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Lecture 9

EESA06H3 Lecture 9: EESA06 – Lecture 9 (Canada’s Geologic Journey

Environmental Science
Course Code
Nick Eyles

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EESA06 Lecture 9 (Canada’s Geologic Journey)
Canada’s Geological History is a Long Journey in 5 Parts (Slide 2)
Main Themes (Slide 3)
- Continents grow through time
o All continents pretty much experiences the same journey; they have all been part of the
same supercontinents that have broken up, gone their separate ways and then those
continents have reassembles as another supercontinent commonality of history between
all these continents
- North American continent has grown in 5 stages:
o Arctica (2.7 Ga)
Crust added to north America
Phases of expansion
o Nena (1.8 Ga)
Crust added to north America
Phases of expansion
o Rodinia (1.0 Ga)
Important to us in southern Ontario because most of the basement that underlies
the southern part of the province, Quebec, Labrador, Texas, some parts of Mexico
were added to North American at the time of Rodinia
o Pangaea assembly (350 Ma): Maritime Canada is added during Appalachians Orogeny
(Appalachian Mountains)
o Pangaea breakup (<200 Ma): British Columbia is added during Cordilleran Orogeny (e.g.
- The first 3 (arctica, nena and rodinia) = craton (Canadian Shield) not directly equivalent but
those 3 phases saw the building of the Canadian Shield; it still underlies a lot of the US and
Greenland because it was a part of Canada until 80 million years ago and broke off when the
Atlantic started to open
o Core of the continent
o The old core of North America and then later stages 4 and 5 were added
- 4 (Pangaea assembly) added much of Eastern Canada, Maritime Canada, the Atlantic Provinces
most of the crust that underlies those were added during Pangaea assembly
- 5 (Pangaea breakup) added Western Cordillera, British Columbia, Yukon, Alaska, much of
- 1,2,3 are the Shield; core of North America (the old rigid core), 4 and 5 saw additional material
being added around the core again
- The last two are given the names of orogenies; mountain building, collision event, it is the event
- Orogen; the rocks/mountains, this is all this crust that was added, highly deformed
Question: The Geology of North America Has Been Known for 100 years But What Put It Together?
(Slide 4)
- Newfoundland added onto North America; much of the island consists of oceanic crust (the floor
of an ancient ocean that closed

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EESA06 Lecture 9 (Canada’s Geologic Journey)
2.5-Billion-Year-Old Zircons (Slide 5)
- Need microscope to see them, produce them by crushing rocks and then you have different fluids
of different density and you float these little crystals off
- They are important because they are produced in magmas; produced by action volcanic colcanos
and they contain unstable uranium isotopes whole basis of radiometric dating is that these
isotopes break down on a steady rhythm
- Uranium breaks down to produce lead and we know the amount of time it takes to complete that
- Zircons contain unstable uranium isotopes, they break down they produce lead
- They are tight; once lead is produced it cannot escape (there are other crystal types that contain
unstable uranium isotopes, but they are leaky so that when you measure the amount of lead, it
won’t be accurate)
- Can actually see how old that rock is, fundamental to reconstruct an ancient environment so you
can date something well
Zircon (Slide 6)
- This is a zircon crystal
- Can see the core of the crystal and growth rings too
- Crystal is growing because it is in a cooling magma and there is very hot fluids moving through the
rock and slowly the crystals are growing in size
- In some studies can actually date the growth of the crystal and find that the crystal will grow
during orogenic events
o When you compress rock, so if you have mountains being formed you get tremendous
forces at work it is like a sponge if you squeeze the sponge it moves the fluids away and it
can move thousands of kilometres away and so they feed on the growth of crystals
- Can read a nice history of tectonic activity in the crystal itself can see a little circular pit where it
has been lasered and then we can measure the amount of lead
- The guy on the picture is standing on Gneiss (banded gneisses up on the shield)
Lithoprobe (Slide 7)
- Making our own seismic waves (earthquakes) so that we can map what is underneath and map
down to about 70 km
Figure (Slide 8)
- This is a seismic record, can see the ground surface (dark line to the left) and then can see
successive layers which have been picked up by the energy going down from one of these trucks
- The energy goes down, we know the velocity of the waves so what we are doing is measuring 2-
way travel time; time it takes for that energy to go down, reflect a hotter layer of a different layer
and then go back to the surface
o If you can measure this accurately, you know how deep that level is
Continents Grow Through Time (Slide 9)
- Oldest rocks anywhere on the plant are in this dark, youngest are green then there is the pink
- Shields are composed of rocks of those 3 types
- The grey is much younger rocks

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EESA06 Lecture 9 (Canada’s Geologic Journey)
- What is the pattern that you see?
o Black surrounded by green then surrounded by pink pattern you see in most continents
- What is the pattern telling us?
o Concentric relationship with the oldest rocks in the middle
o Shows that the continents have grown through time and that the original continents were
quite small that was the original continental crust on the planet about 4 billion years ago;
small, very small land masses that have grown through time
- Accretion; add crust, how land masses grow
- How do you produce continental crust? Subduction
o Importance is that we are converting oceanic crust into continental crust
o Every time there is a collision and oceanic crust is pushed down, starts to melt, goes back
up with magma of very different chemistry and that is continental crust
o Converting basalt into things like granite, andesite, gneiss which we call continental crust
- Whole planet becoming one megacontinent?
o We will run out of heat; whole process is driven by radioactivity and there is a finite
amount of that
- Using zircon age dates, can plot up episodes in history where we see spikes in the production of
continental crust where there was an accelerated production of continental crust
- If we record through time, the amount of continental crust that is being produced we get these
big spikes they are recording amalgamation phases of supercontinents; when collisions are at
their maximum; when widely dispersed continents start to come together to form these
- Ocean floor between the colliding continents are being recycled
- Past supercontinents in our history are really important and they are the main stages where the
continents grow
Acasta Gneiss (NWT) (Slide 10)
- The oldest continental crust found so far? Probably not
- 4 billion years old this is also when formation of planet happened so telling us that there were
continents really early in earth history
- This is older than 4 billion because contain detrital zircons; freshly made zircons in magmas, they
cool, they create volcanic rocks, but then they get weathered, they break up
- Zircon crystal is very hard so can get washed away by rivers, moved by glaciers and then become
parts of sediments and in turn they become part of another rock and get recycled through time
and there are these detrital zircons in these rocks, so it tells us that there was continental crust
older than this
Deconstructing North America (Slide 11)
- This is how it how it was constructed
- 1,2,3 make up Rodina
- Where is Baha? Part of the slave province that is the ancestral north American continent 4 billion
years ago, that was north America
o Rocks are really old here, contain lots of diamonds, called pipes because look like carrots
come up from the lower mantle and then they get the earth’s surface and bring up
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