealed later when erosion wears awaythe rocks above it.

Bedrock The solid rock that lies beneaththe soil.

! The oldest rocks on the Earth are about3800 million years old.

! Feldspar is the most commonlyoccurring mineral on the Earth.

! Diamond is the hardest naturalsubstance found on the Earth. It is a type ofgemstone that is usually clear andcolourless. Diamond forms when carbon,the element that makes up coal, issqueezed in theEarth’s mantle.

! The study ofrocks is calledpetrology. It comesfrom the Greekwords petra,meaning rock, andlogos, meaningknowledge.

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The Giant’s Causeway (above) is the result of lavacooling very slowly into large crystal shapes.

Sea

Sedimentslaid down

Risingmagma

Pressure from Earthmovements deepunderground formsmetamorphic rocks

Older, deeper layers hardeninto sedimentary rock

Glaciers, streamsand rivers carrysediments awayfrom the land

Magma coolsunderground toform intrusiveigneous rock

Volcanoes

Marble

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THE ROCK CYCLE

Mineral A naturally occurringsolid chemical substance.

Rocks are made up of variousdifferent types of minerals, such

as quartz, feldspar and mica.

Moh’s scale A scale used tomeasure how hard a mineral is,

where 1 is the softest and10 is the hardest.

Limestone

Mantle

Subductionzone

Crust

Volcano

1

2

3

4

Laccolith A dome of igneous rock that liesbetween layers of sedimentary rock. It isformed when magma pushes betweenthe sedimentary rock strata.

Limestone A type of sedimentary rock,often made up of the remains of tiny seacreatures that lived millions of years ago.It is made up of calcium carbonate, whichdissolves easily in water.

Certain conditions can make ordinary rocks changeinto gemstones. Diamonds form in the Earth’smantle (1). Rubies and sapphires occur in somemetamorphic rocks (2). Amethyst forms in coolingvolcanic rock (3). Jade (4) is produced in subductionzones ( 11), while topaz is found in granite (5).

5

Diamond ring

Granite

Crystal A substance with a regulararrangement of atoms (the basic buildingblocks of matter). Crystals form whenmolten rock cools and solidifies. The moreslowly the rock cools, the larger thecrystals will be. If the rock cools too quicklythe crystals may not have time to form.

Dyke A sheet of rock formed by magmarising vertically through the Earth’s crustand cutting across surrounding rock strata.

Erosion The transportation by water, windor ice of rock fragments that have beenbroken down by weathering.

Gemstone A type of mineral that is valuablebecause it is rare and beautiful. Diamonds,rubies and topaz are all types of gemstone.

Granite A hard, pink-grey igneous rock,made up of large, clearly-visible crystals.

Igneous rock A type of rock formed whenmagma ( 13) rises to the Earth’s surfaceand solidifies. Intrusive igneous rock, suchas granite, is formed when magmasolidifies beneath the ground. Extrusiveigneous rock, such as basalt, is formedwhen it solidifies above the ground.

Braid A small island in the middle of ariver. Braids form when a river can carryno more sediment, and is forced todump some on the riverbed. Overtime the sediment builds up intobraids. These may split theriver into smaller channels.

Channel The groove in theground along which a river flows.

Column A tall, thinformation of rock thatjoins the roof and floorof a cave. Columnsare formed when astalactite andstalagmitemeet. Levee A bank of

sediment, deposited by ariver either side of its

channel when it floods.

Limestone cave A type of cave made inlimestone rocks by a chemical process.Rainwater naturally contains tiny amountsof acid. When rain trickles into cracks inthe rock it reacts with the rock made ofsoluble calcium carbonate to dissolvethem away. Over thousands of years smallcracks are widened into large caves.

Meander A curve or bend in a river.Meanders form when a river erodessediments from one side of a bend anddeposits them on the other side. Thisoccurs because the water on the outsideof the bend flows more quickly than thewater on the inside. Over time this actionincreases the size of the bend.

Mouth The point where a river flows intoa sea or lake.

Ox-bow lake A curved lake that was oncepart of a meander. It is formed when ariver cuts through the land between twomeanders in order to shorten its course.The old meander is cut off by sedimentsand becomes an ox-bow lake.

Sediment Particles of soil, sand and mudthat are moved from one point to anotherby water, wind or ice. Fast-flowing riverscan transport heavier sediment, such asgravel and rocks.

Source The point where a river begins,often high in the mountains or hills.

! The longest river in the world is the RiverNile which runs through north Africa. It is6695 km long from source to mouth.

! The world delta comes from the Greekletter delta, !, which is the same shape asthe River Nile delta.

! Stalactites and stalagmites can reach 30 m in length.

! The world’s longest cave system isMammoth Cave in Kentucky, America. It has591 km ofpassageways onfive separatelevels.

! The highestwaterfall in theworld is AngelFalls inVenezuela, SouthAmerica. It is 979 m high.

RRIIVVEERRSS &&CCAAVVEESS

Spring A point where water gushes upfrom underground.

