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Lecture 10

Lecture 10 Notes

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Department
Environmental Science
Course Code
EESA06H3
Professor
Nick Eyles

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Eesa lecture 10
Deconstructing: taking something apart and seeing what it’s 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-didn’t invent the technique but got it to the point where it
could be widely used. The amount of lead that’s in the crystal will tell us how
long it’s 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 it’s 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
www.notesolution.com
collided with another one. We also find a lot of green stone rocks in orogens
which are oceanic rocks from oceans that were formerly found between the
continents. Slave province: is really the true core of the shield, the oldest part of
the continent. Notice that Greenland is a part of the north American continent it
broke off about 80 million years ago because of a failed rift. You can see an
orogen in the hudsons bay, and a couple in Greenland. This area is Arctica. The
southern province came in as a major block later on, and you’ll see Nena
columbia the second growth stage. Grenville Orogen is very important to us,
these make up the shield or the craton. The old rocks that make up the rigid
interior of the continent. 4 is labeled as the Appalachian mountains. And then the
cordilleran orogeny added during the breakup.
Slide 10: lithoprobe: literally means probing rocks, these strange vehicles, they
call them dancing elephants, because they park them and then there is a big
hydrolic plate which is pushed down onto the ground and is then subject to very
high vibration. They are making vibrations to try and mimic earthquakes, creating
seismic wave energy which then goes down, reflects back of reflecters (rocks with
different structure, density and type) allows to reconstruct whats down there do
depths of about 70km. the purpose was to better understand the construction of
the Canadian shield and how it had been but together. Called seismic transects,
some of these were 100’s of km in length.
Slide 12: what comes out after looking at 100’s of km of seismic profile. Shows
how Labrador rocks were added to North America. Shows that the ancient earth
behaved in the same way that the modern earth does. Lithoprobe confirms
uniformitarian ideas. There is some evidence that the plates might have moved
faster and been a little thinner in the past because the earth was warmer, the earth
has cooled a lot in the past 2 billion years. We can recognize ancient mountains,
that are akin to the Himalayas today. The Canadian shield is what we can a pene
plain, it means literally almost a plain, there is a little bit of relief elevation and
high areas, but its mostly flat because of millions of years of erosion, the process
might take 300 millions years.
Golden spike: Canadian pacific railroad, the last spike that was put down to
complete the railroad.
Slide 14: obduction is the accretionary process that builds continents from smaller
crustal blocks. Results in cratonization, the building of cratons; the building of
shields.
Slide 15: we get a lot of our mineral wealth from the superior craton, a lot of
which are found in the green stone belt, they are metamorphose basalts, also
known as amphibolite. Because they sat on an ancient ocean floor that was
invaded by hot mineral rich water, this process is known as hydrothermal, these
processes take place at mid ocean ridges. You can see the green stone belts, and
the whole host of smaller land masses that collided together. The same process
obduction created these provinces.
Slide 16: Yellowknife. Great thickness of pillowed basalts between blocks of
continental crust. Tell us that the rocks formed underwater billions of years ago
www.notesolution.com

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Description
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 www.notesolution.com
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