Fossils


 



What are fossils?



Fossils are the traces of ancient animals and plants found buried in rock. Sometimes a fossil retains the shape and structure of the hard parts of an animal, such as fossilized dinosaur bones. These are not the original bones, because minerals have replaced them over millions of years, but they have the same shape. Other fossils are just the impression of an animal or plant, created when the plant or animal was buried in mud that has gradually solidified into rock. Even footprints of animals such as dinosaurs have been preserved. 



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How old are fossils?



Ancient history has been divided up into eras, which are periods of time identified by the fossilized forms of life from that period. The oldest era, called the Palaeozoic, contains fossils ranging from many primitive life forms up to some of the earliest land-dwelling animals. During this era, fishes, amphibians and early reptiles appeared. The Mesozoic era was the age of giant reptiles, when dinosaurs stalked the world. The Cenozoic era in which we still live is the age of mammals and birds. All fossils can be placed in these eras, which are subdivided further into smaller periods.



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The Changing Earth



How do we know that the climate has changed?



In more recent times, geologists and paleontologists have been able to make more accurate measurements of the age of ancient rocks. Trees produce a new ‘ring’ every year, and these can be measured and counted. The width of the ring shows how well the tree grew in any year, reflecting climatic conditions at that time. The rings of even partly fossilized trees buried in bogs can be measured, and these measurements provide accurate records of climate changes over the past 750,000 years. By measuring the radioactivity of once living material, scientists can make accurate measurements up to 40,000 years ago. 



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How was oil formed?



Most scientists think that oil was formed from the remains of tiny plants and animals that inhabited the seas millions of years ago. When these creatures died, their remains were buried in layers of rock. Substances in their bodies gradually altered to form oil. Other scientists disagree, and believe that oil formed from carbon-containing materials that were trapped inside the rock when the Earth was formed. Oil is found in rocks buried beneath the continents, and beneath the shallow parts of the oceans. More than half of the world’s known oil resources are in the Middle East. 



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The Changing Earth


 



 



Why did the world’s climate change?



The appearance of living things changed the Earth’s atmosphere, providing the conditions for climate change. The first living things were plant-like creatures. They used the Sun’s energy to change carbon dioxide and water into sugars, which they used for food, and oxygen, which they released into the atmosphere. Some of this oxygen turned into ozone, forming the layer that now protects us from the Sun’s ultraviolet radiation. 



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What effect did the Ice Age have on the world and its animals?



During an Ice Age, the enormous weight of the ice sheet presses the underlying rocks down, while those around the edges rise to form hills and valleys. The constant scouring action of glaciers shapes the Earth’s surface, wearing away complete mountain ranges. In an Ice Age, animals are forced to migrate to warmer areas. The ones that cannot adapt, for example mammoths and the woolly rhinoceroses, become extinct. The ice sheets on Greenland and in the Antarctic are the remains of the most recent Ice Age. 



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The Changing Earth


Poisonous air



The Earth’s original atmosphere was very poisonous. It contained almost no oxygen. It consisted of gases such as hydrogen, helium, methane and ammonia, as well as carbon dioxide. It would have been impossible to breathe in such an atmosphere Lightning and the Sun’s radiation caused chemical reactions in these gases, producing some of the substances characteristic of life, and this may have triggered the development of the first primitive life forms.



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How has the Earth been reshaped?



Erosion is one of the most powerful ways in which the Earth’s surface has been altered. Moving ice and flowing water wear away the surface of rocks and cut out valleys, as well as wearing down the peaks of mountains. Along the coast, tides and wave action wear away exposed cliffs, and currents carry away sand and mud to be deposited elsewhere. The tides and wind-blown sand polish and wear away rocks and pebbles. 



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The Changing Earth


What is an Ice Age?



Ice Ages were periods when large parts of the Earth’s surface were covered by sheets of ice. Each Ice Age has lasted about 100,000 years, with gaps between of up to 20,000 years when the weather was warmer and the ice melted. The last Ice Age ended about 12,000 years ago. Ice ages appear in groups in geological time, each lasting for 20 to 50 million years. The oldest known glacial periods were as long as 2.3 billion years ago. A minor ice age began in the 1500s and lasted for 300 years, during which glaciers were more widespread than at any time for thousands of years.



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Gigantic waterfall!



More than five million years ago, the Mediterranean Sea was a dry basin. Then, movements in the Earth’s crust opened up the Straits of Gibraltar between the land masses of Europe and Africa. The waters of the Atlantic Ocean poured through the gap, flooding the basin. The result was the creation of a gigantic waterfall, at least 800 m high, which let through so much water that the whole of the Mediterranean filled up in few years. 



