My Science School

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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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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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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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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How can rocks be folded up?

Most rocks are too brittle to be reshaped, but folding is common in sedimentary rocks. Movements in the Earth’s crust force flexible sedimentary rocks into folds. These folds are sometimes so large that the rock lays back on itself in layers. Geologists may find that the layers of rock are actually upside down, with the youngest rocks on top. 

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What is a mountain range?

A group of mountains is called a range. Nearly all large mountains are grouped in ranges. Single high peaks are usually old volcanoes. Mount Kilimanjaro in Tanzania is the highest mountain in Africa. It is a single peak, and is an extinct volcano. These isolated peaks always look much higher than mountains in ranges, because they are not surrounded by the usual lower foothills that disguise their true height.

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Why do clouds form near mountains?

Clouds form when damp air is forced upwards to pass over a mountain range. Air is forced up the face of the mountain because it is unable to go around it. There is usually a high wind on the top of the mountain. As it rises, the air becomes cooler and the moisture in the air condenses to form water droplets, resulting in the formation of clouds. This explains why there is often high rainfall and snow in mountainous regions.

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What is a rockslide?

Rock is broken away from the mountainside by the effects of freezing. Rainwater enters cracks in the rock and as it freezes, it expands and opens the crack wider. As the thaw begins the pieces of rock splinter and separate from the bedrock. Loose rock builds up continuously on a mountainside, but usually only small pieces slide down. This loose broken rock is called scree.

A rockslide takes place when the mass of broken pieces of rock slides down the side of a mountain. This usually happens in very wet weather, when rain lubricates the rock and allows it to slide freely down the mountainside, mixed with a torrent of mud.

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Are climbers able to breathe at high altitudes?

The higher you climb, the thinner the air becomes, until there is not enough oxygen to support life. On the highest mountains, there is just about enough oxygen for life to survive for a short time. Extremely fit mountaineers have managed to climb to the top of Mount Everest without the use of oxygen cylinders.

Our bodies can adapt slowly to a lack of oxygen, which is why expeditions to the Himalayas and other high ranges camp at progressively higher levels to get used to the thinner air. People living at high altitudes in the Andes and the Himalayas develop enlarged chests and changes in their lungs to help them breathe.

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What is chalk?

Chalk is formed from the skeletons of millions and millions of tiny animals called foraminifera. It is a sedimentary rock that formed millions of years ago beneath shallow seas. Chalk is used in the manufacture of rubber goods and paint. 

 

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Which are the highest mountains in the world?

The world’s highest mountains are in the Himalaya-Karakoram range to the north of India. The Andes Mountains in South America cover a larger area but are not so high. The Himalayas contain 79 peaks that are over 7,500 m high. The highest of these is Mount Everest, which is 8,863 m high. At these altitudes, mountains are always covered in snow and ice, and there is little oxygen to breathe. Although Mount Everest is the tallest mountain when measured from sea level, Mauna Kea in Hawaii is actually the tallest if measured from its base. It reaches 5,818 m below the surface of the Pacific Ocean. From its base to its peak, Mauna Kea is 10,023 m high.

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On top of the world

After many years Mount Everest was finally conquered on May 29, 1953, when a Nepalese guide, Tenzing Norgay, and a New Zealander, Edmund Hillary, reached the highest point on the Earth’s surface. Since then, many people have climbed Everest, and all the world’s major peaks have been conquered. It is now possible to map mountains from the air so that mountaineering routes can be carefully worked out in advance. Also, equipment has improved so that sheer rock faces can be readily climbed. Altitude sickness and avalanches of ice and snow are now the main hazards faced by climbers. 

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How are mountains formed?

There are three main ways in which mountains can be formed. Volcanoes form mountains when lava from deep inside the Earth cools and hardens on the surface. Other mountains are formed when layers of rock are forced up into folds, or when rocks fracture to cause faults, allowing slabs of rock to be raised up. These movements are caused by the very gradual shifting of the land areas, which sets up stresses in the Earth’s crust. 

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Can water wear away solid rock?

Water itself cannot wear away rock, but tiny particles of grit carried in the water can eventually wear away the hardest rocks. It is this continuous wearing process that cuts valleys through mountains and hills. The faster the water flows, the more the grit it carries wears away at the rock. You can see the same process in action along the coast. There, rocks are worn away by the action of sand dashed against them by the waves. 

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What is a spring?

Many types of rock contain tiny holes like a sponge, in which rainwater collects. The water gradually sinks down and eventually flows out lower down. Water stored in such porous rocks sometimes bubbles to the surface in springs. They are usually found near the foot of hills. Spring water is often used for drinking because as it passes through rock, any harmful substances or germs are filtered out. On more level ground, spring water is found by digging a well to reach the underground water. Small outflows of water can lead to the formation of isolated bogs or marshes. 

