Category Science

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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