Learning about the Earth Geography is a science that studies the Earth ' s surface . It studies what makes the different shapes and colors of the Earth - the ground , rocks , and water , what does and does not grow . If you look at the Earth as a geographer does , then you might see it as a colorful map . Much more than half of it is blue with oceans , lakes , rivers , and streams . In some places it is tan - colored with the sands of dry deserts . In other places it is green with forests . There are purple - gray mountains and white snowcapped peaks . And there are the soft yellow of grainfields and the light green of leafy crops . Part or learning about the Earth is learning where people can and can ' t live . The different colors of your Earth map can help you discover this . You won ' t find many people in the tan , white , or larger blue parts - desert , the snowfields , and oceans . Not many people live in the deserts , because deserts are hot and dry . Very few plant can grow there . In the high mountains and at the North and South poles , it is very cold . Most plants don ' t like the cold , and most people don ' t either . You will find people in and near the green and yellow parts and the smaller blue parts - the farmlands , forests , rivers and lakes . To those regions you can add brown dots and cluster of dots , for towns and cities . There ' s a lot to learn about the Earth , just as there ' s a lot to learn about a friend . Geography helps you become a friend of the Earth . Did you know ? The " big blue marble " is a nickname for the planet Earth . This is because from space our world looks like a big round marble , all blue with swirling white streaks of clouds . The Largest Pieces of Land The continents are the largest bodies of land on the Earth . Look at a globe . Whatever is blue is water . Most of the rest is land : the continents . There are seven continents . From biggest to smallest , they are Asia , Africa , North America , South America , Antarctica , Europe . and Australia . Some continents , such as Australia and Antarctica , are completely surrounded by water . And some continents are joined together , as Asia and Europe are . Continents are physical bodies , defined by their shape , size , and location . They have mountains , rivers , deserts , forests , and other physical features . But humans have divided them into political groups , called " countries " or " nations . " Large continents , such as Asia , may include both very large countries , such as China , and very small countries , such as Nepal . Australia , the smallest continent , is also itself a country - one of the world ' s largest . North America contains three large countries - Canada , the United States , and Mexico - and a few small countries in a region known as Central America . Europe , on the other hand , is the world ' s second smallest continent but has about 50 countries . Africa , the second largest continent , is believed to be where the very first humans appeared . The continent of Antarctica is all by itself down at the South Pole . It is rocky and is covered by thick ice that never melts . only a few plants and animals can be found along its seacoasts . Earth scientists believe that the continents began forming billions of years ago . Lighter parts of Earth ' s molten core separated from heavier parts and rose to the top . As they cooled off and became solid , the land that would become the continents formed . The continents were probably joined together at first and then drifted apart . one theory supposes that there were once two " supercontinents " Gondwanaland in the south and Laurasia in the north . A Continent of Extremes Antarctica is the coldest , windiest , and highest continent in the world ! It lies at the bottom of the world , surrounding the South Pole . The name Antarctica means " opposite to the Arctic , " referring to the Arctic Circle on the other side of the world . The coldest temperature recorded in Antarctica is also the world ' s lowest , at - 128 . 6 ° F . A sheet of ice covers the entire continent . At its thickest point , the ice is almost 3 miles deep - and much of the world ' s freshwater . Toward the edges of the continent , the ice bicomes glaciers , creeping rivers of ice . Strange and wonderful Antarctica has only one day in the entire year . The Sun generally rises on September 21 and sets on March 22 . This one long day is the summer ! From March 22 until September 21 , the South Pole is dark and Antarctica has its night , or winter . People do not live permanently in Antarctica . only scientists and some adventurous tourists visit . There are , however , 45 species of birds in Antarctica , including the emperor penguin and the Adelie penguin , that live near the seacoast . Also , four species of seals breed only in Antarctica . Whales live in the water around the frigid continent . The killer whale , the sperm whale , the rare bottle nosed whale , the pygmy whale , and seven species of baleen whales can all be found off the coast . Oddly , there are active volcanoes in Antarctica . That means you can find not just the world ' s coldest temperatures here but , deep down , some of the hottest too . Do you know ? Antarctica is a desert - a " frigid desert . " It ' s extremely cold , unlike the more common hot sandy deserts . But like them , it gets so little moisture during the year that very little life can survive . Mountains of Smoke and Fire Deep under the Earth ' s surface it ' s so hot that even rock melts . Sometimes this molten rock , called " magma , " is pushed up to the earth ' s surface . At that point it is referred to as " lava . " And the opening , or vent , that lets the lava out is a volcano . A volcano may explode violently , throwing rocks for miles . Or it might push the lava out so that it flows away , cools , and hardens . Some volcanoes can even do all these things underwater . Most volcanoes have been around for a very long time . Many haven ' t erupted in years and have cooled off . Volcanoes that won ' t be erupting anymore are called " dead volcanoes . " Some volcanoes still let off smoke . These " sleeping volcanoes " may " wake up " someday and start erupting . Mount Besuvius in Italy slept for a thousand years . But one day in AD 79 it suddenly woke up . Its eruption spewed hot ash and rocky fragments that buried the city of Pompeii . A hot mud flow buried nearby Herculaneum . The remains are so well preserved that the area has been named a World Heritage site . But not all volcanoes are destructive . If a volcano spits out enough lava and debris , it piles up into a mountain . The Hawaiian Islands and the Island of Iceland were created this way . Other volcanoes help provide heat and energy . Many Icelandic homes get their hot water from springs heated by volcanic steam . That steam can also be used to produce electricity . Also , plants grow very well in the rich soil left by volcanoes . And valuable gems , such as diamonds , can sometimes be found in the rocks that volcanoes spit out . Building Earth ' s Giant Landscapes What makes mountains ? Several different processes contribute to mountain building . And most mountains are formed by a combination of these , usually over millions of years . Deep inside , the Earth is so incredibly hot that everything is melted , or molten . This molten material , or lava , escapes to the Earth ' s surface when volcanoes erupt . The lava cools and becomes hard and solid . This happens again and again , collecting until there is a volcanic mountain . Mount Fuji in Japan and Mount St . Helens in Washington state , U . S . , are volcanic mountains . There are also many undersea volcanic mountains - much taller than anything on land ! In some cases strong earthquakes caused the surface rock for miles and miles to break . Part of the surface would then be lower and part it higher . More earthquakes moved the lower parts down and the upper parts up . Eventually , the high parts became tall enough to make mountains . Still other mountains were pushed up from the bottom of an ocean when two enormous portions of the Earth crashed together - very slowly , over millions and millions of years . Some of the largest mountain chains formed this way . The Andes of South America are an example . Another mountain building process is called " folding . " If you push a carpet up against a wall , it folds and rumples . That ' s basically the way the Appalachian Mountains in eastern North America were formed . At first most mountains were steep and sharp . But even hard rocks can be worn away . Slowly , with the wind and the rain rubbing at them , steep