Lecture 10.docx

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21 Apr 2012
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EESA06H3 - Introduction to Planet Earth
Lecture 10 March 26/2012
Canada’s Geologic Journey: The Last 4.5 Billion Years (Chapter 20)
Geology of North America known for 100 years but what put it together?
- Tectonic plate paradigm
Paradigm - revolutionary idea that changes
people’s views on data that have been around
for many years
- Cratons (orange colour) Inner core of old crystalline
(igneous metamorphic rocks) rocks from the pre-
Cambrian age (roughly 600 million years old)
Similar to Canadian Shield
- Cover rocks (blue/lime colour) cover the craton
Has a much greater extent because it’s under a
large area of cover rocks
- Continents start off as small piece of crust and by colliding with other micro continents,
they grow in size
Comparable to an onion. Older rocks are at the core (craton) while the newly
formed rocks are at the outer layers
Rocks get younger as you go towards a margin of large continents
- Over the last 4.5 billion years, Earth has been recycling oceanic crust into continental
crusts
Subduction cannot destroy continental crust
Obduction allows for continental crust to grow
Supercontinent paradigm
- Discovered by J. Tuzo Wilson
- Full cycle takes about 500-600 million years to accomplish
Deconstructing the geology of North America
- You will be expected to understand how Figure 20.4 works for the exam
- “What is it made of?” - North America is made of 5 pieces:
1 and 2 make up the Craton
3 and 4 gets added
5 is the last area to be added to North America
Known as the “Left coast”
From Baha, California to Alaska
Continental crust that has arrived fairly
recently
Province long standing term in geology which
refers to areas of fairly uniform geology
Ontario is the superior province
Each provinces are a continent in their
own right and they collided
o All the resulting mountains (resulted from obduction) have
eroded down
o Erogen the deformed rocks that result from erogeny
Are the roots of the mountains that resulted from
province collision
Terrane pushes rocks to the east
Slave province - is the oldest part of North America
Original North American continent
Process of building North America began 4.1 billion years ago
o Still on going as subduction is going on around the Cascadia
Subduction zone
Recycling of oceanic crust and adding of continental crust
Cordillera refers to the western mountain
Cordilleran erogeny led to the formation of western North America
North America came about after collisions of several smaller continents and
formation of new crust
Obduction The accretionary process that builds continents from smaller crustal blocks
- Cratonization - Adding blocks of continental crust to make a bigger one
Political unification of the provinces 1867-1949
- An analog for welding of geological provinces through geologic time
Acasta Gneis the oldest continental crust found so far (4.2 billion years old)
- Very famous rocks and the banding is typical of highly deformed metamorphosed rocks
- Zircons found inside Gneiss are known as detrital zircons
Detrital something that has been derived someplace else by weathering of
rocks
Greenstone belts
- Great thickness of pillowed basalts between blocks of continental crust
- Pillow basalts are indicative of recycled oceanic crust
Remnants of oceans that were pinched in North America
Records the progressive growth of North America by obduction
Economically important because of hydrothermal processes
Sea water goes through the pillow basalts and the minerals are formed
from the pores
o Primary source of Gold and other minerals
We can use plate tectonic models to understand the evolution of the craton
- There is some evidence that the rate of plate tectonics was a bit faster in older days
- Crust was thinner and the mantle was warmer and thus the rate of plate movement was
faster
Oxygenation of the atmosphere
- Occurred 2 billion years ago
- Stromatolies large algal colonies that were around 3.5 billion years ago and are still
around
They like hyper saline waters
Grenville orogeny
- Collision of South America and North America during the formation of Rodinia
- Mountains have been eroded off leading to flat eroded surface
Usually occurs in 200 million years
- Pene plain means “almost a plane
Flat but not quite dead flat
Result of years of erosion of mountains
The rocks that we walk over today originated from the mountains (gneis rocks)
- Mylonites of the parry sound shear zone
Deformed gneis with uniform banding
Deforming occurs because of shearing
- Intra-plate earthquakes
Related to terrane boundaries which are deeply buried
Breakup of Rodinia
- 570 million years ago
- Creation of an independent North America (‘Laurentia’) and the quietest episode in its
long geologic history
No active plate margins/No active tectonics
Floating around in the tropics covered by shallow seas
- Iapetus is the result of the breakup of Rodinia and the Atlantic ocean is the result of the
breakup of Pangea
First multicellular life (550-500 Ma)
- Ediacaran fauna
- Referred to as the ‘Cambrian’ explosion
First sudden appearance of complex organisms
Now known as a slow evolution
Correlated with the breakup of Rodinia
Pangea (300 Ma)
- Eastern North America becomes an active plate margin
- Appalachian orogeny
- Maritime Canada is added (‘Out of Africa’)
- When Pangea is broken apart, Atlantic ocean is formed
Compressing crust, thickens it
- Leads to the formation of mountains
- Depresses the edge of the continent
Forms basins
Canadian Shield A landscape
- Strike is the edge of a layer
Marmara, Ontario
- Old iron mine which is part of buried Canadian Shield
- Limestone at the stop which is the ideal cover rock
Niagara
- Cover rocks: Alternation of limestone and shale