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Crantons and shields

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Post by martinjsto Wed 05 Nov 2014, 9:47 am

great show on SBS the other night on the evolution of continents. it covered the formation and deformation of the continents b4 gowala land formed then split and how they are massive lumps of solid rock called cratons, that plummet over 150km down into the mantle, at 150km down the pressure is ideal for the formation of precious gems like diamonds ETC but the temperature is too hot at that depth, thats where the cantons (massive chunks of precambrian rock dating up to 3.5 billion yrs old) penetration into this layer has a cooling effect and created the ideal environment for precious gems. the plate tectonics pushes up the seabeds surrounding these cratons forming mountain ranges and plateaus and through this and volcanic activity these gems are eventually lifted to the surface to eventually be exposed through erosion. the intersections of these cratons and shields throughout the continent are the major fault lines are pathways for dissolved mineral deposits like gold copper zinc etc are forced through these faults to the surface .
A shield is that part of the continental crust in which these usually Precambrian basement rocks crop out extensively at the surface. Shields themselves can be very complex: they consist of vast areas of granitic or granodiorite gneisses, usually of tonalitic composition, and they also contain belts of sedimentary rocks, often surrounded by low-grade volcano-sedimentary sequences, or greenstone belts. These rocks are frequently metamorphosed greenschist, amphibolite, and granulite facies. Shields are normally the nucleus of continents and most are bordered by belts of folded Cambrian rocks. Because of their stability, erosion has flattened the topography of most of the continental shields; however, they commonly do have a very gently convex surface. They are also surrounded by a sediment-covered platform. By contrast, in a platform, the shield (more accurately referred to then as the crystalline "basement") is overlain by horizontal or subhorizontal sediment. Together, the shield, platform and basement are the parts that comprise the stable interior portion of the continental crust known as the "craton."

The margins surrounding a shield generally constitute relatively mobile zones of intense tectonic or plate-like dynamic mechanisms. In these areas, complex sequences of mountain building (orogeny) events have been documented over the past few hundred million years.(copied the shield bit from wiki)
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enjoy

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Post by Moneybox Wed 05 Nov 2014, 1:56 pm

martinjsto wrote:great show on SBS the other night on the evolution of continents. it covered the formation and deformation of the continents b4 gowala land formed then split and how they are massive lumps of solid rock called cratons, that plummet over 150km down into the mantle, at 150km down the pressure is ideal for the formation of precious gems like diamonds ETC but the temperature is too hot at that depth, thats where the cantons (massive chunks of precambrian rock dating up to 3.5 billion yrs old) penetration into this layer has a cooling effect and created the ideal environment for precious gems. the plate tectonics pushes up the seabeds surrounding these cratons forming mountain ranges and plateaus  and through this and volcanic activity these gems are eventually lifted to the surface to eventually be exposed through erosion. the intersections of these cratons and shields throughout  the continent are the major fault lines are pathways for dissolved mineral deposits like gold copper zinc etc are forced through these faults to the surface .
A shield is that part of the continental crust in which these usually Precambrian basement rocks crop out extensively at the surface. Shields themselves can be very complex: they consist of vast areas of granitic or granodiorite gneisses, usually of tonalitic composition, and they also contain belts of sedimentary rocks, often surrounded by low-grade volcano-sedimentary sequences, or greenstone belts. These rocks are frequently metamorphosed greenschist, amphibolite, and granulite facies. Shields are normally the nucleus of continents and most are bordered by belts of folded Cambrian rocks. Because of their stability, erosion has flattened the topography of most of the continental shields; however, they commonly do have a very gently convex surface. They are also surrounded by a sediment-covered platform. By contrast, in a platform, the shield (more accurately referred to then as the crystalline "basement") is overlain by horizontal or subhorizontal sediment. Together, the shield, platform and basement are the parts that comprise the stable interior portion of the continental crust known as the "craton."

The margins surrounding a shield generally constitute relatively mobile zones of intense tectonic or plate-like dynamic mechanisms. In these areas, complex sequences of mountain building (orogeny) events have been documented over the past few hundred million years.(copied the shield bit from wiki)
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enjoy

Thanks for that Martin. I wish I'd seen it but this retirement thing has knobs on it. I seem to have work book up beyond the foreseeable future and not much time for television.
Anyway we're off this Friday to Hawaii for a few weeks. I'm going to pocket a bit of that bright orange liquid. It should solidify before I get home and then I just have to wait to see what forms. It might be quicker than waiting for me next nugget to appear here.
wegr

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Post by martinjsto Wed 05 Nov 2014, 4:19 pm

have a great trip Phil. there should be a catch up for the show online.

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