sharp mountains grow smoother , shorter , and rounder . Did you know ? To be considered a mountain , the land must rise at least 2 , 000 feet above its surroundings . Mount Everest , the world ' s highest mountain , rises 29 , 035 feet above sea level . The Nitty - Gritty You can find sand at the edge of lakes , the bottoms of rivers , and the seashore . You can find it in mountain valleys , deserts , and , of course , a sandbox . Where does all this sand come from ? Sand is created when rocks break into tiny , pieces . Wind , ice , and rain knock against high mountain cliffs . And slowly , over millions of years , these forces break off pieces of rock . The pieces bounce down the mountainside and break off other pieces of rock - while it ' s also breaking into smaller and smaller pieces itself . It isn ' t sand yet , but it ' s getting there . Rivers and glaciers are also good at making sand . A river ' s water rushes along , carrying rocks with it and breaking them into little pieces . The ice of a glacier grinds away at whatever rocks it slowly rolls across . Another great sand maker is the ocean . Every day , all over the world , tides ride and fall , pushing against rocks over and over . Waves tear at the rocks along the shore , wearing them down . Thanks to the weather , water , and ice , some of these borken rocks finally get so small that they become what we call " sand . " Now that you have all this sand , what can you do with it ? Sand is used for paving roads . Bricks made with sand are harder and stronger than other bricks . Sand is also used to filter ( or clean ) water . When it ' s sprayed with great force against stone or brick , it can grind away thick layers of dirt or even paint in a process called " sandblasting . " And , of course , sand is great for building sand castles ! Lands of Little Water Deserts are places that get very little rain each year - so little rain that most trees and plants cannot grow there . Some deserts will go for years without rain . They are difficult places to live in , and the few plants , animals , and people who live there have to be tough to survive . Every continent except Europe has a desert . Even Antarctica has one , a frigid desert . Most desert , however , are arid , or dry , deserts with mile after mile of sand , baked earth , and barren rock , In the daytime these places look like lost worlds - hot , dry , and silent . Usually , the only plants growing there are low thorny ones . These plants store most of the water they are able to collect . It may be a long time before their next drink . At night it can be quite cold in the desert . That ' s when creatures that have been hiding from the Sun ' s burning reys come out of their homes . Many of the creatures are lizards and insects such as scorpions . There are also different kinds of rats as well as other , larger animals . You can hear the animals squeaking and growling near water holes and springs . That ' s where the coyotes , badgers , bobcats , foxes , and birds gather , all hunting for food and water . When the rare spring does bubble up in the desert , plants and trees begin to grow . An island of green like this is called an " oasis . " Many people choose to live in the desert . In late afternoon the sky turns crimson and gold , and the mountains make purple shadows . And at night the stars seem close enough to touch . Fingers of Land A peninsula is a body of land surrounded by water on three sides . The word " peninsula " comes from the Latin paene insula , meaning " almost an island . " There are peninsulas on every continent , but every one is different . Most peninsulas of any significance extend into oceans or very large lakes . In the United States , Florida is a peninsula . The state of Alaska qualifies as one and has several smaller peninsulas of its own . One of the last great wilderness areas in the United States is on the Olympic Pininsula in Washington state . It is surrounded by the Pacific Ocean , the Strait of Juan de Fuca , and Puget Sound . It has a rainforest , rivers , alpine peaks , glaciers , and such creatures as salmon and elk . In Mexico there are two main peninsulas , the Yucatan Peninsula in the east and Baja California in the west . The Yucatan Peninsula draws tourists to the ruins of great Mayan cities such as Uxmal and Chichen Itza . Another famous peninsula is the Sinai Peninsula of Egypt . It is triangular in shape . The peninsula links Africa and Asia . In Jewish history the Sinai Peninsula is know as the site where God appeared before Moses and gave him the Ten Commandments . Europe too has several peninsulas . In northern Europe the Scandinavian Peninsula contains the countries of Norway and Sweden . Denmark forms another . And the Iberian Peninsula in southern Europe is made up of Spain and Portugal . Italy and part of Greece are peninsulas as well . The world ' s largest peninsula is Arabia , at over a million square miles . Other important peninsulas in Asia include Korea and Southeast Asia . Endangered Ecosystems Imagine a forest with a carpet of wet leaves littering the ground . If you look up , you see only a canopy of broad green leaves . There are wildflowers on the trees . You can hear water drops , insects , birds , and , perhaps , the distant screech of a monkey . The place you are picturing is a rainforest . A rainforest is a kind of ecosystem - a community of all the living things in a region , their physical environment , and all their interrelationships . Rainforests are dense , wet , and green because they get large amounts of rain . The Amazon Rainforest in South America is the world ' s largest rainforest . Other large rainforests lie in Central Africa and Southeast Asia . Northeastern Australia ' s " dry rainforest " has a long dry season followed by a season of heavy rainfall . In a rainforest nothing is wasted . Everything is recycled . When leaves fall , flowers wilt , or animals die on the forest floor , they decay . This releases nutrients into the soil that become food for the roots of trees and plants . Water evaporates in the forest and forms clouds above the trees . Later this water falls again as rain . Rainforests are rich in jplants and animals . Many have not even been discovered yet . Some rainforest plants have given us important medicines . These include aspirin , which is a pain reliever , and curare , used to help people relax during medical operations . Unfortunately , the rainforests are being destroyed rapidly . The trees are felled for timber and to create land for farming . Animals living in these forests are facing extinction . And once lost , these animals and forests cannot be replaced . Grassy Wetlands A marsh is a wetland , an area of land containing much soil moisture that does not drain well . Swamps are also wetlands . The main difference is that while trees grow in a swamp , grasses grow in a marsh . Marsh grasses have shallow roots that spread and bind mud together . This slows the flow of water , which creates rich soil deposits and encourages the growth of the marsh . There are two main types of marshes , freshwater marshes are found at the mouths of rivers . These marshes are famous as bird sanctuaries and are an important habitat for many birds , mammals , and insects . If we didn ' t have the marshes , then we would lose many of these animals . There simply isn ' t anywhere else where they can survive . The Amazon in South America , the Congo in Africa , the Nile in Egypt , the Tigris and Euphrates in Iraq , and the Mekong in Vietnam all have large freshwater marshes . Did you know that the rice you eat grows in freshwater marshes ? Rice is the most important of all marsh plants . It provides a major portion of the world ' s food . Salt marshes are formed by seawater flooding and draining flat land as tides go in and out . The grasses of a salt marsh will not grow if the ground is permanently flooded . Salt marshes are found along the east coast of the United States , in the Arctic , in northern Europe , in Australia , and in New Zealand . Did you know ? The largest marsh in the world is the Florida Everglades . This marsh - swamp combination is somewhat more than 4 , 300 square miles and is home to many extraordinary animals , including the very rare Florida panther . Waterlogged Forests People once believed that drinking the tea - colored water from the Great Dismal Swamp in the eastern coastal United States was magic . They believed it prevented illness and made people live longer . The swamp water wasn ' t magic , of course . But its peculiar color , plus the mysterious swampland ' s exotic beauty , made an ideal setting for such folklore . Swamps are special wetland areas found throughout the world . They are usually very wet , wide , low , and green and have many trees . Swamps are found in areas where the water doesn ' t drain and thus keeps the ground waterlogged . Swamps are different from marshes and other kinds of wetlands by having trees as their major form of plant life . Freshwater swamps tend to develop in low - lying regions around rivers . The trees in a swamp lack deep - growing roots . Few kinds of plants can live in swamps . But some swamps support a variety of plants and a great number of animal species as well . For example , in the Okefenokee Swamp of the southeastern United States grow such trees as the giant tupelo and the bald cypress . spanish moss , brush , and vines grow on these trees . There are exotic flowers such as lilies , rare orchids , and floating hearts . Wildlife is varied and plentiful too . There are 175 species of birds and at least 40 species of mammals , including raccoons , black bears , white - tailed deer , bobcats , foxes , and otters . Alligators also live also live there , as do mosquitoes , which breed in standing water . Besides river swamps , there are saltwater and mangrove swamps . Salt swamps are formed by flooding seawater . Mangrove trees are very hardy and can survive in sandy , salty areas . The deltas of the Mekong , Amazon , Congo , and Ganges rivers have large mangrove swamps . Did you know ? Mangrove trees have a special way of surviving the watery , salty conditions of the swamps they live in . They have an aboveground root system that allows the trees to take in air . The Power of Flowing Water It seems pretty obvious what rivers are for . They give us water to drink and fish to eat . They do these things for many animals too . But it might surprise you to learn that rivers have some even bigger jobs . For one thing , rivers deliver water to lakes and oceans . Another major task is changing the face of the land , and this second job makes a huge difference . No other force changes as much of the world ' s surface as running water does . In fact , the world ' s rivers could completely erode the face of the Earth , though it might take them 25 million years to do it . We can see rivers ' handiwork all around us . Valleys are carved out when rivers slowly cut through rock and carry off dirt . Canyons and gorges are young valleys . Another impressive bit of river handiwork is the waterfall . Waterfalls happen when a river wears away soft rock and then drops down onto hard rock that it can ' t erode . Some falls are harnessed to produce electricity . The world ' s tallest waterfall is Angel Falls in Venezuela . It drops and incredible 3 , 212 feet . Khone Falls in the Mekong River in Southeast Asia send 2 1 / 2 million gallons of water over the edge every second - the most of any falls and nearly double the flow of North America ' s Niagara Falls . The world ' s longest river is the Nile in North Africa . The Amazon in South America is a little shorter but carries more water than any other river . Did you know ? People first looking at Mars through modern telescopes thought that it was covered with rivers or canals . Satellite photographs now suggest that parts of Mars once had flowing water . Water could have meant there was life on Mars . Engulfed by Water Take a small bowl and place a sponge in it . Now slowly pour water into the bowl . The sponge 내만 up the water . But once the sponge is full , the bowl begins to fill up with water . If you pour more water , the bowl will overflow . This is what happens in a flood . The ground is like a giant sponge that soaks up rainwater until it is full . Some of the water dries and goes back into the air . The rest , called " runoff , " can ' t be soaked up and can cause floods . There are different types of floods . Spring floods occur when heavy winter snows melt rapidly . Floods caused by heavy rains can occur at anytime of the year . Rivers overflow their banks , and the ground can ' t soak up the extra water . The rain and wind accompanying hurricanes ( or typhoons , in the Pacific Ocean ) can also cause floods . Huge ocean waves overwhelm coastal towns , and the heavy rains cause rivers and streams to flood nearby areas . Such hurricane - created floods struck Central America in 1998 , killing more than 20 , 000 people and leaving one and a half million homeless . A flash flood , however , comes without warning . When a cloudburst occurs in hilly country or in a dry riverbed , the runoff is fast . The ground doesn ' t have time to soak up the rainwater . Destructive flash floods happen when a great deal of water overflows all at once . Volcanic eruptions and earthquakes at sea may cause huge waves , called " tsunamis , " that may swamp seacoasts . The volcanic eruption of Krakatoa in 1883 formed waves that flooded whole districts in Indonesia . Water in the Desert Probably the most precious thing in the world is fresh water . If a person was lost in a desert without any special equipment or supplies , he or she would soon die from lack of water . It is therefore not surprising that very few people live in the desert . But some people do . Where do they stay ? Obviously , they stay where there is water . A place in the desert with a natural supply of fresh water is called an " oasis . " An oasis has enough water to support a variety of plants . Most oases ( the plural of " oasis ") have underground water sources such as springs or wells . AI - Hasa is the largest oasis in the Middle Eastern country of Saudi Arabia . It has acres and acres of palm groves and other crops . But not all oases have a constant supply of water . Some areas have dry channels called " wadis " where springs sometimes flow . And desert areas at higher elevations sometimes receive extra rain to support plant life . In the Sahara people can live year - round in the oases because the water supply is permanent . The oases allow crops to be watered , and desert temperatures make crops grow quickly . The date palm is the main source of food . However , in its shade grow citrus fruits , figs , peaches , apricots , vegetables , and cereals such as wheat , barley , and millet . The Siwa Oasis in western Egypt has about 200 springs . It is a very fertile oasis , and thousands of date palms and olive trees grow there . In fact , the people living in this oasis export dates and olive oil to other places in the world . Did you know ? Few people realize just how extreme desert weather can be . The hottest desert temperature recorded is 136 ° F , in Libya . And in Chile there is a desert that apparently hasn ' t had any rain for the last 400 years . The World of Water Did you know that nearly three - fourths of the Earth ' s surface is underwater ? And almost all of that water is in one of the four major oceans . From biggest to smallest the oceans are : the Pacific , the Atlantic , the Indian , and the Arctic . Seas , such as the Mediterranean and the Caribbean , are divisions of the oceans . The oceans are in constant motion . The gravity of the Moon and the Sun pulls on the oceans , causing tides - the regular rising and falling of the ocean along beaches and coastlines . The Earth ' s rotation makes the oceans circulate clockwise in the Northern Hemisphere and counterclockwise in the southern Hemisphere . And winds cause waves to ripple across the ocean surface , as well as helping currents to flow underneath . Currents are like rivers within the ocean . Some are warm - water current , which can affect temperatures on land , and some are cold - water currents , which generally flow deeper . Major ocean currents , such as the Gulf Stream off the North American coast , also make for faster ocean travel . We know less about the oceans than we do about the Moon . The ocean depths hide dramatic deep trenches and enormous mountain ranges . The Mid - Atlantic Ridge extends for about 10 , 000 miles . It follows a curving path from the Arctic Ocean to the southern tip of Africa . Oceans affect our lives in important ways . They provide fish to eat . They add moisture to the air to form clouds . And the clouds then make the rain that helps plants grow . Some scientists are even working on affordable ways to turn salt water into fresh water for drinking , cooking , washing , and watering crops . If they succeed , it will be one of the most important inventions of our time . Did you know ? The Mariana Trench near the island of Guaam has th deepest spot measured so far , at nearly seven miles . The world ' s highest mountain , Mount Everest , could sink in that spot and still have a mile of water above it . The Youngest