Our Round Earth - How It Changes (1971)

[Music] [Music] change the earth is a place of continual change in a few hours a sand castle is washed away by the sea it takes longer for the sea to wear down Solid Rock but it happens waves pound against the shore washing bits of rock into the ocean the same forces that wear away the shore wear away other parts of the land slowly very slowly over thousands and millions of years a river can cut through solid rock forming a deep valley or a canyon the surface of the Earth is continually being worn down and not just where there are rushing rivers or ocean waves but all over even mountains are acted on by forces that wear down the land it may seem hard to believe when we look at tall angular mountains which haven't been worn down very much these aren't as old as these mountains which are millions of years older see how much lower and more rounded they are they've been worn down what wears down a mountain well changes in weather for one thing at times the weather may be sunny and warm other times it may get quite cold this can wear down a mountain how well suppose you place a bottle of soda pop in a freezer if you forget about it and leave it in too long the change in temperature will cause the liquid to freeze and expand breaking the glass into many pieces cooling of water on a mountain side can do the same thing to rock breaking it into many pieces something else that breaks Rock apart is plant life roots of plants can grow into small cracks as the roots grow they push against rock hard hard enough to make small cracks larger hard enough to make large rocks split how does all all this splitting of rock wear down a mountain what happens to rock once it's split loose rock is pulled down the side of a mountain by gravity wind wears down a mountain too water also helps wear down the land not just river water or ocean water but rain water water washes huge amounts of rock and soil and Clay into Mountain rivers and streams what happens to the material that's washed into rivers much of it piles up at the mouths of rivers where they empty into the ocean causing changes in the land along the shore the shoreline changes also when materials along the ocean floor are pushed by waves forming sand bars waves also push sand onto beaches wind acts upon sand too blowing it from one part of the land to another forming sand dunes we've seen how mountains can be worn down how are they built up how are mountains formed we can get a hint from the great damage caused by earthquakes earthquakes are sudden movements in the earth movements quick and large enough to destroy houses buildings sometimes entire cities other things that show us that the Earth has moved are giant cracks in the earth cracks where the Earth has pulled apart here we see a part of the earth that has been pushed upward such movements may happen quickly causing an earthquake but usually such movements happen more slowly over thousands and millions of years in this way great masses of the Earth's crust have moved upward forming mountains we can get an idea of how this happens with layers of clay arranged like layers of rock for forces within the Earth press harder against one part than against the other such forces tear the Earth and tilt it see how this mountain might have formed not all mountains though are formed by Earth movements there's another way sometimes hot melted rock pours from the earth rock known as lava the lava erupts from an opening in the earth known as a volcano volcanoes appear on land and at Sea volcanic islands are formed by lava that erupts from the ocean bottom creating new land that gets higher and higher until it rises above the water a volcano may be active for a time then quiet down then become active again volcanoes usually build up for many years eventually though volcanoes stop being active and are worn down like other mountains all mountains remember are the result of great changes in the earth the earth is continually being built up in some places and worn down in others these changes are going on all the time usually very [Applause] slowly not all changes in the Earth are the result of the forces we've seen man himself is responsible for many things the change the [Applause] [Music] [Applause] [Music] land are the changes made by man as big as those made by Nature are they as important what do you think [Music] all