My Science School

A monsoon is a season of very heavy rain falling is tropical countries. Monsoons are most common near the Equator. They occur when seasonal winds spring up carrying moist air in from the sea. Important crops such as rice depend entirely on the monsoon season to provide the right growing conditions.

If the monsoon rains fail, famine often follows. There are also some special winds called monsoon winds. These are winds that change their direction with the season. In India, the monsoons blow south as hot, dry winds in the wintertime, and blow north in the summer, bringing heavy rainfall.

Fact File: The Aborigines believe that if a possum is left cooking by the water’s edge, a sizzling sound is produced. The noise irritates the rainbow snake, who they believe crawls underground towards the source of the noise. As it moves along, the weather turns stormy, and the monsoon rains begin.

 

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Mount Everest, which is in the Himalayas, is 8,863 metres high. At these altitudes, mountains are always covered in snow and ice, and there is little oxygen to breathe. 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 now been conquered.

Fact File: Mountain peaks are often seen to be surrounded by layers of cloud. This is because as winds laden with water are blown towards the mountains, they are forced to rise and the temperature drops. The water condenses into clouds at these higher altitudes.

 

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Mountains can be formed in three different ways. Volcanoes form mountains when lava from deep inside the Earth cools and hardens on the surface.

Other mountains are formed when two plates move towards each other under pressure or where an oceanic plate is pushed under a continental crust. The pressure causes the ground near the joining plate margins to fault and fold. The ground is forced upwards to form mountains. The Rockies, Alps, Andes, Urals and Himalayas were all formed in this way.

Finally the Earth’s crust can fracture and create faults which mean that large blocks of land can be moved upwards or downwards. Faults in the rocks normally occur when there is a lot of pressure on the rocks. Mountain building is a slow process and happens over centuries.

Fact File: Rock slides are common where forests have been destroyed on mountainsides. There are no longer any tree roots to stabilize the loose material.

 

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The island of Krakatau, Indonesia (west of Java) was a small volcanic island. The volcano itself had laid dormant for over 200 years until August 1883. On May 20 one of the cones erupted violently and three months later the whole island blew up. It was the biggest explosion in recorded history. For two and a half days the island was in total darkness because of the amount of dust in the air. A cloud of ash rose 80 km into the air. The eruption caused a tidal wave which killed 36,000 people. The explosion could be heard and felt in Australia, 3,500 km away.

Fact File: On August 24 in the year AD 79, there was a great eruption of Mount Vesuvius, a volcano in southern Italy. The lava, stones, and ashes thrown up by the volcano completely buried two nearby towns.

 

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The word dormant actually means ‘sleeping’. So when people talk about a volcano being dormant, it really means it is temporarily sleeping and might erupt at any time in the future. An extinct volcano, on the other hand, is one that will not become active again. Like earthquakes, volcanoes mainly occur along fault lines. Earth’s crust to release the pressure that has built up. When there is very little pressure the volcano can remain in a dormant state for many, many years. On the island of Maui there is a volcano called Haleakala which rises to a height of about 10,025 feet. It is the world’s largest inactive volcano. Its crater is about 20 miles around and some 2,720 feet deep.

Fact File: A major volcanic eruption can hurl boulders high into the air. These boulders, called volcanic bombs, can be very large indeed.

 

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Sometimes there is a big news story about an earthquake that happened in some distant part of the world. Although people living in that area didn’t feel the earth shake, scientists probably made a complete and exact have special instruments called seismology. The seismograph picks up the one rock mass rubbing against the other. The energy of this rubbing is changed to vibration in the rocks. This vibration can travel many thousands of miles.

Fact File: The edges of the Pacific plate are surrounded by earthquakes, volcanic activity and hot springs, caused by the crust shifts and hot lava rises near the surface.

 

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Our planet is a very restless place. Every 30 seconds, the ground suddenly rumbles and trembles. Most of the movements are so slight that they are not felt. Others can be so large they cause complete disaster. Big cracks appear in the land, streets buckle and buildings simply crumble. In fact whole towns and cities can be destroyed. These are called earthquakes and the reason they occur is because the Earth’s crust is made up of moving parts called plates. When these plates slide past or into each other, the rocks jolt and send our shock waves.

Fact File: Both mining and tunneling operations are known to have caused earthquakes in areas that are already under tension due to movements in the Earth’s crust.

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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.

Sometimes the flow is reversed naturally, but this only happens 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.

Fact File: Tidal bores happen in the Amazon in south America, where there is a bore as high as 4.5 metres. A smaller bore travels up the River Severn in England.

 

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

Fact File: When water seeps through the ground and reaches hot rock or volcanic gases, it boils violently this produce steam that can shoot the water out of cracks, causing a geyser.

 

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Some sort of force or energy is needed to start a wave, and it is the wind that provides this energy in the water. Wind blows the surface layers of the sea, gradually a rolling movement of waves. As these waves near the coast, the sea-bed interrupts their rolling movement and they mount up and break onto the beach. The water inside a wave moves round and round in a circle. Near the shore, the circular shape of the wave is changed and it becomes squashed. The top of the wave becomes unstable, so when it hits the beach, it topples and spills over. On beaches with a shallow slope the waves pile up to a great height before breaking, causing surf.

Fact File: For centuries sailors have been in terror of being sucked into a whirlpool that would swallow them and their ships. Whirlpools happen when opposing currents meet.

 

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In Antarctica and the Arctic, the oceans freeze because the temperatures are so low. Seawater, unlike fresh water, continues to increase in density as its temperature decreases towards freezing point (about -1.8 ). In fact Antarctica contains 90 per cent of all the ice on the Earth. Scientists have calculated that should all this ice melt, sea levels would rise by 60 m causing world-wide flooding. An accumulation of sea ice can cause an iceberg which floats in the sea because it is less dense than water.

Fact File: Over 360 million square kilometres of the Earth’s surface area are covered by oceans and seas, with the Pacific accounting for nearly 36% of the total.

 

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 The Moon is 400 times smaller than the Sun but about 400 times closer. So we see them both as about the same size.

A solar eclipse is when the Moon blocks out the light from the Sun. This can make the day turn dark as all the light from the Sun is blocked out in a total part of the light is blocked out.

The Sun is our nearest star. It is 149.6 million km (92.9 million miles) away from Earth. It is the heat and light from the Sun that makes life on earth possible. The huge gravity pull of the Sun keeps the planets of our solar system orbiting around it.

Fact File: The daily rise and fall of the ocean’s tides occur because of the pull from the Moon and the Sun. There is a smaller pull from the Sun. when the Moon and Sun are in line there are high spring tides.

 

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The Earth casts a long shadow into space, and when this shadow passes over the Moon it can cause an eclipse.

A lunar eclipse can only happen during a full Moon, when the Sun is directly behind the Earth. It happens only occasionally, because the orbit of the Moon is slightly tilted and so the shadow usually misses it. Lunar eclipses can last for over one hour, but they do not completely black out the shadowed part of the Moon. Some sunlight always filters through, making the shadow look a reddish-brown colour. Eclipses can only be seen from certain parts of the world.

Fact File: The distance of the Moon away from the Earth was settled once and for all after the Apollo astronauts left a small reflector on the Moon’s surface. This distance is 384,000 km, though it varies slightly because the Moon does not have a perfectly circular orbit.

 

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The shortest day of the year is in the winter solstice which is 21 December. When the Northern Hemisphere is turned toward the Sun, the countries north of the equator have their summer season, and the countries south of the equator have their winter season. When the direct rays of the Sun fall on the southern hemisphere, it is their summer and it is winter in the northern hemisphere. There are two days in the year when night and day are equal all over the world. They come in the spring and fall, just halfway between the two solstices. One is the autumnal equinox in September, and the other is the spring equinox in March.

Fact File: Mars is known as the ‘Red Planet’ because it is covered by a stone desert that contains lots of iron oxide, making it appear rusty red. The water and oxygen that used to exist on Mars are now locked up in these iron deposits; nowadays the planet has hardly any atmosphere.

 

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The earth revolves around the Sun and at the same time it revolves on its own axis. As it moves around the Sun, it is also spinning like a top. If the axis of the earth were at right angles to the path of the Earth around the sun, all the days of the year would be the same length. However, the earth is tilted at an angle of 66.5 . In June the northern hemisphere is tilted towards the Sun and it receives more sunshine during a day. This is its warmest season called summer. On the 21 June the Sun is directly over the Tropic of Cancer and it is midsummer in the Northern hemisphere. This is the time known as the summer solstice.

Fact File: The Sun is the source of light and heat for the solar system. The four planets closest to the Sun are small and solid, the closest being Mercury. An asteroid belt separates these from the larger planets which are made up of gas.

 

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