Ocean The Atlantic Ocean is the world ' s second largest ocean , after the Pacific . It covers nearly 20 percent of the Earth . If you tasted water from all the oceans , you ' d find the Atlantic to be the saltiest . And even though it is very old , it is actually the youngest ocean . The Atlantic Ocean lies between Europe and Africa on one side of the globe and North and South America on the other . It reaches from the Arctic Ocean in the north to Antarctica in the south . Like all oceans , the Atlantic has large movements of water circulating in it called " currents . " Atlantic water currents move clockwise in the northern half of the world , but counterclockwise in the southern half . The Gulf Stream , a powerful and warm current in the North Atlantic , moves along the east coast of North America . There and elsewhere , the Gulf Stream has important effect on the weather . Millions of tons of fish are caught each year in the waters of the Atlantic Ocean . In fact , more than half of all the fish caught in the world come from the Atlantic . The Atlantic is also used for activities such as sailing , windsurfing , and whale watching . But despite the usefulness and magnificence of the Atlantic Ocean , the level of pollution has increased . People have allowed fertilizers , pesticides , and waste from toilets and sinks and factories to get into the ocean waters . As people and businesses try harder to stop pollution , the Atlantic will again become a healthier home for its animal and plant life . Largest Ocean in the world The pacific Ocean is the largest ocean in the world . It covers nearly one - third of the Earth . The Pacific Ocean lies between the continents of Asia and Australia on the west and North and South America on the east . The Pacific ' s deepest parts are the ocean trenches . These trenches are ling , narrow , steep , and very deep holes at the bottom of the ocean . Of the 20 major trenches in the Mariana Trench . Part of the trench is nearly 7 miles deep . There are also many islands in the Pacific Ocean . Some islands were once part of the continents . Some that were part of Asia and Australia include Taiwan , the Philippines , Indonesia , Japan , and New Zealand . Other Pacific islands have risen up from the floor of the ocean . Many of them are born from volcanoes . These islands are built over thousands of years by the lava that comes out of the volcanoes . The Hawaiian Islands and the Galapagos , for example , started as volcanoes . The Pacific Ocean is very rich in minerals . It also has large supplies of oil and natural gas . And there is rich marine life in the Pacific . Fish such as salmon in northwestern America , bonito and prawns in Japan and Russia , and anchovy in Peru are all major food sources for people worldwide . Ocean Between Many Continents Millions of years ago , there was one huge mass of land in the Southern Hemisphere . It was the continent of Gondwanaland . But over many , many years Gondwanaland slowly broke up into the continents of south America , Africa , Antarctica , and Australia , as well as most of India . The water that filled the growing space between these continents is now the Indian Ocean . The Indian Ocean is a huge body of salt water . It is the third largest ocean in the world about five and a half times the size of the united States ! People from India , Egypt , and ancient Phoenicia ( now mostly in Lebanon ) were the first to explore this ocean . Later , Arabian merchants set up trade routes to the east coast of Africa . And Indian traders and priests carried their civilization into the East Indies . The dependable winds from the rainy season known as the " monsoon " made these voyages possible . Today the Indian Ocean has major sea routes . They connect the Middle East , Africa , and East Asia with Europe and the Americas . Ships carry tanks of crude oil from the oil - rich Persian Gulf and Indonesia . The oil is important to modern society , but spills from these oil tankers can endanger ocean life . The Indian Ocean is alive with plants , as well as animals such as sponges , crabs , brittle stars , flying fish , dolphins , tuna sharks , sea turtles , and sea snakes . Albatross , frigate birds , and several kinds of penguins also make their home there . The Sea in the Middle of Land The Mediterranean Sea gets its name from two Latin words ; m edius , meaning " middle , " and terra , meaning " land . " The Mediterranean sea is almost entirely surrounded by land . It ' s right between Africa , Europe , and Asia . The Mediterranean is a bit larger than the African country of Algeria . But more important than its size is its location . Its central position made the Mediterranean an important waterway for a number of ancient cultures , such as those of Italy , Greece , Egypt , and Turkey . Many channels connect the Mediterranean with other bodies of water . The Strait of Gibraltar connects the Mediterranean with the Atlantic Ocean . the Dardanelles and the bosporis connect it with the Black Sea , between Europe and Asia . And the Suez . Canal is a man - made channel connecting the Mediterranean Sea with the Red Sea , which lies between the Arabian Peninsula and North Africa . Three major rivers also lead into the Mediterranean Sea : the Rhone in France , the Po in Italy , and the Nile in Egypt . But the water from most of the rivers evaporates very fast . Instead , the Mediterranean Sea gets most of its water from the Atlantic Ocean . So Mediterranean water is very salty . There are many popular tourist resorts along the Mediterranean . These include some of the Mediterranean ' s many islands , such as Corsica , Sardinia , Sicily , Malta , Crete , and Cyprus . Tourists often like to take a cruise across the Mediterranean . They get to visit many different countries , try lots of different food , and see the remains of various ancient civilizations . The Ocean ' s Rise and Fall Perhaps you have been to the beach and put your towel really close to the water . Then , when it was time to leave , the water seemed to have shrunk and was now far away from your towel . What actually happens is even more surprising . At high tide the water creeps up the beach . At low tide the water slips down . so the water really doesn ' t shrink : it simply goes away ! But how , and where ? Most seashores have about two high tides and two low tides per day . It takes a little more than 6 hours for the rising waters to reach high tide . It takes another 6 hours for the falling waters to reach low tide . This 12 - hour rise and fall is called the " tidal cycle . " Tides are caused mainly by the gravity of the Moon and the Sun pulling on the Earth . This causes ocean waters to pileup in a big bump of water directly beneath the Sun and th Moon . As the Earth rotates , the tidal bumps try to follow the two heavenly bodies . The Sun and the Moon are in line with the Earth during a full moon or a new moon . Their gravity added together causes higher - than - normal high tides called " spring tides . " When the Moon and the Sun are farthest out of line , their gravity forcers offset each other . This causes lower - than - normal high tides , called " neap tides . " The tides in the Bay of Fundy in Canada rise higher than 53 feet . Beach towels and umbrellas at the Bay of Fundy don ' t stand a chance ! Did you know ? Some narrow rivers that empty into the sea develop large waves when extremely high tides rush into them . These waves , called " tidal bores " force the river ' s flow to change direction as they pass . Movement on the Seas The ocean never seems to sit still . Its waves rise and fall . on beaches they push forward and fall back . But what makes ocean water into waves ? Most waves are created by the wind . The wind blows along the surface of the water and forces waves in the same direction . The top of a wave is called the " trough ". When waves roll through the open ocean , they ' re called " swell . " As they reach the shore , their crests get higher and closer together and finally topple over . Then they ' re called " breakers " or " surf . " A gentle wind makes long waves that don ' t rise very high . But stronger winds push harder on the water and create taller waves . Big storms mean strong winds , and that means huge , powerful waves . Major ocean storms , called " hurricanes " or " typhoons , " can cause enormous waves . Some are so big they can smash oceanside houses into pieces or tip over ships that get in their way . During violent storms waves have been known to reach to the tops of lighthouses and to toss boats completely out of the water . The most destructive waves are tsunamis , but