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This programme is highly integrated with the text of Unit 12 which deals with the science of 'Plate Tectonics'. It discusses the climatic and paleo-magnetic evidence for the theory of continental d...rift, first proposed as a hypothesis by Wegner in 1915. Wegner was led to this concept of drifting continents chiefly from indicators of past climatic evidence gathered from fossils in dated rocks. But it wasn't until the 1950s when measurements of past magnetic fields in rocks gave over-whelming support which led to general acceptance of the theory. It was discovered that material was bursting out of the Earth's interior in the middle of the Atlantic and other oceans. The concept of 'huge plates' of stable areas of oceanic and continental crust being forced apart by seafloor spreading is of crucial importance to the theory of plate tectonics. This programme uses model and extensive animation sequences to teach the essential physics: ocean floor spreading, continental and oceanic collisions which produce earthquakes and volcanoes, and the concept of a dipolar magnetic field which periodically 'flips' from North to South, leaving evidence in rocks formed at the time. It also looks at evidence of past climates in Britain which show the European continent has been moved extensively across the globe from polar regions to desert regions. The actual physical mechanism responsible for continental drift is, however, still in the realms of conjecture.
Metadata describing this Open University video programme
Module code and title: U202, Inquiry
Item code: U202; 05
First transmission date: 20-05-1981
Published: 1981
Rights Statement:
Restrictions on use:
Duration: 00:24:00
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Producer: Tony Jolly
Contributor: Chris Wilson
Publisher: BBC Open University
Keyword(s): Continental drift; Earthquake zones; Plate tectonics; Volcanic zones; Wegener
Footage description: The programme begins with shots of volcanoes erupting and earthquakes. Chris Wilson describes the causes of these phenomena. He explains that the behaviour of the Earth's crust is now described in terms of Plate Tectonics and then goes on to describe the immediate evidence for this argument, the shapes of the continents, volcanic and earthquake activity confined to certain zones, near continental oceanic boundaries. Prior to 1915 however, these factors were thought to be evidence that the Earth's crust was contracting. Wegener's arguments about continental drift were greeted with scepticism. Wegener supported his hypothesis with various arguments including the distribution of indicators of past climatic conditions. To develop this line of argument Chris Wilson looks at evidence of past climatic change in Britain. Shots of mountains and U-shaped valleys; these were caused by coverings of thick ice sheets 25,000 to 300,000 years ago. Chris describes the type of materials left by the departing ice-sheets. Shots of clay, rocks and boulders, melting ice and streams. Boulder clay is the end result of this process. Fossil equivalents of this kind of deposit would tell us about the climate in Britain hundreds of millions of years ago. Shots of rocks in Scotland which show that Britain experienced an Ice Age climate 700 million years ago. Chris is shown standing in an open cast coal mine. He describes a coal seam, 300 million years old and the rocks and shale that separate the seams. He reveals some fossils found in the rocks which indicate the type of plant material which make up the coal. These are evidence of a warmer climate in Britain. Shots of tree fossils preserved in a Glasgow park. They look like the type of trees found in hot swampy conditions. This indicates that the coal in Britain grew in hot equatorial conditions. Shots of sand-dunes in the desert. Shots of sandstone deposits in Britain, which were once desert sands 270 million years ago. Chris recaps on the evidence for Britain's extremes of climate. He argues that this shows that Britain has moved northwards through geological time, crossing the equator to do so. For evidence of continental drift Wegener examined the distribution of fossil climatic indicators all over the earth. Shots of globe with continents assembled into one supercontinent showing in the South evidence of cold glacial conditions. When the continents are placed together the ice sheet assumes a reasonable size. Chris argues that this is strong evidence for the drift hypothesis, but it was not accepted until the 1950s, when detailed studies of fossil magnetism in rocks enabled the position of the poles to be determined through time. Using a compass and a globe containing a magnet, Chris shows how magnetic readings can give a measurement of latitude across the surface of the Earth. Cooling lavas freeze the magnetic field within them as they cool. This allows us to plot the past positions of the North Pole back in time. He then shows diagrams giving the polar wandering curves for Britain and two continents. Due to the similarity of the last two curves geophysicists believe that the two continents moved in unison until about 15O million years ago. This provided strong evidence for the continental drift theory. Next Chris looks at the magnetic patterns produced by rocks under the oceans, which differ markedly from those on land. Shots of magnetic field contours on the Pacific Ocean floor. The field pattern for ocean ridges is symmetrical on either side. Using the globe and the compass, Chris shows how the Earth's magnetic field has reversed on different occasions in its history. This pattern of reversed magnetic fields has been mapped out in the rocks on ocean ridges. This discovery fitted with Holmes suggestion that drift was caused by convection currents in the Earth's mantle. In the 60s Hess further developed this idea, arguing that mantle material brought up beneath the ocean ridges cools there and moves aside as further material is emplaced in the same area. Animated shots of the process, known as sea floor spreading. Using a model Chris shows how the magnetic field reversals found on land map to the patterns found above the sea-bed. Chris now considers what happens to the crust as it spreads out from the ocean ridges. Animation of ocean crust diving beneath the continental crust. The denser crust is pushed down by lighter continental crust, this results in seismic or earthquake activity, and later, as the crust melts, to volcanic activity. Hence these activities mark the site where oceanic crust is being destroyed. Chris looks at the spread of these activities across the globe. The vast stable areas are known as the tectonic plates. Plate boundaries can occur within continental areas. Animation of a continental collision of the plates. As they collide the crust thickens to twice the normal amount, creating mountain ranges such as the Himalayas. Computer animation of accepted pattern of continental drift. Chris sums up the development of the drift concept and argues that the true nature of the driving mechanism causing the drift is still not properly understood.
Production number: FOUD118R
Videofinder number: 1301
Available to public: no