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Lecture 10

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Environmental Science

 How North America got built, the basic process of deconstruction, it means taking something apart and seeing how its built and what its made out of. Thats what we're doing with continents.   We now have the tools to figure all this out, the interpreted tool is PLATE TECTONICS for understanding how NA got built. We can do exactly the same thing with any continent, cuz basically the same process everywhere.   India colliding with Asia is OBDUCTION   There are five main growth stages for NA, oldest one is called Artica (2.7 Giga annum), then Columbia (1.8Ga), and very important one Rodinia (1.0Ga, where life evolved), the first three combined to create the Canadian Shield (the Craton) the core of NA   Then we have the East side (Maritime Canada 350Ma, when pangea is forming, during the Appalachians Orogeny- Appalachian Mountaints) and then the west (British Columbia-200Ma, as Pangea broke up (The cordilleran Orogeny-eg.Rockies)   SLIDE with NA MAP, red area is the shield, blue area is appalachian belt.   Zircons (2.7bya) are used for uranium lead dating, which is a relative cheap technique that is quite precise and can be done in large numbers. (this is one of the breakthrough methods) Tom Crowe got this technique to the stage where it can be more easily used. (developed it), First u find the amount of lead in the crystals using lasers and mass spectrometry and u use that to find the age. Zircon are detrital, which means they are secondary and derived from something else, zircons originate in igneous rock and then become part of sedimentary rock, and then metamorphosed.   Any continent that has a shield has the oldest rocks, Acasta Gneiss in Yellowknife is about 4 billion years old. (oldest part of the NA continent) The oldest rocks are in a swiss roll pattern, continents have grown through time by old rocks embedded in the interior. Greenland broke off about 80 million years ago and is only separated by a failed rift.   Province is a large area defined my similar rocks and the same geology, but very sharp boundaries. (microcontinents like India)   Origin is the rocks that result from orogenic processes, highly deformed rocks (like gneiss), (after the continents collided, the deformed rocks that result in between like the himalayas)   1.Artica is the earliest NA landmass. 2.Nena Columbia is the second growth stage. (refer to slide), 3. then the Grenville orogeny, (1,2 and 3 = craton), 4 and 5 are same as above.   LITHOPROBE literally means probing rocks, vehicles called dancing elephants have hydrolic plates that are placed on the ground and exhibit very strong vibrations, it tries to mimic earthquakes by producing seismic waves which then goes down, reflects back up by rocks of different hardness, different density, structures, shape, and so allows us to see underground up to about 70km. The purpose was to better understand how the canadian shield had been put together. They conducted and put together what we call a Seismic Transects (100's in length) leads to cross sections of the shield in long distances. Uniformitarianism means the present is the key to the past, so ancient earth behaved just like modern earth. The earth has cooled though quite a bit since the past but its all minor stuff. A PENE plane means almost a plane, broadly speaking the shield is flat as a result of millions of years of erosion that literally wiped out ancient mountains. Takes about 300 million years before mountains are completely wiped out.  The golden spike in british columbia was the last spike to build the railroad. Obduction: the accretionary process that builds continents from smaller crustal blocks. Cratonization is the building of cratons/shields. A lot of ontario's mineral wealth is from the superior craton, green colored metamorphose basalts, sometimes amphibolites, all from greenstone belt, this is because of rocks being cooked on the sea floor in ancient times (Hydrothermal processes) Where exactly? Mid ocean ridges. Magma comes up in the mid ocean ridges and minerals are precipitated in the rock during this process.  Pillow basalts show us that rocks formed underwater billions of years ago but have been metamorphosed, found between blocks of continental crust, lots of gold hence the name yellowknife Gunflint Chert is 1.1Ga and is oldest bacteria record in ontario, and simple bacteria is 3.3Ga, bacteria form communities in stromatolites which are circular masses of bacterial colonies that are very successful, still here today and will be here when we're gone. live in very hyper saline lagoons where there is no predators (eg. Shark Bay) The sudbury crater is a big depression in the surface of the shield, once thought to be volcanic, now realized that it is a product of a huge meteorite strike. Was the second largest meteorite impact (10km in diameter) on the planet and was big enough to go all the way through the earth's crust, largest one was south africa. Sudbury now famous for its Nickel resources. This crater was made about 2 billion years ago. Most craters are round, this one wasn't because its been squeezed, as southern ontario rocks got added to north america, the grenville orogeny from 1bya. We're getting younger and younger through time (ONTARIO)     Most craters are circular but Sudbury meteorite crater is not..why? o Its been squeezed when rocks to the south (southern Ontario) added to North America (Grenville orogeny) about a billion years ago..deformed and squeezed o Other lines are shield areas or fracture patterns (softer rock because it was broken) are old o Ice sheets have come down scoured the sheet surface o Patent on shields o Wanapitei Lake-young crater, 37 million yrs old (fluke)  Laurentian University in Sudbury o Main axis road, cone structures found o Shatter cones o Can map the structures in rock to distance of 200 km away from Sudbury o Record intense impact stresses created by meteorite strike o Same thing found when looking at testsites of atom bomb  Continents would’ve looked completely different before because all of eastern, western, maritime in North America di
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