they ' re quite different from other waves . Tsunamis - also wrongly called " tidal waves "- are not caused by tides or by the wind . These huge waves are created by underwater earthquakes or volcanic eruptions . Waves of Destruction A powerful earthquake struck the coast of Chile in 1960 . Many people were frightened and got into their boats in the harbor to escape the disaster . Soon enormous waves caused by the earthquake rose up from the ocean . These violent waves , each more than three stories high , destroyed all the boats and killed the people in them . The waves then traveled for 15 hours across the Pacific Ocean to Hilo in Hawaii , where they destroyed more property . These waves are known as " tsunamis , " from the Japanese for " harbor wave . " A tsunami is a large destructive wave created by the shock from an earthquake or volcanic eruption . The impact of a meteorite could also create a tsunami . Tsunamis travel fast and have the force to destroy entire coastal communities within moments . A tsunami can travel at speeds of 450 miles per hour or more ( as fast as a jet plane ) and packs tremendous force . As the tsunami approaches land , it grows larger . It continues to travel until its energy is completely used up . All low - lying coastal areas are vulnerable to a tsunami disaster . In December 2004 an earthquake caused a major tsunami in the Indian Ocean . The earthquake struck off the coast of Indonesia . Two hours later , waves as high as 30 feet devastated coastal areas some 750 miles away . The tsunami killed more than 200 , 000 people in at least 10 countries . The Biggest Ice Cubes Icebergs are simply broken - off pieces of glaciers or polar ice sheets that float out into the ocean . Very big pieces . Even little icebergs called " growlers " are as big as a bus . Big ones are longer than a freight train and as high as a skyscraper . One especially surprising thing about an iceberg is that the part you see above the water is only a small bit of the whole iceberg . Most of the iceberg is underwater . You can see the way an iceberg floats by doing an easy experiment at home . Fill a clear glass half full of very cold water . Drop an ice cube into the glass . Notice how most of the ice cube stays below the water . The ice cube floats just the way an iceberg floats . And as the cube melts , it turns over , just as an iceberg does . Icebergs melt when they float away from freezing waters into warmer waters . Icebergs always start in the part of the world where it stays cold all the time , near the North or the South Pole . Icebergs can be very dangerous when they float , big and silent , into the path of a ship . In the past many ships were wrecked because they hit an iceberg . fortunately , this hardly ever happens anymore . This is because most modern ships have radar that finds the icebergs before they become a problem . In addition , special airplanes from the International Ice Patrol watch for icebergs in likely areas , and satellites scan the oceans every day . Maps and warnings are regularly sent by radio to all the ships in nearby waters . Rivers of Ice In high mountains there are places that are packed full of ice . These ice packs are called " glaciers " and look like giant frozen rivers . And like rivers , glaciers flow - but usually so slowly you can ' t see them move . It takes a long time to make a glacier . First , snow falls on the mountains . It collects year after year , until there is a thick layer called a " snowfield " In summer the surface of the snowfield melts and sinks into the snow below it . There it freezes and forms a layer of ice . This too happens year after year , until most of the snowfield has been changed into ice . The snowfield is now a glacier . The snow and ice in a glacier can become very thick and heavy . The glacier then begins to actually move under its own weight and creeps down the mountain valley . It has now become a valley glacier . The valley glacier moves slowly but with enormous force . As it moves , it scrapes the sides of the mountain and tears off pieces of it . Sometimes it tears off chunks as big as a house . As the glacier moves down the mountain into warmer regions , the ice begins to melt . The icy water fills rivers and streams . Many thousands of years ago , much of the Earth ' s surface was covered with moving glaciers . This period is sometimes called the Ice Age . As the world warmed , most of the ice melted away and formed many of the rivers , lakes , and seas around us today - including the Great Lakes in North America , which have an area greater than the entire United Kingdom . Floating Water Have you ever looked up at the clouds and wondered what they ' re made of ? Well , they ' re made of water - thousands of gallons of water , floating high in the air . It ' s easier to believe this when you know that cloud water takes the form of tiny droplets . The droplets are so tiny that you couldn ' t see one if it was separated from all the others . Sometimes the water droplets join together around tiny pieces of dust in the air . These drops get bigger and bigger as more droplets collect . When they become too heavy to float , they fall -" plop " That ' s rain ! There are three main kinds of clouds . " Cumulus " refers to the small puffballs or great wooly - looking clouds that are flat on the bottom . " Stratus " are low clouds , usually streaky or without much shape . And " cirrus " are light feathery clouds , like the ones in the photo . Sometimes cirrus clouds are so high , where the air is very cold , that the whole cloud is made of ice . Adding " nimbus " to any of these names changes it to mean a rain cloud . Tall white cottoy rain clouds are called " cumulonimbus " or " thunderheads . They often bring thunderstorms . Flat gray rain clouds are called " nimbostratus " They usually bring only rain . Snow , sleet , and hail also fall from clouds . Snow and sleet fall only on cold winter days . Hailstones can fall even on a warm summer day . And you may not realize it , but you ' ve probably been right inside a cloud yourself . A cloud so close to the ground that we can walk through it is what we know as " fog ". Nature ' s Fireworks It can be fun playing in a gentle rain , splashing in puddles and chasing raindrops . But this would be a dangerous thing to do when there are thunderheads above . Thunderheads are the large , dark , often fast - moving clouds that come out during storms . Thunderheads rumble mightily during storms , and that rumbling indicates the presence of lightning . The rumbling is the sound lightning make as it arcs across the sky . During a thunderstorm , electricity collects in the clouds . And often that electricity gets released as lightning . It ' s dangerous to be out when lightning is a possibility because lightning can quite easily kill from miles away . People have died from lightning strikes even though the storm the lightning came from was barely visible on the horizon . Lightning bolts frequently race to the ground , drawn by objects such as trees and lamp posts that are especially good conductors of electricity . Lightning is most attracted to tall objects , which is why trees , buildings , and radio towers are often struck . Actually , there are two parts to a lightning strike . The bolt from the sky is the part we don ' t see , because it is so fast and faint . The part we do see is the return strike . This is a bright flash of lightning that jumps up out of the ground to meet the lightning coming down and then races up to the base of the clouds . Lightning can hurt or kill people who are struck by it . If you ever are caught in a lightning storm , get inside quickly . Or get into a car . Lightning that hits a car will travel into the ground harmlessly . Did you know ? Florida is known as the " lightning capital of the world . " Every year lightning strikes in Florida more often than any other state in the United States , Also , lightning kills more people in Florida than in any other state . Nature ' s Fury A cyclone is a rotating storm that can be hundreds of miles across . These storms can be very destructive . The winds in a cyclone usually blow at more than 75 miles per hour . When a cyclone starts in the warmer waters of the Atlantic Ocean , it is called a " hurricane . " In the western Pacific Ocean , it is known as a " typhoon . " From above , a cyclone looks like a huge spinning doughnut of clouds . The center of the storm , the doughnut hole , is called the " eye . " The eye is quiet and cloudless . When the eye passes overhead , it might seem like the storm has ended . But within an hour or two , the eye passes and