Stalactite An icicle-like formation of rockthat hangs down from the ceiling of acave. As water drips from the ceiling of acave, the minerals dissolved in it hardenvery gradually into this shape.

Stalagmite A pointed column of rock thatpoints upwards from the floor of a cave. Itis formed from the drips falling fromstalactites above it.

Stream A small, shallow flow of water.

Tributary A small river that flows into alarger river.

Underground lake A large pool of waterbeneath the Earth’s surface. It formswhere a river flows into a cave, or whererain water gradually fills a cave.

A LIMESTONECAVE

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The Grand Canyon: the Colorado River once flowedacross level ground (1), but as the land rose (2) itwas forced to cut a deeper and deeper valley inorder to maintain its course (3).

1 32

Glacier

Waterfall

Gorge

Braids

Meander

Levee

Delta

Angel Falls

Columns

Limestone hills

Undergroundstream

Stalagmite

Stalactite Column

Tributary

Plunge pool The deep pool at the base ofa waterfall. It is carved out by the force ofthe falling water.

Rapids A fast-flowing, turbulent stretch ofriver. Rapids occur where the river isshallow and the riverbed slopes steeply.

River erosion The gradual wearing downof the land by a river. Rocks, sand and soil,carried away by the force of runningwater, scrape against the sides of theriver, acting like sandpaper.

INSIDE A CAVE

1918Confluence The

point where tworivers meet.

Delta An area ofland sometimesformed at the

mouth of a river.Deltas develop whena river loses itsspeed and depositsits sediments. Theseform braids anddivide the river into

several channels.

Gorge A deep, steep-sidedvalley, sometimes called a

canyon. Gorges may be cut into theground by a river or may form when a

cave roof collapses.

Lake A large, still, pool of water. Manylakes are filled by rivers. Some are

filled by rain, or melted glaciers.

Meander

Limestone caves are often madeup of a series of chambers,linked by tunnels and shafts.

Waterfall A cascade of water thatoccurs where a riverbed changes fromhard to soft rock. The river wears awaythe softer rock more quickly, so a “lip”of hard rock forms on the riverbed,gradually becoming a cliff over whichthe river waters plunge.

Undergroundlake

ChamberTunnel

Shaft

Sea

Ariver is a natural channel of waterthat runs downhill. It starts as aspring, as melted water from a

glacier ( 14), or as rainwater collecting on sodden ground. Near its source, riveris a fast-flowing stream. Lower down itslows and widens as other streams join it. The river eventually flows into a lakeor sea. A cave is an underground hole inthe rock. Most caves are formed whenstreams or rivers, flowing below ground, hollows out soluble rocks, such as limestone.

Cold desert An area with less than 25 cmof snow or rainfall a year, and with anaverage temperature of less than 10°C. For example, Antarctica is a cold desert,as it receives very little snow.

Desert pavement A hard, thin desertsurface of densely packed pebbles androcks. Desert pavements form when sandand small rocks are blown away, leavingbehind heavier lumps of rock.

Erg A flat, sandy area of desert.

Hamada A flat, rocky area of desert, withlittle or no sand covering.

Sand Tiny pieces of rock. Sand is formedwhen larger rocks are worn down by windand rain.

Sand dune A heap, or bank, of sand thathas been shaped by the wind. Sand dunesform where sand, blown by the wind, isstopped by an obstacle, such as a boulder,a plant or a change in the shape of theshape of the land.

Wadi A steep-sided gorge. It is formedwhen fast-flowing water from rarerainstorms wears away desert rocks.

Wind erosion The gradual wearing awayof the land caused by the wind. Winderosion gradually blows away soil andsand. Strong winds may hurl sand againstrocky surfaces, where it acts likesandpaper, wearing away the rock.

Plateau A flat, raised area of land. Plateaus form when the movement oftectonic plates ( 10) pushes up wholesections of flat rock. Buttes and mesas areboth types of plateau.

Playa A flat, dried-up lake, sometimescalled a salina. During a rare rainstorm, theplaya may briefly fill up with rainwater.

DDEESSEERRTTSS

Arid Very dry. Arid regions, such as deserts,have too little water to support plant life.

Barchan A crescent-shaped sand dune thatslowly shifts along with the wind.

Butte A desert hill with very steep sidesand a flat top. A butte is made up of hardrock that does not easily wear away. It islike a mesa but smaller.

Adesert is a dry area that receivesless than 25 centimetres of rain peryear. Most people think of deserts

as vast sandy regions, but only 20 per centof the world’s deserts are sandy. The restare bare rock, or covered with gravel.When rain does fall in occasional storms,there is no soil to soak it up so it quicklydries up. Some deserts are hot all yearround and some are always cold. In manydeserts, strong winds blast sand at therocks, carving out amazing shapes.

! Deserts and arid regions cover one-eighth of the world’s land area.

! The largest hot desert in the world is theSahara in Africa, which is over 5000 km wideand covers 9 million square km.

