The Wonders of Planet Earth

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'The perpetual cycle of change which has created the face of the Earth, with all its rugged and Fascinating variety usually happens too slowly to be noticed. But occasionally it is rapid and Violent. Volcanoes disgorge molten lava, earthquakes rip open the landscape, landslides, carry away Whole mountainsides. Then human beings become aware of the awesome forces that are shaping Their planet. These forces are fuelled by three powerful sources of energy - heat from within the Earth, heat from the Sun, and the force of gravity. Every landform in the world has been shaped by these ttu.ee energy sources. 'the continents that drift across the surface of the globe, setting off volcanoes and earthquakes and Building mountains. are driven by heat from the Earth's interior which has a temperature of about 5000"C (9000'F).

Most of this heat is created by the breakdown of radioactive elements. Earth is unique among the planets of the Solar System in having liquid water on the surface and water has a major role in shaping the planet. The warmth of the Sun evaporates water from seas and lakes. The vapor rises and condenses to form clouds and then falls again as rain and snow. It is then that its landscaping powers begin, weathering rocks and washing away the loose material. or grinding down the landscape under the power of a glacier. The Sun's heat also produces the rain and the waves that scour the land.

The third force - gravity - causes the tides, which nibble away at the edges of continents, and landslides. which alter the shape of mountains. Under the influence of gravity. rain works its way downwards as streams and rivers, carving the terrain. On its journey, it carries fragments of rock and sand to be deposited on the ocean floor. And over thousands of years more rock which may then be buckled and lifted up by movement of the Earth's crust to form new mountains. {source: Readers Digest, Discovering the Wonders of our World A guide to natures Sciences marvels]
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