the other side of the storm hits . With its strong winds a cyclone also brings flooding rains and sometimes very high ocean waves . When a cyclone hits land , it causes severe damage . The combination of wind , rain , and waves can knock down trees , flatten houses , and wash away roads . Most cyclones start over tropical oceans because areas of warm water are their source of energy . Strong rotating winds that start on land are called a " tornado . " a tornado , such as the one pictured here , starts for different reasons and is smaller than a cyclone . But a tornado also has very strong winds , so it too can be very destructive . It can knock a train off its track or lift a house straight into the air . Fortunately , tornadoes usually die soon after they start . Did you know ? The best way for scientists to learn a cyclone ' s size and strength is to fly a plane through it . That ' s the most sure way - but certainly not the safest ! Arcs of Color If you ' ve ever looked at a rainbow and wondered how all those bright colors got in the sky , you ' re not alone . The ancient Greeks thought these arcs of color were signs from the gods to warn people that terrible wars or storms were going to happen . The Norse people believed a rainbow was a bridge the gods used to walk down from the sky to the Earth . Other legends said there was p pot of gold waiting at the end of a rainbow . But as beautiful as rainbows are , they aren ' t magic . And they aren ' t solid enough to walk on . In fact , a rainbow is just colored light . The seven colors are always the same and appear in the same order : red , orange , yellow , green , blue , indigo ( a very deep blue ) , and violet . The name " Roy G . Biv " helps you remember the first letters and the order of the colors . Rainbows foten appear after or at the end of a storm - when the Sun is shining again but there is still some rain in the air . The sunlight looks white , but all seven rainbow colors are mixed together in it . so when a beam of sunlight passes through raindrops , it ' s broken into the seven different colors . But you don ' t have to wait for rain to see rainbows . They can show up in the spray of a fountain or a waterfall , or you can make your own with a hose . Set the nozzle to create a spray , aim it away from the Sun , and then stand between the Sun and the spray . You ' ve got an instant rainbow ! Did you know ? In spite of some legends , there really is no " end " of a rainbow . Rainbows are actually full circles . but because we can see only a limited distance , to the horizon of Earth and sky , we see only part of the circle . Diamond Drops of Water Susan and her mother had come to the park for an early morning walk . The weather had been nice and warm recently . The nights were still , and there was hardly a cloud in the sky . The park ' s grass glittered and winked . " Are those diamonds ? " Susan asked . It looked as if someone had sprinkled tiny diamonds all across the grass during the night . Susan bent down to touch one of the glittering points . " It ' s water ! " she cried out in surprise . " How did it get here ? Did it rain last night ? " No , this isn ' t rainwater . It ' s dew . What ' s dew ? susan was eager to know . " It came from the air . All air has some water in it , you know , " said Mother . " So how does the water get onto the grass ? " " You know that steam turns into water again if it touches something cold , right ? " Susan nodded . " Well , on certain nights the air is warm and full of moisture , " Mother continued , " but the grass and the ground are cool . So when the vapor in the warm air touches these cooler surfaces . " " It changes to water drops on the grass , " finished Susan . " That must be why sometimes in the morning our car is covered with tiny drops of water . " " That ' s right , " Mother smiled . " Now let ' s get going on that walk ! " The Science of Their Changing Colors Trees that shed their leaves every year are called " deciduous " trees . Their leaves grow back again in spring . Scientists think that plants get rid of things they can ' t use . After flowers have helped make new seeds for a plant , its petals fall off . And soon after leaves have lost their green stuff , called " chlorophyll , " they fall off . A leaf ' s chlorophyll uses sunlight to make sugar out of water and carbon dioxide , a gas in the air . Plants need carbon dioxide to live and grow . When leaves use carbon dioxide , another gas , called " oxygen , " is left over . The plant can ' t use the oxygen . So it lets the oxygen go . Animals and humans need oxygen to live . Their bodies use the oxygen , and what do you think they have left over ? Carbon dioxide . When animals and humans breathe out , they let the carbon dioxide go . It ' s easy to see that plants and animals and humans help each other this way . In places where it gets cool in autumn , a plant loses its chlorophyll , and its leaves may turn yellow or red . The yellow was there all summer , but there was so much green that the yellow didn ' t show until the green went away . Yellow leaves turn red only if they have lots of sugar in their sap and the sunlight shines on them . The more sugar a leaf has , the redder it becomes . If a leaf is kept in the shade , it will stay yellow even if it has a lot of sugar . Did you know ? Deciduous forests are one of the world ' s six major life zones : the often frozen tundra , the mostly evergreen taiga , temperate ( mild ) deciduous forest , tropical rainforest , grassland and savanna , and desert . Sounds That See in the Dark " Helloo - o ! " A boy hears the echo coming from the hills . " Echo , talk to me , " he calls . "... to me , " repeats the echo . "... to me . .. to me . .. to me . " What is an echo ? It ' s a sound you make that bounces back to you from hills or other surfaces . But how can a sound bounce ? It ' s not a ball . Actually , sound is a wave in the air . If you could see air the way you see water , you ' d see the waves that sounds make . Sound waves bounce only if they hit something big and solid like the side of a hill or the walls of a cave . What if nothing stops the sound waves ? Then they just get smaller and smaller . Or they are absorbed by soft things such as carpets , draperies , or large pieces of furniture . That ' s why we don ' t usually hear echoes in the house . Did you know that echoes can help some animals " see " in the dark ? In pitch - dark caves bats fly easily , never bumping into anything and even catching tiny insects in the air . As they make tiny whistlelike sounds . These sounds bounce back to them . The direction of the echo and the time it takes for it to return tell the bats exactly where things are as they fly . Human beings have learned to harness the power of echoes for navigation too . Submarines traveling underwater use sonar to bounce sounds off of solid objects so that they can tell where those objects are located - sort of like undersea bats ! Did you know ? It is that a duck ' s quack is the only sound that doesn ' t echo . If you happen to have a duck and a long hallway , you could test this theory yourself . Killer Downpour Rain seems to make thing cleaner , doesn ' t it ? Rain helps flowers grow and helps keep plants green . It washes the dust from cars and houses . It leaves roads shiny . And it leaves a fresh smell in the air . But rain can be dirty . That ' s because as the rain falls , it gathers up any pollution that ' s in the air . It can leave cars looking streaky and windows spotty . Some rain will even ruin the paint on cars . It will damage or kill the plants it falls in and the fish living in lakes that are fed by rain . Such rain is called " acid rain . " This is how it happens . We burn fuels such as coal , gas , and oil in our factories . This releases gases containing elements such as sulfur , carbon , and nitrogen into the air . These combine with moisture in the air to form damaging substances such as sulfuric acid , carbonic acid , and nitric acid . When it rains , these acids fall to earth with the water . Acid doesn ' t fall to earth only in the form of rain . It can also fall as snow , sleet , and hail . It can even be in fog . Acid rain harms many forms of life , including human life . It also damages buildings . The acid eats through stone , metal , and concrete . Acid rain has injured some of the world ' s great monuments , including the cathedrals of Europe , the Colosseum in Rome , and the Taj Mahal in India . Harming Our Environment Have you ever seen black smoke spilling out of factory chimneys , turning the sky a dirty gray ? This is air pollution . Cars , trucks , buses , and even lawnmowers release gases and particles that pollute the air too . Smoke from fires and barbeque grills pollutes the air . Land pollution , water pollution , and even noise pollution are also big problems . Both factories and ordinary citizens may thoughtlessly dump trash and waste on land or in water . When farm chemicals that kill insect pests or holp crops grow sink into the ground and water , they pollute too . And noise pollution is created by loud machines and honking cars . Ocean life isn ' t safe from pollution . The picture you see here shows a cleanup crew at a polluted seashore after an oil spill . Ships carrying petroleum sometimes have accidents that dump their oil into the ocean . Dirty air , land , and water are dangerous . Dirty air , or smog , is hard to breathe and makes people and animals sick . Dirty water makes jpeople and animals sick when they drink it or wash or live in it . It also kills planes . If land takes in too much waste , nothing will grow on it , and it becomes unfit to live on . Stopping pollution isn ' t easy . Most people find it hard to change the way they live , even if they want to . And governments and big companies find it even harder to change , since the changes are often unpopular or costly . Even small changes help , however . Reusing things instead of throwing them away helps . Using less water each day helps . So does recycling . And perhaps the future will find people using cleaner forms of energy , such as wind power and solar energy . Studying the Earth How did the Earth get its shape ? What was the world like millions of years ago ? What is the Earth made of ? Why do earthquakes happen ? These are some of the many questions that geologists try to answer . Geologists are people who study the Earth ' s form and its history . The word " geology " comes from Greek words meaning " earth science . " Geology is an important science . Geologists help others to find useful fossil fuels such as oil and coal that lie hidden in the Earth ' s crust . Geologists also help figure out where earthquakes are likely to happen . This helps people know where it ' s safest to put up buildings . Because there are so many things about the Earth that geologists study , geology is divided into many individual areas . For instance , the study of physical geology looks at the changes that take place inside the Earth and the reasons for those changes . Geochemistry is concerned with the chemical elements that make up rocks , soil , and minerals . Petrology deals with rocks themselves . Did you know that paleontology is a form of geology ? Paleontologists study life forms that existed on Earth millions of years ago , from the tiniest bacteria to the most enormous dinosaurs . But because these creatures died so many millions of years ago , their bodies have turned into fossils - living things preserved as rock . The Earth ' s Building Blocks You might think that rocks are pretty dull . But rocks tell the history or the Earth , including stories of giant explosions , mountains rising from the sea , and buried forests turning to stone . Rocks are made up of one or more minerals . Most minerals are inorganic , which means that they are not made from living things . They substances that occur naturally in the earth . Some minerals are metals , such as iron and gold . Others are nonmetallic , like quartz and calcite . Some minerals contain the hardened remains of animals and plants . Limestone rock , for example , is made up largely of bits and pieces of shells and skeletons of sea creatures . All rock fall into one of three groups , depending on how they are formed . Igneous rocks are formed from cooling magma , which is the lava released in a volcanic eruption . The earliest rocks on Earth were igneous . Rocks don ' t stay the same forever . They break down into small pieces because of the effects of wind , water , and ice . When small pieces of rock settle together , they ' re known as " sediment " As layers settle on other layers over many years , their weight squeezes the pieces together into solid sedimentary rock . Both photos show the very common sedimentary rock called " sandstone . " which is cemented sand . The third group of rocks gets its name from the word " metamorphosis , " which means " change . " Metamorphic rocks are created when extreme temperatures or pressures cause changes in igneous or sedimentary rocks . Marble is a metamorphic rock formed from intensely squeezed and heated limestone . And limestone , you ' ll remember , began as seashells and skeletons . This is another amazing Earth story told by a rock ! Did you know ? Gold is the most easily shaped and stretched of all metals . It can be hammered as thin ad 4 millions of an inch . And an ounce of gold can be drawn into a wire more than 40 miles long . When Water is Stronger than Stone Caves are natural opening in the Earth large enough for a person to get in . Most have been made whin rainwater or streams have worn away rock - usually a softer rock such as limestone . The wearing - away process is called " erosion . " Slowly , over millions of years , the water works away at the soft rock , making a small tunnel - like opening . As more and more rock wears away , the opening grows wider and deeper . Soon even more water can flow in . In time , many of these openings become huge caves , or caverns . Mammoth Cave - Flint Ridge in Kentucky is a linked system of caverns . It is 345 miles long , one of the longest in the world . In france the Jean Bernard , though much shorter ( 11 miles long ) , is one of the world ' s deepest caves , reaching down more than 5 , 000 feet . Some caves have beautiful craggy formations called " stalactites , " like those pictured here , that hang from the cave ' s roof . These are made by water seeping into the cave . Each drop leaves a very tiny bit of dissolved rock on the ceiling of the cave . After thousands and thousands of years , an icicle - shaped stalactite forms . When water drips to the cave ' s floor , it deposits small particles of solids . These slowly build up into a stalagmite , which looks like an upside - down icicle . There are other kinds of caves that are made in different ways . When lava flows out of a volcano , it sometimes leaves gaps , making volcanic caves . When ice melts inside a glacier , glacier caves result . And ocean waves pounding on the shore year after year can wear away a cave in the face of a cliff . The Hardest - Working Gemstones in the World Diamonds were made millions and millions of years ago when fuming volcanoes melted the element called " carbon " inside some rocks . Gigantic masses of earth pressed the carbon tightly . The hot melted carbon was squeezed so tightly that by the time it cooled , it had changed into the hard gemstones called " diamonds . " Some diamonds are found in the gravel and sand near rivers . Others are left in the mountains by glaciers . Most diamonds are mined from rocks deep underground , usually in Africa . The country of South Africa is the major source for the diamonds used in jewelry . Diamonds usually look like pieces of glass or dull stones when they ' re first taken out of the ground . They must be cut and shaped to be used in jewelry . And diamonds are so hare that nothing can cut them except another diamond . Using diamond - edged tools , the diamond cutter carefully shapes and polishes the diamond so that it has straight edges and smooth surfaces . These edges and surfaces help the diamond reflect light so that it sparkles and flashes with tiny bursts of color . Diamonds often seem to flash like white fire . But there are diamonds that have other colors . Red , blue , and green diamonds are difficult to find . Yellow , orange , and violet diamonds are more common . Sometimes people even find black diamonds . Only the clearest diamonds become glittering gems . But because of their hardness , even dull - looking diamonds are still valuable as cutting tools . These are called " industrial diamonds . " only about 25 percent of all diamonds are fine enough to become jewels , so most of the world ' s diamonds are the hard - working industrial type . The Remains of Tiny Shells The material we call " chalk " had its beginnings during the time when dinosaurs lived on Earth . At that time the oceans were rising higher and higher until finally they covered most of the Earth ' s land . In those oceans lived billions of tiny animals . They were so small you could not have seen them - even smaller than the period at the end of this sentence . These tiny creatures had shells made of the element calcium . When these animals died , their shells