! Antarctica is the largest desert in theworld. It covers over 13 million square km. It receives very little snow each year—theequivalent of just 5 cm of rain.

! The continent with the largestproportion of desert—about one half itsarea—is Australia.

! The driest desert is the Atacama in Chile,South America, with an average of less than1 mm of rain per year.

! “Hot” deserts can be bitterly cold atnight. The Takla Makan Desert in China can be a scorching 40°C by day yet plunge totemperatures of - 40°C at night.

Zeugen A mushroom-shaped rock in thedesert. It is formed by wind erosion. Mostof the sand carried by the wind is foundabout a metre above the ground. Zeugensform when wind-blown sand blasts awaythe base of a boulder, leaving a narrow,top-heavy rock formation. Zeugens aresometimes called rock pedestals.

Rain shadow An area of land that receiveslittle rain because it lies on the far side of amountain range to the prevailing winds.Winds blow moist air over the mountains,where moisture condenses in the cold andfalls as rain or snow. By the time windreaches the far side of the mountains, ithas lost most of its moisture.

Salt flat A layer of salt crystals that formsin a dried-out lake. During a briefrainstorm, salts in the rocks of the desertdissolve in the warm rainwater. A pool ofwater may collect in an area of lowground. When it dries out, a layer of salt isleft behind. After each rainfall the layer ofsalt becomes thicker.

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MAP OF DESERT FEATURES

Naturalarch

Wadi

Salt flat

Zeugens

Oasis

Sand dunes

Natural arch

The Olgas, in western Australia, are the result ofextreme daily heating and cooling. These cause rocklayers to flake away, leaving smooth, rounded blocks.

A rock formation in the Valley of Fire park inAmerica (above). The park is named after itsdistinctive bright red sandstone. Monument Valley in America (below) has manyflat-topped buttes and mesas.

An oasis

2120

Mesa

Barchan dunes

Hoodoo

Hoodoo A strangely shaped column ofrock. Hoodoos are made up of layers ofsoft and hard rock, which the wind andsand erodes at different rates, producingbizarre shapes and colours, andsometimes a stripy appearance.

Hot desert An area with less than 25 cm ofsnow or rainfall a year, and with anaverage temperature of over 20°C.Hot deserts are cold at night because theydo not have any cloud cover to trap heat.

Mesa A desert mountain with a flat topand very steep sides. Mesas are formedwhen layers of soft rock are worn away bywind and water, revealing a block ofharder rock. Mesas take their name, theSpanish word for “tabletop”, from theirdistinctive, flat-topped appearance.

Natural arch A rock that is shaped like anarch. Natural arches form when the windforces open a crack in a rock face.

Oasis A lush area of water and trees in themiddle of a desert. An oasis forms wherean underground river comes to thesurface at a spring.

Seamount An underwatermountain formed by volcanic

eruptions. It may rise 1000 m or moreabove the sea floor but does not emergeabove the surface of the water.

Submarine canyon A deep, V-shapedvalley on the continental shelf or slope. Itis formed when currents carrying rockswear away the ocean floor. Submarinecanyons are often found near the mouthsof large rivers.

Mid-oceanic ridge A longmountain range running alongthe ocean floor. It is formed wherelava erupts between tectonic plates ( 11).A series of cracks, called transform faults,runs at right-angles to the ridge.

Seafloor spreading The process by whichthe ocean floor is steadily growing. Thisoccurs when tectonic plates ( 11) beneaththe ocean pull apart. Molten rock wells up in the gap, forming new ocean floor.

Current Themovement of oceanwater around the Earth.Currents are driven by the winds,and move in circular patterns, calledgyres. They move clockwise in thenorthern hemisphere and anticlockwise inthe southern hemisphere ( 28).

Deep-sea trench A long, very deep valleyin the seabed, plunging to depths of 6000-10,000 m. Deep-sea trenches occurwhere two tectonic plates push together, forcing one plate underneaththe other one ( 10).

Gulf Stream A warm current that flowsnorth-east from North America tonorthern Europe. It brings mild, wetweather to north-west Europe.

OOCCEEAANNSS

Abyssal plain A flat region of the oceanfloor that lies 4000-5000 m below theocean’s surface. It forms about 40% of theEarth’s surface.

More than two-thirds of theEarth’s surface is covered byvast expanses of salty water

called oceans. The Earth has five oceans:the Pacific, the Atlantic, the Indian, theArctic and the Southern. Areas of theoceans that are partly enclosed by landare called seas. The ocean floor lies at anaverage depth of around 4500 metres. It is mostly made up of the flat abyssalplain. Rising from the abyssal plain aremountains, volcanoes and ridges. In some places, deep trenches plummetdeep into the plain.

! About 71% of our planet is covered with water.

! The largest ocean is the Pacific, whichcovers 166 million square km—almost thesame area as all other seas and oceans added together.

! Every mid-oceanic ridge joins together toform the longest mountain range in theworld, stretching 65,000 km.

! The oceans hold about 97% of theworld’s water.

