Class Notes (921,913)
CA (542,705)
UTSC (32,886)
EESA06H3 (610)
Nick Eyles (539)
Lecture

Chapter 20

9 Pages
160 Views

Department
Environmental Science
Course Code
EESA06H3
Professor
Nick Eyles

This preview shows pages 1-3. Sign up to view the full 9 pages of the document.
Chapter 20 Geological History
CANADA: A YOUNG NATION, BUT AN OLD COUNTRY
The North American Continent was assembled by plate tectonics processes that brought
together many smaller land messes. When fused and locked together, they have created
the geological mosaic of the present-day continent. The process of continental building has
not been a simple one and has taken more than 4 billion years to accomplish.
Construction of North America began at least 4,000 million years ago with the formation
of Acasta Gneiss of the North-west Territories, which now forms part of the Slave
Province of the Canadian Shield.
The building of North American was essentially complete 65 million years ago, although
the modern landscape is the result of geologically recent glaciations that have occurred in
the last 2.5 million years. The last ice sheet left the southern portions of the country only
12,000 years ago, and finally melted in Labrador 6,000 years ago. Remnants of this vast
ice sheet still survive on Baffin Island today as the Penny and Barnes Ice Caps.
WHAT ARE THE MAIN GEOLOGICAL BUILIDING BLOCKS OF NORTH
AMERICA?
A Geological Jigsaw
The northern part of the continent, in Canada, is underlain by the exposed part of the
ancient core or craton of North America; this exposed part is called the Canadian Shield
and consists predominantly of very old, Archean and Proterozoic rocks (figure 20.3 A,
B). These rocks range in age from 4 billion to approximately 1 billion years old and are
largely devoid of fossils.
The craton is composed of a complex assemblage of several distinct geologic provinces.
Geologic provinces are broad regions of similar rocks, usually covering many thousands
of square kilometers, with characteristics that differ significantly from rock types present
in adjacent areas. Individual geologic provinces have been subdivided into smaller unites
called sub-provinces, which are fault-bounded unites containing similar rock types,
structures and mineral deposits.
Provinces and sub-provinces are now widely recognized to be terranes. Terranes are
discrete fragments of oceanic or continental material that have been added to a craton at
an active margin by accretion.
These likely originated as small continents and remnants ocean-floor crust, each with its
own complex geological history, and were welded together by plate tectonics processes to
form the North American craton.
The full geographic extent of the craton is not immediately apparent from a map of the
geology of North Americas as its outermost margin s are buried by layer of the younger
cover rocks that reach thicknesses of more than 10km.
www.notesolution.com
The North American Craton versus the Canadian Shield
The former refers to a large, continent-sized block of distinct geology marking up the
basement of much of North America (and Greenland). The Shield is the exposed part of
the craton, and consists of a gently undulating surface that rises inconspicuously, almost
like an arch, in its centre.
The Canadian Shield is a large landform called a peneplain, which is a surface of low
relief and great areal extent and age. Erosion and beveling of the ancient rocks of the
craton had created this peneplain by about 800 million years ago. The outer, gently
sloping margins of the shield are buried below younger rocks, and the ancient peneplain
surface now forms an unconformity between the craton below and younger rocks above.
The cover rocks that bury the outer margins of the North American craton are
fossiliferous sedimentary strata of Paleozoic and Mesozoic age. These sedimentary rocks
were deposited when the outer margins of the craton were depressed and flooded by
shallow sears. This is occurred primarily during mountain-building episodes (opogenies)
that resulted from the collision of other land masses with the craton. During these events,
the outer margins of the craton were depressed by the great weight of large mountain
belts (including volcanoes) and their thick piles of sediment. Many other continents on
Earth show the same basic anatomy of a central, ancient craton created by the fusion of
many separate geological provinces (micocontinents), buried around its margins by
younger sedimentary cover strata.
The Geologic Jigsaw of the North American Craton
Early geologists recognized that, in general, rocks got younger as one travelled away from
the geographic interior of North America to its coasts. Also, rocks that were uniform in
character over enormous distances changed abruptly across very sharp boundaries.
Famous Canadian geologist Sir William Logan was the first to write of geological
provinces in the early 1860s; the term is still in use to describe areas of the shield with
distinctive geological characteristics.
It is not realized that these processes have been operating on the Earth in some form over
the past 4 billion years and are responsible for bringing together the various geological
provinces of Canada. Processes operating in the modern wourld are thus being used to
explain the evolution of the ancient North American craton, a working principle known as
uniformitarianism.
HOW DID THE NORTH AMERICAN CONTINENT EVOLVE?
(Figure 20.4)
1.The original North American continent, Arctica, which started to form about 2.5
billion years ago from smaller continents and was completed by about 1.9 billion
www.notesolution.com
years ago when old Archean cratons (e.g. Slave, Nain Provinces) were welded
together by the trans-Hudson Orogen and others.
2. Added to the North American continent during the formation of Nena about 1.8
billion years ago after Penokean Orogeny.
3.Added during the formation of Rodinia about 1.3 billion years ago during the
Grenville Orogeny.
4.Added during the formation of Pangea abou 600 million 300 million years ago.
5.Added after the breakup of Pangea about 250 million years ago.
Stages in the Evolution of the North American Continent
The orogens consist of crushed and deformed rocks that represent the remains of
mountain belts or volcanic arcs formed during collision.
The earliest part of Canadas geological history is not well known and therefore is subject
to much debate. North American continent can be broken down into five different stages.
Each stage is characterized by a major plate tectonics event, when ancestral North
American either collided with or ripped apart from other land masses. This processes of
repeated continental aggradation and breakup is known as the Wilson Cycle (see
Chapter 2), and has resulted in the development of Supercontinents at certain times in
Earths history.
Stage 1 Arctica: North America in the Archean
The formation of the central part of the North American continent spans the entire
Archean Era (4-2.5 billion years ago). Some of the oldest rocks so fat dated on Earth are
found in the Slave Province (Acasta gneiss of the Northwest Territories, see chapter 10)
and are thought to be between 3.96 and 4.05 billion years old. These rocks formed part of
an ancient continent that some have named Arctica.
Stage 2 Nena and Rodinia: North America in the Proterozoic
The nest events in the development of the North American continent span the entire
Proterozoic Era (from about 2.5 billion to 570 million years ago). The southern part of the
Canadian Shield, including the Southern Province and the Yavapai and Mazatzal
Orogenies, had been added to Arctica by about 1.9 billion years ago or shortly thereafter.
Stage 3 The Grenville Orogeny and Formation of Rodinia
The Grenville Orogeny was the result of the long lived collision between ancestral South
and North America between 1.3 and 1 billion years ago. Much of what is now wastern
north American from southern Greenland to northern Mexico was formed at this time by
www.notesolution.com

