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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