! Through seafloor spreading, the AtlanticOcean is widening by about 5 cm every year.

! The world’s deepest trenchis the Marianas Trench in

the Pacific Ocean. Its deepest point is calledChallenger Deep, and is 11,035 m deep.

Atoll A ring of coral islands. An atoll forms where a coral reef has grownaround a volcanic island ( 12). When thevolcano becomes extinct, it will sink backinto the ocean floor. If it does this slowly,the coral reef will keep growing upwards.When the island disappears, the atoll is allthat remains.

Black smoker A dark, rocky chimney,several metres high, found nearhydrothermal vents. It emits mineral-richwater from beneath the Earth’s crust.

Continental rise The gentle slope thatleads from the steeper continental

slope down to the flat abyssal plain.

Continental shelf The part of acontinent that lies beneath ocean waters. It is no more than

200 m deep.

Continental slope Theouter edge of the

continental shelfthat plungesdown towardsthe abyssal plain.

Coral reef Anunderwater

structure formedfrom the hard

skeletons of tinyanimals called polyps.

Coral reefs are found inwarm, clear, shallow waters.

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Transformfault

THE OCEANFLOOR

Continental shelf

Continental slope Continental rise

Seamounts

Abyssal plain

Oceanic islands

Mid-oceanic ridge

Guyot A flat-topped ocean mountain.Guyots form when an island, originally avolcano, sinks into the ocean floor and itstop is worn flat by waves.

Hydrothermalvent A crack in the

seabed through whichhot water spurts out. Water

that has seeped into the rocks ofthe ocean floor is heated and blasted

back up through the vents. The water isrich in minerals from the Earth’s crust.

Mid-oceanic ridge

OCEAN CURRENTS (above): red arrowsshow warm ocean currents;blue arrows show coldcurrents.

FORMATION OFAN ATOLL Reef forms Volcano sinks Atoll remains

AFRICA

EUROPE

ATLANTIC OCEAN

Mid-Atlanticridge

ARCTIC OCEAN

SOUTH AMERICA

NORTH AMERICA

Marianas Trench

PACIFIC OCEAN

NORTH AMERICA

2322

MAP OFOCEANFEATURES

Hawaiian Islands

Emperor Seamounts

Aleutian Trench

East Pacific Rise

Continental shelfContinental

slope

Deep-seatrench

Magma

Estuary The wide part of a river near itsmouth where it meets the sea and freshand salt water mix.

Headland An area of land that projectsout into the sea. It is sometimes called apromontory. It is formed from hard rockthat resists coastal erosion.

Intertidal zone The part of the coast thatis submerged by the sea during high tideand is exposed to the air during low tide.

Island An area of land that is completelysurrounded by water.

Lagoon A shallow pool of seawater cut offfrom the sea by a spit or coral reef ( 22).

Coastal erosion The gradual wearingdown of the coast by waves and the wind.Waves pick up rocks from the bottom ofthe sea and hurl them at the cliffs,knocking off fragments of rock.Sometimes air is trapped between a waveand a rock face. The force of the wavepushes the air into cracks in the rock,making the rock split open further.

Tide The rise and fall of the sea usuallyevery 12 hours 30 minutes. Tides arecaused by the gravitational pull of the Sunand Moon. Ocean waters on the side ofthe Earth closest to the Moon (and theopposite side) bulge outwards, causing ahigh tide. At the same time, the rest of theEarth has a low tide. The Sun may eitherincrease or lessen the effect of the Moon.

Wave The surging motion of the sea as itapproaches the shore. Waves are causedby wind moving over the surface of theocean. The water itself does not move butspins around on the spot. When a waveapproaches the shore, the lower partdrags on the seabed, while the upper partmoves on. Eventually it topples over, orbreaks, on the shore.

Blowhole A hole in the top of a sea cave.When waves pound at the cave, theypush water and air up through theblowhole. It bursts out above the groundas a spray of water.

Cliff A steep, nearly vertical, wall of rock,often found along coastlines. Cliffs areshaped both by the waves and byrockfalls and landslides.

Spit A mound of sand or pebbles thatextends out from the land into the sea.Spits are formed by currents that carrysand and shingle along the coast. When the current’s path is interrupted, by meeting a headland or entering anestuary, the current weakens and dropsthe sediment.

Spring tide A tide with a very largedifference between high tide and lowtide. Spring tides occur when the Sun andMoon are in line. The Sun’s gravitycombines with that of the Moon toincrease the pull on the water.

Stack A column of rock, created when theroof of a sea arch collapses, leaving oneside of the arch standing alone.

Strait A narrow stretch of sea betweentwo areas of land.

CCOOAASSTT

Bar A mound of sand and pebbles thatbuilds up offshore. It is formed by wavesand currents moving sand from one placeto another.

Bay A wide, curving area of sea betweentwo headlands. Bays form where soft rockhas been worn away by the sea.

Beach An area of sand or pebbles on theseashore. Beaches are formed whenwaves wear cliffs down into sand andpebbles, which they then pile up on thewave-cut platform, the flat area betweenthe cliff and the sea.

The coast, or seashore, is the placewhere land meets sea. It can takethe form of rocky cliffs, beaches or

marshy wetlands. The shape and featuresof a coastline depend on its rocks, windsand currents. As powerful waves crashagainst cliffs, they gradually wear awaythe rocks, sometimes carving out caves,or stone arches. When pieces of rock fallinto the sea, the waves grind them downinto pebbles or sand and lay them downin sheltered places to form beaches.

Very hard rocks erode slowlyand stand out as high headlands.Even hard rocks are gradually wornaway over time, especially during storms,when high winds whip up huge wavespowerful enough to smash pebbles andboulders against the shore.

! The highest coastal cliff in the world ison the east coast of Moloka’i in Hawaii. The cliffs are 1010 m high.

! The largest lagoon in the world is inLagoa dos Patos in southern Brazil. It is 280 km long and is separated from the Atlantic Ocean by a sand bar about 5 km wide.

! The tallest sea stack in the world is Ball’sPyramid in the Pacific Ocean. It is 562 m talland is named after its pyramid shape.

! The highest tidal range in the worldoccurs in the Bay of Fundy, a strip of waterthat lies between Novia Scotia in Canadaand Maine in the United States of America.The sea level here changes by up to 16 mduring one tidal cycle.

Longshore drift The sideways movementof sand and pebbles along the coast. It iscaused by waves hitting the shore at anangle instead of head-on.

Neap tide A tide with a very smalldifference between high and low tide.Neap tides occur when there is a quarter Moon. The Sun and Moon are atright angles to one another, pulling indifferent directions, so that the force oftheir pull on the tides is lessened.

Pebble A small, round rock. Pebbles areformed when rocks that fall into the seaare worn down and smoothed by waves.

Peninsula An area of land that extendsinto the sea with water on three of its sides.

Salt marsh A low-lying area of coastlinethat is often flooded by the sea. Saltmarshes usually form in sheltered areas,such as deltas. They are home to grassesand shrubs that grow in salt water.

Sea arch A section of headland that isshaped like a arch. A sea arch is formedwhen a sea cave is gouged through.

Sea cave A cave near the base of a cliff.Sea caves form when the sea crashesagainst cracks in a cliff’s surface, pushingwater and air into the cracks and forcingthem to open wider.

FF AA CC TT FF II LL EESpring tides Neap tides

Low

High

Low

High

Moon

Earth

Sun

Earth

Moon

Sun

Spring tidesNeap tides

The twelve apostles are a set of giant rock stacksthat line part of the coast in Victoria, Australia.

StackArch

Cliffs

SpitBar

Sand dunesEstuary

Headland

Salt marsh

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Circularmotion

How waves break onthe seashore

Beaufort Scale A scale used to measurethe strength of the wind, where 0 iscomplete calm and 12 is a hurricane.

Cloud A visible cluster of water droplets,ice crystals or both. The droplets are sosmall that they can remain in the air.Different types of cloud form at differentheights and under different conditions.

Condensation The process of a gaschanging into a liquid when it is cooled.Water vapour condenses more easily if theair contains small particles, such as dust,salt or smoke for it to collect around.

Dew Droplets of water that cover theground overnight. Dew forms when theground cools down faster than the air,causing moisture in the air to condense.

Rain Liquid water that falls from clouds.

Rainbow An arch of colours in the sky thatforms when the Sun shines from behindyou at rain falling in front of you. Theraindrops split the light into all thedifferent colours of the spectrum.

Sleet Snow that falls through warm air andis partially melted when it reaches theground.

Snow Precipitation that falls to the groundas clumps of ice crystals, called snowflakes.All snowflakes have six sides, but no twoare exactly alike.

Thunder The loud sound that accompaniesa flash of lightning. The heat of the flashmakes the air around it expand so fast itmakes a loud boom.

Air The mixture of gases, mostly nitrogenand oxygen, that surrounds the Earth.

Air mass A large body of air of a similartemperature and moisture content. Airmasses are separated by fronts.

Air pressure The weight of air pressingdown on the Earth. Air pressure is lowwhere air masses are rising. The rising airwill cool and condense, forming rainclouds. Pressure is higher where airmasses are sinking. High air pressure isusually accompanied by dry weather.

Barometer An instrument that measuresair pressure.

WWEEAATTHHEERR

Weather describes the conditionsand changes that take place inthe lower atmosphere ( 28), up

to about 20 kilometres above the Earth’satmosphere. It includes temperature,wind speed and direction, cloud type andcover, rain, hail, snow, frost, ice, droughtand storms. The study of the weather iscalled meteorology.

Lightning A visible spark of electricityproduced during a thunderstorm.Electricity can build up inside clouds when ice and water molecules bumpagainst each other and becomeelectrically charged. The light we see is astrip of air being suddenly heated by theelectricity. Fork lightning travels fromclouds to the ground. Sheet lightningoccurs within clouds.

