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John Wright shows how progressive regional metamorphism affects clays and pelitic rocks. An outcrop of Dalradian rocks in Scotland is followed from Aberfoyle near Glasgow to Aberdeen to show the pr...ogressive mineral and textural changes from slate, to phyllite, to garnetiferous mica schist to sillimanite gneiss. Specimens are examined at outcrop, in very big close up and in thin section. This is the classic area where Barrow worked out his sequence of metamorphic zones, These are explained by means of an animated diagram, John Wright empahsises that these changes in mineral composition and texture are isochemical. Dr Peter Francis shows the unusual behaviour of a piece of silicone plastic, it bounces, stretches, flows and deforms but when it is tugged rapidly it snaps, and when hit with a hammer it shatters. He suggests that rocks behave in a similar way, that their behaviour is time dependent. To study deformation in rocks he introduces the strain ellipsoid, and shows some experiments to study deformation in the limbs of folds. These experiments can be repeated at home using a telephone directory, a piece of sponge rubber and a pack of cards. Dr Chris Wilson shows how faults can be studied on geological maps. The 'Moreton in the Marsh' sheet is used to explain fault trend, downthrow, direction and amount, and the age of faulting
Metadata describing this Open University video programme
Item code: S23-; 08; 1973
First transmission date: 13-05-1973
Published: 1973
Rights Statement:
Restrictions on use:
Duration: 00:24:26
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Producer: Peter Clarke
Contributors: Peter Francis; John Wright
Publisher: BBC Open University
Keyword(s): Borrow; Clays; Dalradian rocks; Limbs of folds; Outcrop; Pelitic rocks; Progressive regional metamorphism; Scotland; Sequence of metamorphic zones; Silicone plastic
Footage description: Part 1. Metamorphism and metamorphic rocks: John Wright with a piece of clay and a brick. He uses this as an example of thermal metamorphosis of clays. John Wright with a map of Scotland. He points out the outcrop of Dalvadian rocks between Aberfoyle and Aberdeen along the Highland boundary fault. This is the area which he will examine for evidence of progressive regional metamorphosis. John Wright at a quarry at Aberfoyle. He examines a slate specimen, points out its characteristics and explains how it acquired these. Shots of a micrograph showing mica crystals in the slate. Film shots of John Wright driving to another site at Stonehaven near Aberdeen. Still shot of John Barrow the geologist who first established that progressively increasing grades of regional metamorphosis could be defined by the appearance of specific minerals in the rocks. Wright examines rock samples at Stonehaven. He points out their characteristics and compares them with the slate from Aberfoyle. Wright walks to a site higher up the coast to examine samples which have metamorphosed at higher pressures and temperatures. Shots of the samples. Wright points out their characteristics. A micrograph of a sample shows that Wright is in the Garnet zone. Wright in the studio with a rock sample taken north of the above locality. He points out its characteristics. This sample represents a clay which has been metamorphosed in the highest region of Barrow's tones, Wright sums up using the map of Scotland again. Animated diagrams show what happens to the clay at each of the metamorphic zones. Peter Francis bounces a ball of silicone plastic in the studio, He demonstrates that it stretches, flows, deforms, snaps and shatters under various conditions. Francis suggests that this time dependent behaviour models that of rocks. He points out some of these apparent characteristics in rock samples. Francis introduces the strain ellipsoid and shows some experiments to study deformation in the limbs of the folds. Micrograph of oolitic limestones illustrate the technique. Francis demonstrates further with a piece of sponge rubber, a telephone directory and a pack of cards as models. Students are asked to do these at home. Part 2. Mapwork: looking at faults. Chris Wilson begins by explaining the sorts of things one can discover about faults from mapwork. Wilson uses a map of the Moreton in the Marsh district to examine trends in faults. He points out the fault trends on the sheet. Wilson then examines downthrow. Photographs of a cliff near the Severn Bridge shows a triassic fault. Wilson explains how the downthrow directionm is worked out using both the photograph and the map. Wilson then explains how faults can be dated by mapwork.
Master spool number: 6HT/70880
Production number: 00525_1060
Videofinder number: 1716
Available to public: no