fell to the bottom of the oceans . After thousands of years there were many layers of shells on the ocean floor . As more and more of the tiny shells pressed down from the top , those on the bottom became harder and began to stick together . Eventually the shells changed into a mineral called " calcite . " the main ingredient of the rock known as " limestone . " Many millions of years passed after the first chalk was made . The Earth ' s surface changed its shape , and the land and sea developed new shorelines . This left many chalk layers on dry land , both in the middle of continents and along coastlines . In some parts of England there are chalk cliffs 800 feet high . These are the famous White Cliffs of Dover , and they are almost solid chalk ! If you had a piece of chalk from those cliffs , you could use it to write on a chalkboard . But the chalk that you now use in classrooms is not a piece dug from the cliffs or the ground . It is made in factories by mixing several other materials together . Ancient Life in Stone Would you like to see something that lived millions of years ago ? You can if you find a fossil . The remains or traces of plants , animals , and even bacteria that are preserved in stone are called " fossils . " If you ' ve ever pressed a coin into clay and then removed it , you ' ve seen the sort of image you ' ll find in many fossils . The original thing isn ' t there anymore , but there ' s an impression of it left in the stone . Many fossils are easy to recognize as the living things they once were . The plant fossil you see in the smaller photo here , for example , looks like the tracing of a fern leaf . Usually , the harder portions of an organism are what last long enough to become fossils . Sometimes the hard structures are preserved almost whole . For instance , entire fossilized dinosaur bones have been petrified , or changed to a stony substance . Fossils are not always easy to find . only a small fraction of all ancient life ever became fossils . And the fossils that did form are often buried deep underground . You can tell the fossils in the big photo used to be fish . After they died - millions of years ago - they sank to the riverbed and became covered with soft mud . Their flesh wasted away , but their bones were held together by the mud . Eventually , the river dried up . It was filled with dust and dirt blown by the winds . The bones of the fish stayed where they were . Slowly , the mud of the riverbed turned to stone . Finally , somebody found this fossil while digging where the river once was . Ancient Elephants Believe it or not , thousands of years ago some elephants wore heavy fur coats . Actually , the mammoth was an ancestor of the modern elephant . And mastodons were distant relatives of the mammoth . Neither animal is around today . But at one time they roamed the Earth in great numbers . We know a lot about these ancient creatures because scientists have found many frozen mammoth bodies , especially in the icy area of Russia known as Siberia . Both beasts largely died out at the end of the last Ice Age , about 10 , 000 years ago . Mastodons and mammoths were a lot alike , but the mastodons were on the planet first . They appeared about 20 million years ago . They were smaller than mammoths and had thick legs like pillars . Mastodons were covered with long reddish brown hair . The mammoth didn ' t show up until about 1 1 / 2 million years ago . Mammoths were the size of modern elephants . The woolly mammoth had a thick furry yellowish brown undercoat with longer bristly hair over it . Like the mastodon , the mammoth had small ears and very long tusks . Despite these dangerous tusks , both animals ate only grass and other plants . The tusks may have been for shoveling snow and ice to uncover food . Mastodons and mammoths were around at the same time as early humans . The people of the day hunted the animals , but hunting didn ' t wipe them out . Scientists think that the mastodon and the mammoth vanished because the glaciers of the Ice Age destroyed much of the vegetation they relied on for food . Did you know ? In 1816 , when coal - gas lights were first being used , one of the first museum exhibits to be lit with the new invention was a mastodon skeleton . Giants of the Past The word " dinosaur " means " terrible lizard . " It is a name given to lizard - like animals that lived long , long ago . Many of the dinosaurs were the largest and scariest creatures that ever walked on land . All of them , large and small , were part of the animal group known as " reptiles . " The dinosaurs were the ancient cousins of today ' s crocodiles , snakes , and lizards . You may be familiar with the brontosaur , or " thunder lizard . " What you may not know is that this dinosaur is now called apatosaur , meaning " dishonest lizard . " A mix - up in fossil bones gave scientists the wrong idea of what it looked like . The apatosaur was still pretty impressive : it was as much as 70 feet long . No matter what you call it , this creature certainly must have sounded like thunder when it walked . The apatosaur was a plandt - eating dinosaur . Others , including the tyrannosaur was thought to be the largest carnivore ever to have walked on Earth . But the giganotosaur was an even larger carnivore ! The anatosaur is called the " duck lizard " because it had a bill like a duck - though there were hundreds of un - duck - like teeth in its cheeks . There were many other kinds of dinosaurs - more than 1 , 000 different species . And they once lived almost everywhere in the world . A Mystery Disappearance Many of the dinosaurs that once roamed the Earth were so big and strong that they didn ' t need to be afraid of any living thing . So why aren ' t there dinosaurs today ? Some scientists think that when new kinds of plants began to grow on Earth , dinosaurs couldn ' t eat them . New kinds of animals smaller than dinosaurs also appeared during this time . They may have been able to survive better than the dinosaurs . It ' s also possible that disease killed dinosaurs by the millions . Not all scientists think that all dinosaurs died at once . Another explanation is that a changing climate killed them . We know that when they were living , the weather began to change . Summers grew shorter and winters grew colder . In some places heat waves dried up rivers and swamps . Elsewhere , new lakes and rivers appeared , and many places were flooded . Some dinosaurs may have died because it gradually became too cold or too hot for them . Many scientists believe that dinosaurs died because an asteroid struck the Earth about 65 million years ago . The dust raised by the impact would have blocked out sunlight for months , so that plants stopped growing and the temperature dropped . When plant - eating dinosaurs died from lack of food , so would the meat eaters that hunter them . Some scientists think that many dinosaurs evolved into birds . So the next time you see a robin , consider that you may be looking at a dinosaur ' s relative . The Tyrant King It was longer than a bus , weighted more than four tons , and had teeth up to a foot long . The tyrannosaur may have died out 65 million years ago , but it is still one of the largest meat - eating land animals that ever lived . It ' s no wonder that the first scientist who discovered this frightening creature ' s bones called it Tyrannosaurusrex : " tyrant lizard king . " Dinosaurs were not true lizards . When scientists first discovered tyrannosaur fossils , however , they did believe that such a dangerous - looking animal would have been a powerful and mean bully among the dinosaurs . The tyrannosaur ' s jagged teeth and huge jaws make it clear that the tyrannosaur was a powerful carnivore , or meat eater . Tyrannosaurs lived mainly in what is now North America and Asia . The creature was about 40 feet long from its head to its thick and heavy tail . The tyrannosaur probably stooped forward , with the big tail balancing its weight when it walked . The tyrannosaur had large , powerful rear legs but small front arms . These forearms wouldn ' t even have been able to reach its mouth . So the tyrannosaur probably planted its clawed rear feet on a dead animal , bit hard , and ripped the flesh away from the carcass . The tyrannosaur is one of the most popular of all dinosaurs , thanks to movies and books . But scientists still don ' t know a lot about the beast . Did it hunt by sight or by smell ? Was tyrannosaur a hunter at all , or did it just eat dead animals it found ? Was it a fast runner ? With so many questions , we ' re still getting to know the tyrannosaur - but from a safe distance !