Mist A thin layer of cloud lying at groundlevel. Mist is thinner than fog.

Precipitation Water that falls in the formof rain, hail, sleet or snow. Thetemperature inside and outside a clouddetermines which form it will take.

Air spiralsupwardsaround eye

Air sinks intoeye

Warm, moist air flows towards centreof low pressure

The centre or“eye” of the stormis an area of calm. Whole hurricane

moves along

Upcurrents

A hurricane easily tosses a car across a field.

Sea

Evaporation

Clouds

Rain or snow

Water flows to the sea inrivers or through the ground

A tornado

Hail Precipitation that falls as ice. Hail formsin large cumulonimbus clouds where waterdroplets are tossed about by strongupward-moving air currents. Each time araindrop is pushed into the freezing top ofthe cloud a thin layer of ice forms around it.When the drop is too heavy to be lifted bythe currents, it will fall to the ground.

Humidity The level of moisture in the air.Air with lots of water molecules in it is veryhumid. Warm air can hold more moisturethan cool air.

Hurricane A massive, spinning storm ofwind and rain. Hurricanes form whenwarm, moist air from the oceans is stirredup into a huge, spinning mass. Strongwinds and rain swirl around a calm centre,the “eye” of the storm. The spiralling windsare caused by the force of the Earthrotating on its axis.

Fog A thick layer of cloud-like moistureformed by water vapour cooling at groundlevel. Fog is thicker than mist.

Front A boundary between two air masses.A warm front occurs when warm air flowsup and over colder, heavier air. This makesthe moisture in the rising air condense andfall as rain. A cold front occurs when coldair pushes against warm air, forcing itupwards where it forms clouds.

Frost A thin layer of sparkling ice crystalsthat forms when the air is below freezingand dew freezes on the ground.

THE WATERCYCLE

12,000 m

10,000 m

8,000 m

6,000 m

4,000 m

2,000 m

Cirrus

Cirrostratus

Cirrocumulus

Cumulonimbus

Altostratus

AltocumulusStratocumulus

StratusNimbostratus

Cumulus

Ice crystal

Waterdroplet

2726

CLOUD TYPES

Drought A period where there issignificantly less rainfall than normal.

Evaporation The process in which a liquidchanges into a gas when it is heated up orwhen the air above it is dry.

Flood An overflow of water on to areas ofsurrounding land. Flooding is usuallycaused by particularly heavy rain swellingthe level of water in river channels. Therivers burst their banks and flood the land.

Outwardflowing winds

FF AA CC TT FF II LL EE! Lightning heats the air to around30,000°C—five times the temperature onthe surface of the Sun.

! The winds inside a tornado are thefastest winds ever recorded on Earth. Theycan travel at speeds of up to 500 km/h.

! Spinning, circular storms are known ashurricanes in the Caribbean, the northAtlantic and the north east Pacific oceans. In the west Pacific Ocean they are known astyphoons, and in the Indian Ocean they areknown as cyclones.

! The heaviest ever hailstone weighedabout 1 kg. It fell in Bangladesh in 1986.

Tornado A twisting column of air thatextends from the base of a cloud to theground, like an upside-down funnel. Its winds are so powerful, it can pick uppeople, cars and even houses.

Water cycle The process by which watercirculates from the land or oceans to theatmosphere and back again. When it iswarmed by the Sun, water evaporates andrises. As it does so it cools and condenses,forming clouds. Rain and snow fall to theground, where it flows into rivers, lakesand seas and the cycle continues.

Wind Air moving from one place toanother. The Sun makes some regions ofair warmer than others. When warm airrises, cooler air flows in to take its place.

! The troposphere contains 99% of thewater vapour in the atmosphere.

! This air in the atmosphere is a mixtureof gases. About four-fifths of the air inthe atmosphere is nitrogen. Oxygen

makes up about one-fifth of theatmosphere. The rest is made up of

tiny amounts of argon, carbondioxide, hydrogen, ozone and

water vapour.

! El Niño is Spanish for “the Christ

Child”, named becausethe weather change

normally occurs around thetime of Christmas.

FF AA CC TT FF II LL EE

INDIAN MONSOON: 1 In winter, dry air flowssouth, there is no moisture and there is

drought. 2 In summer, the land heats, and the airabove it becomes warmer than the air over the

ocean. Moist air flows in, bringing heavy rains.

Stratosphere The layer of the atmospherethat lies 11-50 km above the ground,between the troposphere and mesosphere.Jet planes sometimes fly in this layer toavoid storms in the troposphere.

Temperate climate A pattern of weatherwith warm summers and cool winters.

Thermosphere The layer of theatmosphere that lies 80-140 km above theground, between the mesosphere andexosphere. Temperatures can rise to 1400°Cdue to the Sun heating the thin air.

Trade winds Winds that always blow in thesame direction and with the same strength.Near the equator, the air is warmed by theSun so that it rises and spreads out. Cool airfrom the north and south poles blows into replace it. The winds are shovedsideways by the Earth’srotation so that they alwaystravel slightly east or west.