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The Changing Earth


 



What was the Earth like when it was formed?



The Earth had probably existed for around 3,000 million years before the first signs of life appeared. As it cooled after forming from a ball of hot gases, the Earth would have been a relatively smooth sphere. It was covered with shallow seas that contained a mixture of dissolved chemicals, and surrounded by an atmosphere of mixed gases. The atmosphere was full of swirling clouds of vapour, which probably caused huge electrical storms. 



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When did life first appear?



The first signs of life are thought to have appeared about 3,500 million years ago. Some rocks from this time contain substances that are characteristic of life, although fossils appear from much later. Scientists believe that primitive life forms altered the original poisonous composition of the atmosphere, eventually releasing the oxygen that we need to survive. The seas were probably once filled with a mixture of chemicals from which life may have developed. It is more likely, however, that life first appeared in areas such as those around undersea vents. These first forms of life would have been bacteria and microscopic blue-green algae. 



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Reshaping the Earth


What is irrigation?



Irrigation is a method of supplying water to crops artificially. It was used in ancient Egypt, and possibly even earlier. Water is pumped out of rivers or wells and fed into channels or canals that carry it for long distances. Modern irrigation systems are usually very extensive.



Elaborate irrigation was used centuries ago in some parts of the world. Native Americans cut long canals along the foot of the Andes to irrigate their crops, which they grew on artificial terraces. Even today, many Asian hillsides are covered with flooded terraces where rice is grown. The terraces are irrigated by complicated canal systems. 



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How do dams affect the environment?



Most dams are built to meet people’s growing demand for water, but sometimes dams cause unforeseen environmental damage. Dams are usually built across valley entrances, creating enormous reservoirs of water. Some are built to provide water for hydroelectric power stations while others control the flow of rivers that are liable to flood. Unfortunately, the best places to build dams are usually in the most picturesque parts of the countryside.



The Aswan High Dam was built to control the flow of the River Nile. However it has prevented the river’s annual flooding, which covered agricultural land with silt.



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Reshaping the Earth


What are the effects of deforestation?



Many areas of tropical rainforest are burned to provide farmland, but the effects of this deforestation can be disastrous. After the forest is burned, new fertile ground is exposed. But after one or two years this new land is exhausted. The clearing process is repeated, eventually destroying the forest and its wildlife. The bare ground becomes eroded because there is little vegetation to slow the run-off of rainwater. The water strips away the topsoil, dumping it into rivers. The end-result is useless, infertile land that is prone to flash floods. Also, the smoke from burning forests contributes to global air pollution. 



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Can rivers run backwards?



In the former Soviet Union, the direction of several rivers was diverted or even reversed to provide water for irrigation. Some of the rivers running into the Aral Sea were diverted northwards in a huge water management project to irrigate land north of the region. In some cases the direction of their flow was reversed. The result was that the Aral Sea began to dry up, because no more river water flowed into it. 



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Reshaping the Earth


 



Can we push back the sea?



The action of the waves continually erodes beaches and cliffs, so sea walls are often constructed to protect them. It is very expensive to control the action of the sea in this way. Much of the Netherlands originally lay under the North Sea. For centuries, the Dutch have built enormous dykes (banks) to hold the water back. They have also reclaimed land from the sea, increasing the size of their country by almost one-third.



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Hole in one



In the 1970s, scientists discovered a gap in the protective ozone layer around the Earth. Ozone, a form of oxygen, filters out more than 90 percent of the Sun’s damaging ultraviolet rays. So when a gap in the ozone layer was found over Antarctica, scientists were very concerned. Increased ultraviolet radiation can cause skin cancer. The hole in the ozone layer has been blamed on our use of chemicals called chlorofluorocarbons (CFCs). They were widely used in refrigerators, freezers and aerosol cans. Although the use of CFCs is now heavily restricted, it may take years before the ozone layer repairs itself. 



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Reshaping the Earth


 



 



What is acid rain?



Rain containing dissolved acids falls to the ground, damaging trees, crops and other wildlife as well as harming water supplies. Acid rain is caused by air pollutants such as nitric and sulphuric acids, which are released by burning coal, oil or petrol. Damage due to acid rain has destroyed huge areas of trees in North America, Europe and parts of Asia, and has had serious effects on forestry. Attempts are being made to limit the damage by removing nitrogen and sulphur from fuels before they are burned, and by reducing emissions from factories. 



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What was the Chernobyl disaster?



