Volcanoes


Why do geysers spout hot water?



When water seeps through the ground and reaches magma, hot rock or volcanic gases, it boils violently. This produces steam that can shoot the water out of cracks, causing a geyser. Geysers can be very spectacular, and some shoot water as high as 500 m into the air, although most are much less powerful. Sometimes they erupt at regular intervals, but most erupt only occasionally. 



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What happened when Krakatoa erupted?



The eruption of Krakatoa, in Indonesia, in 1883 was the biggest explosion in recorded history. Krakatoa was a small island between Java and Sumatra, on which there was a small cluster of volcanoes that were not dangerously active. On May 20, 1883, one of the cones erupted violently, and three months later the whole island blew up. The explosion could be heard in Australia, 3,500 km away, and a cloud of ash rose 80 km into the air. For two and a half days the island was in total darkness because of the amount of dust in the air.



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Volcanoes


Volcanic bombs



A major volcanic eruption can hurl boulders high into the air. These boulders, called volcanic bombs, can be very large. Most of the material thrown out of the erupting volcano is ash, which forms a huge cloud. Steam and sulphurous gases are also released, and these can be very dangerous to bystanders. Pumice is a kind of foamed rock that is often thrown out during an eruption. It is full of tiny holes, making it very light. It is formed when gases in molten rock are boiled off; creating foam that solidifies as it cools. 



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What is the Ring of Fire?



All around the rim of the Pacific Ocean is a circle of places of volcanic activity known as the Ring of Fire. The Ring of Fire is caused by shifts in the giant plates that make up the Earth’s crust. The Pacific plate is gradually disappearing under the surrounding continental plates, and volcanic activity marks the points where this movement is taking place. Frequent earthquakes accompany the volcanic action in the countries surrounding the Pacific. Also, ocean trenches and underwater volcanoes form around the Ring of Fire. 



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Volcanoes

What is lava?



Lava is molten rock that reaches the Earth’s surface. Its temperature may be as high as 1, 200 degree Celsius. Lava may be as runny as water or so thick that it scarcely moves at all. The liquid type of lava that flows from Hawaiian-type volcanoes runs downhill in sheets and streams, travelling very fast. Sometimes the surface of these streams cools and hardens, and the liquid lava continues to flow underneath, eventually producing lava tubes when it cools. The thicker type of lava inches its way down the side of a volcano like a very slow avalanche, carrying with it anything in its path, as well as red-hot lava. 



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What happened to Pompeii and Herculaneum?



Two busy Roman towns were completely buried in lava and ash by an eruption of Mount Vesuvius in AD 79.



A burning rain of ash, lava and volcanic debris covered Pompeii to a depth of more than 3 m. Nearby Herculaneum was buried under about 18 m of ash. Many people were overcome while trying to escape the eruption. In Pompeii, cavities were found in the ash where their bodies had lain and then rotted away. The two towns preserved everyday Roman life, and they are still being excavated. 



 



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Volcanoes


What is a volcano?



Volcanoes are openings in the Earth’s crust through which molten lava, red-hot rocks, steam and fumes pour out. We usually think of a volcano as a smoking mountain that erupts explosively, but most volcanoes are simply cracks in the crust through which lava flows continuously.



The more familiar kind of volcano has a single central pipe though which lava reaches the surface, building up into a cone as it cools. The cone consists of layers of lava and volcanic ash. There is usually a crater at the centre. Most active volcanoes smoke and spit out occasional pieces of lava, and dramatic eruptions are rare.



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How fast do volcanoes grow?



Unlike most geological happenings, the growth of a volcano can be very rapid. In 1943, a farmer in Mexico noticed smoke coming from a crack in the ground in his cornfield. Lava began to ooze out, and six days later it had piled up into a volcanic cone 150 m high. By the end of the year the cone had grown to 450 m. The eruption finally stopped in 1952, when the volcano Paricutin had reached a height of 2,808 m above sea level. The volcano had buried two whole villages. 



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Our Planet Earth



 



What are metals?



Metals are one of the major groups of elements. Most of them are shiny and hard. Mercury is the only metal that is a liquid at normal temperatures. Most metals can be bent and stretched, and they can be mixed to make alloys.



Metals are usually found with other elements in the form of compounds. Bauxite is an ore, or mixture of substances, that contains aluminium, the commonest metal. It makes up about eight percent of the Earth’s crust. Osmium is the heaviest metal, and is twice as heavy as lead. Lithium is the lightest metal. It is half the weight of the sane volume of water, so it floats in water.



 



 



 





 



 



How much of the Earth is covered by water?



More than two-thirds of the Earth’s surface is covered by seas and oceans. About 72 percent of the Earth’s surface is water. This water is either in the oceans, locked away as ice at the poles, or held as water vapour in the atmosphere. All of the Earth’s water is known as the hydrosphere. 



 



 



 



 



 





 



 



 



Why is the Earth round?



Gravity pulled the Earth and the other planets into a sphere when they were being formed. The Earth is not truly round, but is slightly flattened making it bulge out at the Equator. This effect is due to the speed with which the Earth spins, causing the Equator to try to fly out from the axis of the spinning Earth. It is rather like a heavy object whirling round on the end of a piece of string. The shape of some of the other planets is distorted in the same way.