Tropic of Cancer Animaginary line that runsaround the Earthbetween the equator andthe north pole.

Monsoon Winds that change directionbetween summer and winter. In summerthe winds bring torrential rain with them.Monsoons occur where there are greatdifferences in temperature betweenoceans and land.

AATTMMOOSSPPHHEERREE,,SSEEAASSOONNSS &&

CCLLIIMMAATTEE

Altitude Height above sea level.

Atmosphere The blanket of gasessurrounding the Earth.

Climate A long-term pattern of weather.An area’s climate mainly depends on itslocation on the Earth’s surface, but is alsoaffected by altitude, and by winds and

ocean currents, which carryair masses ( 26) around

the globe.

Desert climate Apattern of very dry

weather in an areawhere less than

25 cm of rain fallsin one year.

Tropical

Desert

Temperate

Cool temperate

Polar

Mountain

21st March

22ndDecember

Equator

Spring in thenorth; Autumnin the south

Tropic ofCapricorn

Winter in thenorth; Summerin the south

23rd September

Autumn in thenorth; Springin the south

Earth’s axis

Equator

SunSummer in thenorth; Winter in thesouth

Tropic ofCancer

21st June

THE LAYERS OF THEATMOSPHERE

TRADEWINDS

THE SEASONS

2928

The atmosphere is the blanket ofgases surrounding the Earth. It hasseveral different layers. About 800

kilometres above the ground, theatmosphere fades away completely andspace begins. The seasons are timeperiods in the year, marked by changes inthe weather. The climate is the pattern ofweather over a long period.

Mountain climate A pattern of cold, windyweather that occurs in mountainous regions.

Ozone layer A thin layer of ozone gas that isfound in the stratosphere. It absorbs 97-99% of the Sun’s ultraviolet rays.

Polar climate A pattern of cold, dry, windyweather that occurs near the poles.

Seasons Time periods in the year, marked bychanges in the weather. The seasons occurbecause the Earth is tilted on its axis. The half of Earth tilted towards the Sun hassummer, while the other half has winter.

Tropic of Capricorn An imaginary line thatruns around the Earth between theequator and the south pole.

Tropical climate A pattern of hot, wetweather throughout the year. Tropicalregions lie close to the equator where theSun shines directly overhead. There areonly two seasons: wet and dry.

Troposphere The layer of the atmospherethat is closest to the Earth. It is sometimescalled the “weatherlayer” because itcontains the Earth’sclouds and winds.

Wet season A long period of very wetweather. It is one of two seasons that occurin tropical climates, where it is dry for halfthe year and wet for the other half.

MAP OF CLIMATE ZONES

Clouds formas moist aircondenses

INDIANOCEAN

INDIA

Warm, moistwinds blow infrom ocean

Cool, dry air fromnorthern Asia

INDIANOCEAN

INDIA

1

2

EarthTROPOSPHERE

STRATOSPHERE

MESOSPHERE

THERMOSPHERE

Meteors

EXOSPHERE

Dry season A long period of very dryweather. It is one of two seasons thatoccur in tropical climates.

El Niño A climate pattern that occurs offthe west coast of South America every four to seven years. Ocean currents change and warm water moves east. The air above the ocean is warmed, causing rain and storms. More distantareas may suffer droughts as a result.

Equator An imaginary line that runshorizontally around the centre of the Earth.

Exosphere The outermost layer of theatmosphere about 900 km above us. The air in the exosphere is so thin that oneparticle of gas may travel hundreds ofkilometres before hitting another particle.

Hemisphere One half of the Earth’ssurface. The equator divides the Earth intonorthern and southern hemispheres.

Mesosphere The layer of the atmosphere50-80 km above ground level, between thestratosphere and thermosphere. This layeris where meteors burn up.

Microclimate The climate of a particularlocal area. For example, towns are oftenwarmer than the countryside around them.This is because concrete absorbs heat inthe day and releases it through the night.

A S I A

A F R I C A

A U S T R A L I A

S O U T H

A M E R I C A

N O R T H

A M E R I C A E U R O P E

AAaa 12abyssal plain 10, 22, 23aftershock 10air 26air mass 26, 28air pressure 26altitude 28arête 14arid 20ash 12-13asthenosphere 8, 10atmosphere 7, 26, 28-29atoll 22-23aurora borealis 8-9axis 8

BBbar 24barchan 20barometer 26basalt 16batholith 16bay 24beach 24Beaufort scale 26bedrock 16bergschrund 14black smoker 22block mountain 14blowhole 24braid 18butte 20-21

CCcaldera 12canyon 18cave 18-19, 24channel 18, 26cirque 14cli! 2, 19, 24-25

climate 28-29clinker 12cloud 20, 26-27, 29coast 11, 24-25coastal erosion 24cold desert 20column 18-20condensation 21, 26-27confluence 18continent 8-11, 21continental crust 10-11continental drift 10continental rise 22-23continental shelf 10, 22-23continental slope 22-23convection current 8-10convergent boundary 10-11coral reef 22-24core 8-9, 11crater 12crevasse 14-15crust 8-10, 12-13, 15-16, 23crystal 16-17, 21current 22-25

DDdeep-sea trench 10, 22delta 18, 25desert 20-21desert climate 28-29desert pavement 20dew 26divergent boundary 10-11dome mountain 14dormant volcano 12drought 26drumlin 14-15dry season 28dyke 16

EEearth, formation 6-7earthquake 10-11, 13El Niño 28epicentre 10equator 28-29erg 20

erosion 16-18, 20 eruption 12, 23esker 14-15estuary 24-25evaporation 26-27exosphere 28-29extinct volcano 12extrusive igneous rock 16

FFfault 10-11, 14-15, 17fissure 12fjord 14flood 26focus 10-11fog 26-27fold 11fold mountain 14fork lightning 27front 26frost 26

GGgemstone 16geyser 12-13glacial erosion 14-15glacial valley 15glacier 14-16, 18gorge 18, 21graben 15granite 16-17gulf stream 22guyot 23gyres 22

HHhail 26-27hamada 20hanging valley 14headland 24-25hemisphere 9, 22, 28hill 19-20hoodoo 20horst 14

hot desert 20hot spot 12-13humidity 27hurricane 26-27hydrothermal vent 22-23

IIice cap 14-15iceberg 14-15igneous rock 16-17intertidal zone 24intrusive igneous rock 16island 23, 24

LLlaccolith 16lagoon 24-25lake 15, 18, 21, 27landslide 24lava 12-13, 17levee 18lightning 27limestone 16-19limestone cave 18lithosphere 8lithospheric plates 11longshore drift 25

MMmagma 10, 12-13, 16-17, 23magma chamber 12-13magnetic field 9magnetic poles 9magnetosphere 9mantle 8-10, 13, 15, 16-17, marble 17marsh 24meander 18meltwater 14-15mercalli scale 11mesa 20-21mesosphere 28-29metamorphic rock 16-17meteor 28

microclimate 28mid-oceanic ridge 10, 13, 23mineral 13, 16-17, 19, 22-23mist 26-27Moh’s scale 17moho 9monsoon 28-29moraine 15

lateral 15medial 15terminal 15

moulin 15mountain 10, 12-15, 19, 21,

22-23, 28mountain climate 28-29mouth (river) 18

NNOOnatural arch 20neap tide 24-25nunatak 15oasis 20ocean 11, 16, 22, 27, 28ocean currents 28oceanic crust 10oceanic plate 13ore 17ox-bow lake 18ozone layer 28

PPpahoehoe 13pebble 14, 20, 24-25peninsula 25pillow lava 13plateau 21playa 21plunge pool 19polar climate 28-29post-glacial rebound 15precipitation 27primary waves 11promontory 24pumice 13pyroclastic flow 13

RR

rain 18, 20-21, 26-28rain shadow 21rainbow 27randkluft 15rapids 19Richter scale 11rift 11rift valley 15ring of fire 12-13river 16, 18-20, 23, 24, 26-27river erosion 15, 19roche moutonnée 15rocks 14, 16-17, 20-21, 24-25rock arch 20rock cycle 16-27rock pedestal 21rollers 9

SSsalina 21salt flat 20-21salt marsh 24-25San Andreas Fault 11sand 19, 21, 24-25sand dune 20-21, 24sandstone 17sea 18, 22, 24, 26-27sea arch 24-25sea cave 24-25seafloor spreading 23seamount 10, 23seasons 28-29secondary waves 11sediment 16, 18-19, 25sedimentary rock 16-17seismic wave 11seisometer 11serac 15sheet lightning 27shield volcano 13silicates 12-13sill 17slate 17sleet 27snout 15snow 14, 20-21, 26-27source (river) 19spit 24-25spring 18-19

30 31

spring tide 24-25stack 24-25stalactite 18-19stalagmite 18-19stone arch 24storm 20, 26strait 25strata 16-17stratosphere 28-29stratovolcano 13stream 16, 19subduction 11subduction zone 10-11, 16submarine canyon 23summit 12, 25

TTtectonic plates 10-11,

14, 21, 22-23temperate climate 29temperature 20, 26terminus 15thermosphere 28-29thunder 27tide 24-25tornado 27trade winds 28-29transform fault 10-11, 23tributary 18-19tributary glacier 14-15Tropic of Cancer 28-29Tropic of Capricorn 28-29tropical climate 28-29troposphere 28-29tsunami 11tu! 13

UUVVultraviolet rays 28underground lake 19valley 14-15, 18, 22-23vent 12-13viscosity 13

volcano 10-12, 16, 22-23

WWZZwadi 20-21water cycle 26-27waterfall 18-19wave 11, 23, 24-25wave-cut platform 24weather 17, 22, 26-29weathering 16-17wet season 29wetlands 24wind 20-22, 24-29wind erosion 21zeugen 20-21

IINNDDEEXX