In 1986 an explosion in a nuclear power station in the Ukraine was the worst ever nuclear accident. Large amounts of radioactive material were released into the atmosphere, and many people died. Large areas of the Ukraine and neighbouring Belarus were contaminated, and there has been a huge increase in the number of cancers and abnormal births in the region. The radioactive fallout was carried by the wind right across Europe into Sweden and the UK. It is still too dangerous to live in the area around Chernobyl because of the continuing high levels of radioactivity. 



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Reshaping the Earth


What is global warming?



Since the late 1800s, the temperature of the Earth’s surface has risen, and it is predicted that this increase will continue. This phenomenon, known as global warming, is thought to result from human activities. By burning wood and fossil fuels such as coal, we have increased the amount of carbon dioxide in the atmosphere by 25 percent. At the same time the average temperature of the world’s surface has increased by about 0.8 degree Celsius. It seems likely that there is a connection between this temperature increase and increased levels of greenhouse gases, such as carbon dioxide. 



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What causes air pollution?



Air pollution is mostly a result of human activities. The exhausts from motor vehicles are one of the main pollutants. They contain greenhouse gases that are thought to contribute to global warming. Exhaust gases also contain substances such as sulphur and nitrogen oxides that can damage the lungs. Recently, vast clouds of choking smoke have appeared over parts of Asia. The smoke comes from the rainforest fires that clear land for farming.



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Ice and Snow


 



Where does most snow fall?



Snow can fall wherever the weather is very cold, and it falls throughout the year in the Arctic and Antarctic.



Most snow falls in temperate climates during the winter, especially in the Alps in Italy and Switzerland and in the Rocky Mountains in the USA. In fact, almost every mountain range is associated with heavy snowfalls. Snow can even fall on the Equator on mountains more than 4,880 m high, where the air is very cold. 



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How thick is the ice at the South Pole?



The thickest ice at the South Pole is 4,800 m deep — that’s ten times the height of a tall skyscraper. The ice covers a huge area of 14 million sq km, which is bigger than the whole of Europe. However, the land area under the ice is much smaller because the ice sheet extends out over the sea. Antarctica contains 90 percent of all the ice on the Earth. Scientists have calculated that should all this ice melt sea levels would rise by 60 m causing world-wide flooding.



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Ice and Snow


How does frost break up rocks?



Frost can shatter rocks, making it one of the most important causes of erosion in cold regions. Water seeps into small cracks in rocks and freezes at night, when frost appears. As water freezes it expands slightly, opening up cracks in the rock until parts of it flake away. This process continues until large masses of rock are broken up. It is the cause of the piles of shattered rock, or scree that are commonly found at the foot of mountains. You can see the same process at work when a clay flowerpot left in the garden in winter begins to flake and crack. 



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Why do icebergs float?



Ice floats because it is lighter than water. Most objects shrink when they get cold, but when water freezes it expands slightly. In this way ice becomes lighter than water. It is only slightly lighter however, and so nine-tenths of a floating iceberg lies under the water. 



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Ice and Snow


 



Frozen



Glaciers and ice sheets sometimes give up their secrets centuries later. Thawing ice has revealed the bodies of people who fell into crevasses hundreds of years ago. The ice has preserved their clothes and their internal organs. Even more astonishing are the bodies of mammoths found occasionally in the permafrost under Alaska and Siberia. Scientists are trying to extract their DNA so they can study these extinct beasts in minute detail. 



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How big are icebergs?



Some icebergs are bigger than an entire country. The biggest iceberg ever recorded was larger than Belgium, with an area of 31,000 sq km. These giant icebergs break away from the polar ice sheets when the rise and fall of the tide crack the ice. 



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Ice and Snow


Is there such a thing as red snow?



Red snow is found in many parts of the world, when tiny plants grow over the snow and ice. Microscopic algae growing on the surface of snow and ice can give them a red colour. This rare event happens when the weather warms up slightly, and the algae that have been dormant throughout the cold period suddenly start to grow again. 



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What are glaciers?



Glaciers are large masses of ice that form on land and move slowly under their own weight. They are found in cold mountainous places or Polar Regions. Some glaciers are huge continuous sheets of ice, such as those covering Antarctica and Greenland. But most are rivers of ice that move slowly through valleys until they melt and become rivers. Such glaciers are common in the Himalayas, Alps and Rocky Mountains. Because of its enormous weight a glacier scrapes off large amounts of rock and debris, wearing away the valley floor and sides until it carves out a deep ‘U-shaped’ valley. Glaciers formed the deep fjords that line the coast of Norway.



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