Loved by over 2.2 million students

Over 90% improved by at least one letter grade.

Leah — University of Toronto

OneClass has been such a huge help in my studies at UofT especially since I am a transfer student. OneClass is the study buddy I never had before and definitely gives me the extra push to get from a B to an A!

Leah — University of Toronto
Saarim — University of Michigan

Balancing social life With academics can be difficult, that is why I'm so glad that OneClass is out there where I can find the top notes for all of my classes. Now I can be the all-star student I want to be.

Saarim — University of Michigan
Jenna — University of Wisconsin

As a college student living on a college budget, I love how easy it is to earn gift cards just by submitting my notes.

Jenna — University of Wisconsin
Anne — University of California

OneClass has allowed me to catch up with my most difficult course! #lifesaver

Anne — University of California
Description
Chapter 20 Geological History CANADA: A YOUNG NATION, BUT AN OLD COUNTRY The North American Continent was assembled by plate tectonics processes that brought together many smaller land messes. When fused and locked together, they have created the geological mosaic of the present-day continent. The process of continental building has not been a simple one and has taken more than 4 billion years to accomplish. Construction of North America began at least 4,000 million years ago with the formation of Acasta Gneiss of the North-west Territories, which now forms part of the Slave Province of the Canadian Shield. The building of North American was essentially complete 65 million years ago, although the modern landscape is the result of geologically recent glaciations that have occurred in the last 2.5 million years. The last ice sheet left the southern portions of the country only 12,000 years ago, and finally melted in Labrador 6,000 years ago. Remnants of this vast ice sheet still survive on Baffin Island today as the Penny and Barnes Ice Caps. WHAT ARE THE MAIN GEOLOGICAL BUILIDING BLOCKS OF NORTH AMERICA? A Geological Jigsaw The northern part of the continent, in Canada, is underlain by the exposed part of the ancient core or craton of North America; this exposed part is called the Canadian Shield and consists predominantly of very old, Archean and Proterozoic rocks (figure 20.3 A, B). These rocks range in age from 4 billion to approximately 1 billion years old and are largely devoid of fossils. The craton is composed of a complex assemblage of several distinct geologic provinces. Geologic provinces are broad regions of similar rocks, usually covering many thousands of square kilometers, with characteristics that differ significantly from rock types present in adjacent areas. Individual geologic provinces have been subdivided into smaller unites called sub-provinces, which are fault-bounded unites containing similar rock types, structures and mineral deposits. Provinces and sub-provinces are now widely recognized to be terranes. Terranes are discrete fragments of oceanic or continental material that have been added to a craton at an active margin by accretion. These likely originated as small continents and remnants ocean-floor crust, each with its own complex geological history, and were welded together by plate tectonics processes to form the North American craton. The full geographic extent of the craton is not immediately apparent from a map of the geology of North Americas as its outermost margin s are buried by layer of the younger cover rocks that reach thicknesses of more than 10km. www.notesolution.com
More Less
Unlock Document


Only pages 1-3 are available for preview. Some parts have been intentionally blurred.

Unlock Document
You're Reading a Preview

Unlock to view full version

Unlock Document

Log In


OR

Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit