Category Our World

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.

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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.

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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.

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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.

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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.

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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.

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