What is a delta?

As a river reaches the sea its flow becomes slower, forming an area known as a delta. As the water slows the tiny particles of silt carried by the current settle to the bottom, forming mud banks. These make the river-bed shallower, so the water spreads out to form a wide estuary. In large rivers like the Danube, islands form among these exposed mud banks, and the river water trickles through a maze of small waterways. This region, which is rich in plants and other wildlife, forms the river’s delta.

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Uphill flow!

Water always flows down a river, but occasionally the flow is reversed. This happens only in very large rivers, when very high tides overcome the normal river currents. In narrow parts of the river valley the water begins to pile up, and eventually a wave called a tidal bore passes back up the river, sometimes for a great distance. This happens in the Amazon in South America, where there is a bore as high as 4.5 m. A smaller bore travels up the River Severn in England.

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Which is the world’s longest river?

The Nile in Africa and the Amazon in South America are the world’s longest rivers. The Nile is 6,670 km long, and the Amazon is 6,448 km long. It is difficult to give an exact length because these huge rivers split into tiny tributaries near their sources, and some of these dry up or change course.

The Amazon is by far the largest river because it carries more water than the Nile, Mississippi and the Yangtze put together. The Amazon is 140 km wide at its mouth, and up to 91 m deep in places.

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How does a river begin?

Most of the rainwater that falls on the land returns to the sea by way of rivers. Rainwater first soaks into the ground, but once the ground is saturated the excess water begins to run off in small streams. More water comes from melting ice and snow. Small streams gradually merge and become bigger, until they eventually form a river. The tiny streams that are the source of a river often begin in damp areas such as swamps or bogs, or may flow out of a pond or lake. Usually they are in hills or mountains, where the rainfall is heaviest. 

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What causes the tides?

Tides are caused by the gravity of the Moon. As the Earth spins round, the water in the oceans is ‘pulled’ towards the Moon slightly, making a bulge. There is a corresponding bulge on the other side of the Earth. Wherever the bulges are positioned it is high tide. In between the water is shallower and so it is low tide. High tides occur every 25 hours, because at the same time that the Earth is spinning on its axis, the Moon is travelling around the Earth once every 27  days. This means that high tide is about one hour later every day.

 

 

 

 

What is the continental shelf?

The shallow underwater plateaus of land surrounding the continents form the continental shelf. They slope gently down to a depth of about 180 m, after which they slope more sharply towards the deep ocean. The continental shelf can extend out from the coast for long distances, or it may be narrow, for example off the coast of Chile where the Andes Mountains plunge down into the Pacific with hardly any shelf. Usually the shelves are wider off low – flying regions. The continental shelf is a valuable resource, as most fish are found here. Also, there are plans to mine some of the minerals on the sea – bed in these shallow waters.

 

 

How deep are the oceans?

The depth of the oceans varies widely, but the deepest part found so far is called the Marianas Trench. This trench is a deep cut in the bed of the Pacific Ocean, and its depth is as much as 11,034 m. Even at this enormous depth, signs of marine life have been seen in the trench. In 1960 Dr Jacques Piccard and Lt Donald Walsh reached a depth of 10,911 m in a US Navy vessel, Trieste. The Pacific Ocean is home to nine of the ten deepest sea trenches. The exception is the Puerto Rico trench in the western Atlantic Ocean, which has a maximum depth of 9,220 m.

 

 

 

 

Why did sailors fear the Sargasso Sea?

Sailors feared that the Sargasso Sea was haunted by sea monsters and filled with seaweed that would entangle their ships and prevent their escape. The Sargasso Sea is a patch of ocean in the North Atlantic, about 3,200 km west of the Canary Islands. The ocean currents in this area are very slow, although they are surrounded by fast moving currents. This means that any seaweed floating in the Sargasso Sea will not be carried away. Patches of floating weed can be as large as 30 m in diameter. Despite the legends, they are never thick enough to entangle a ship.

 

 

Why is the sea salty?

A mixture of salty substances is washed out of rocks by rivers and deposited in the sea. When rain falls on the land it gradually dissolves minerals. The dissolved salts slowly enter rivers and streams, and are carried down eventually into the sea. This process is happening all the time, and so the oceans and seas gradually become more salty.

 

 

 

 

 

 

What is the Gulf Stream?

The Gulf Stream is an ocean current that controls the climate and weather of the whole of Europe. The current starts in the Gulf of Mexico, carrying warm water across the Atlantic and travelling northeast until it reaches the European coast. Water evaporates from the surface of the Gulf Stream causing rainfall in Western Europe.

 

 

 

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