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Mans lifespan is so short, in geological terms that many processes occurring at the earth's surface can only be comprehended with skilful use of the imagination. The weathering of granite which is ...examined in this programme is a case in point. In theory, granites which form at depth in a high temperature and pressure environment should be chemically unstable at the earth's surface and yet old granite buildings and artefacts often appear unaltered. This apparent contradiction, and the actual processes of granite weathering are examined in a classic area, Dartmoor, and the fate of the rocks and its weathering products is followed from high on the moor to the sea. The chemical theory underlying the field evidence is also examined and the concept of the rock cycle introduced.
Metadata describing this Open University video programme
Module code and title: S237, "The Earth, structure, composition and evolution"
Item code: S237; 10
First transmission date: 01-08-1981
Published: 1981
Rights Statement:
Restrictions on use:
Duration: 00:24:00
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Producer: John Simmons
Contributors: Julian Pearce; John Wright
Publisher: BBC Open University
Keyword(s): Elocculation; Granite; Rocks; Sediment; Suspended load; Weathering
Footage description: John Wright, on Dartmoor, introduces the programme. He looks at some granite- building stones and notes how well preserved they are. Julian Pearce, in the studio, examines a sample of granite from Dartmoor. He looks at a fresh edge of rock and points out the three main minerals, quartz, feldspar and biotite mica. He then compares this with an edge which has been weathered and where, as a result, the minerals have been changed considerably. Julian Pearce goes on to look at the chemical reactions which take place when the granite weathers. Animated captions give the equations for some of these reactions. Finally, Pearce explains that, by looking at the chemical equations, one can predict what will happen when granite reacts with rainwater. He mixes minerals together in a beaker of rainwater as he talks. John Wright at a granite quarry on Dartmoor, looks at some of the quarry faces which have weathered for some 60 years. He explains how rainwater gets into the granite so that reaction can take place and points out some severely weathered granite which has become quite crumbly. Wright goes on to examine a soil profile nearby on Dartmoor. He explains that weathering of granites is associated with soil formation and points out the mineral particles in the soil. Shots of a stream. Wright explains that many of the clay mineral particles from weathered granites are carried downstream in suspension. Shots of the same stream about 10 km downstream. Wright points out the fragments of feldspar, quartz, mica and tourmaline which have settled out on the bed. Finally, Shots of estuarine mud flats. Wright explains that most of the particles in suspension settle out here. Julian Pearce, with the aid of a diagram of an estuary and some animated graphics, discusses the effects of the increased salinity of the estuary waters on the rate of particle drop-out from suspension. Pearce goes on to conduct an experiment on the effect of seawater on particles in suspension. He adds some seawater to a beaker containing a solution of ferric iron and observes the results. With the aid of animated diagrams, Pearce explains why seawater has this effect on suspended particle. Shots of a sedimentary deposit which was formed by rivers flowing off Dartmoor some 40 million years ago. Commentary point out its composition John Wright summarises the programme.
Production number: FOUS171J
Videofinder number: 1598
Available to public: no