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Lecture 10

Lecture 10 Notes

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University of Toronto Scarborough
Environmental Science
Nick Eyles

Eesa lecture 10 Deconstructing: taking something apart and seeing what its made of. We now have the tools to be able to do this for understanding how continents were built. All continents are built similarly. Most important process is obduction. We can recognize five main growth stages, the oldest one is Arctica (2.7 Ga) then Columbia (1.8 Ga) Rodinia abundant life arrives at this time (1.0 Ga [ga=giga annum) these three are the Canadian Shield (the craton=the core of north america). Pangea assembly (350 Ma): maritime Canada is added during Appalachians Orogeny (Appalachian Mountains), Pangea breakup (<200 Ma) British Columbia is added during Cordilleran Orogeny (eg. Rockies). You can go anywhere in the world and use this 5 part scheme. The shield is made up of old hard rocks fairly rigid; much more younger mostly sedimentary rocks added on either side, maritimes and the west. Slide 4: Geology of North America known for 100 years but what put it together? The shield on this map is the brown rocks. Then blues and lime greens wrapped around this core, the blue area on the right hand side is the Appalachian belt. Then you can see a very large wide belt from Alaska to Mexico that was added on when Pangea broke up. This map was hard to interpret because there was no model for how these were put together, it was only with plate tectonics in the 60s that this complex picture could begin to be interpreted. Slide 5: zircons used for uranium lead dating. This technique is now relatively cheap so that we can do large numbers of age determinations, they are quite precise, and interpreting that map in terms of age in now quite straight forward. Slide 6: Tom Krohm-didnt invent the technique but got it to the point where it could be widely used. The amount of lead thats in the crystal will tell us how long its been sitting there. Slide 7: Oldest continental crust in Canada is 4 billion years old. This is a gneiss. When we date it and say that its the oldest rock this is not correct, because it was a pre-existing rock. If you look at the zircons in here they are detrital, meaning they are secondary (having been derived from something else). The zircons originate in igneous they are then released and become part of sedimentary rock they are then metamorphosed to a gneiss. So there must have been older rocks. The earth is only 4.5 billion years old so this suggests that we were forming crust pretty early on. Slide 8: oldest rocks are in the centre and as you come out it gets younger and younger. Old rocks imbedded in the interior of continents. Slide 9: superior province an old term used to identify rocks that extend over large area that are similar in appearance. Very sharp boundaries. Orogen: the rocks that result from orogenic activity, a zone of folded highly deformed rocks. The most common rock type within the orogens is gneiss. The provinces are separated from each other by the orogens. The provinces were little micro-continents and the orogens were all of the folded rocks that resulted when that smaller continent
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