4.3 To analyse the structure of the earth and to measure movements  (Page 2/3)

Use a ball of red clay for the centre of the earth:

When the model is complete, change it into a cut-away model:

• Cut the ball into two halves along the equator to make a northern and a southern hemisphere.
• Cut the northern hemisphere in half to make two quarter spheres.
• Place one of these back onto the southern hemisphere.

Comment: ____________________________________________________________

EARTHQUAKES

Scientists believe that in the past all the continents belonged to a single landmass that they have called Pangea. About 300 million years ago this landmass split into pieces that have drifted away from each other to the present position of the continents.

• The pieces into which the crust has divided are known as plates and each plate is about 40 km thick. They float because the rocks they consist of are lighter than the melted section of the mantle underneath. Sometimes they collide with each other and this causes deep trenches in the ocean floor and high mountains on land. The Himalayas, the highest mountain range on Earth, is still being pressed upwards by the collision between India and Asia. The Great Rift Valley, a tear that stretches across a large part of Africa, was formed when the surface sank along the line of two plates that were separating. Strong earthquakes can damage buildings, bridges and roads. Well-constructed buildings will suffer the least damage.
• Seismologists are scientists who study earthquakes. They measure the strength of an earthquake on the Richter scale. On this scale 1 equals a small earthquake that is felt as a light trembling on the surface. A large earthquake of 7 will cause huge damage to buildings, power supplies, roads etc. The place where the waves of the earthquake first reach the surface of the earth is called the epicentre. The effect of the earthquake is the greatest here. Sometimes the epicentre is under the sea. Then huge waves called tsunamis are caused.

TASK 2:

On 29 September 1969 an earthquake caused severe damage in the Boland towns of Ceres, Tulbagh and Wolseley. Read the news report below, printed in Die Burger of 9 September 1989, and summarise it. Make sure that you include all the main points.

THE NIGHT WHEN THE BOLAND MOUNTAINS RUMBLED

EARTHQUAKE: TWENTY YEARS LATER.

When light tremors rolled over the mountains of Tulbagh, Wolseley and Ceres on the morning of 29 September 1969, few people thought that their homes and farms would be badly damaged, or even destroyed. And today, twenty years after the disastrous spring evening when an earthquake of 6.5 on the Richter Scale shook the earth here, people are still talking about it.“It was chaos,” said Mr Dirkie Hougaard (49) of Tulbagh. I was working at the telephone exchange at the time and was alone when the earth began to rumble. But an earthquake was the last thing that I thought of. Not many people thought it was.Shortly before ten o’clock he became worried when the rumbling became louder and more regular. Before anyone knew what was happening there was a gigantic quake. It was four minutes past ten.“The whole earth shook. The lampshades were shaken from the roof, walls wobbled and in some places collapsed. Taps were shaken loose and water streamed everywhere. I realised that I had to run outside. Outside it looked misty as a result of the dust cloud that hung over the village. Everyone was in a panic.”Mr Hougaard managed to start the emergency generator of the Post Office and help was summoned from the Police, Divisional Council and later the Army.