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Chapter 20

Detailed Chapter 20 Notes

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Department
Environmental Science
Course Code
EESA06H3
Professor
Nick Eyles

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Introduction to Planet Earth - Chapter 20
-The construction of North America began about 4,000 million years ago with the
formation of the Acasta Gneiss of the North west Territories, which now forms
part of the Slave Province of the Canadian Shield.
-The building of North America was essentially complete by 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.
-The northern part of the continent, in Canada, is underlain by the exposed part of
the ancient core of craton of North America; this exposed part is called
the Canadian Shield and consists predominantly of very
old, Archean and Proterozoic rocks. These rocks range in age from 4 - 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 units
called subprovinces, which are fault-bounded units containing similar rock types,
structures, and mineral deposits.
-Provinces and subprovinces are now widely recognized to be terranes.
-Terranes are discrete fragments of oceanic or continental material that have been
add to a craton at an active margin by accertion.
-These likely originated as small continents and remnants of ocean-floor crust, each
with its own complex geological history.
-Most of the North American craton was assembled between 1 and 4 billion years
ago.
www.notesolution.com
-Full geographic extent of the craton is not immediately apparent from a map of
geology of North America as its outermost margins are buried by layers of younger
"cover rocks" that reach thickness of about 10km
-North American craton is largest in the world extending from the Atlantic sea
board, west beneath the Rocky Mountains, and as far south as Texas. Greenland is
a detached part of the craton; detached 80 million years ago
-The shield is the exposed part of the craton, the craton is the entire extension
mentioned in the latter point.
-Eduard Suess: introduced the term shield around 1912, because of its dome like
shape resembling a warriors shield.
-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 bevelling of the ancient rocks
of the craton created this peneplain by about 800 million years ago.
-The outer, gently sloping margins of the Shield are buried below younger
sedimentary rocks, and the ancient peneplain surface now forms an unconformity
between the craton below and younger rocks above.
-Can be seen in the walls of the grand canyon in Arizona, where it separates
metamorphic rocks of the craton from overlying Paleozoic cover rocks. This is the
same surface that is exposed many hundreds of kilometers to the north as the
Canadian Shield.
-Many other continents on Earth show the same basic anatomy of a central,
ancient craton created by th fusion of many separate geological provinces buried
around its margins by younger sedimentary cover strata.
-Rocks got younger as one traveled 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.
-Sir William Logan: Was the first to write about of "geological provinces" in the early
1860's; the term is till used to describe areas of the Shield with distinctive geological
characteristics.
www.notesolution.com
-Sir William Logan: created a great map of the geology of Canada (actually parts of
Manitoba, southern Ontario and Quebec) was published in 1869 and was a major
achievement.
-A.C. Lawson declared in 1913 that "no one geologist, not for more that matter two,
can hope to become familiar with the details of more than a very small proportion of
the entire field.
-Processes operating in the modern world are thus being used to explain the
evolution of the ancient North American craton, a working principle known
as uniformitarianism.
-Geologic provinces can be regarded as the remains of individual continents that
collided.
-The orogens consist of crushed and deformed rocks that represent the remains of
mountain belts or volcanic arcs formed during collision.
-Some geologists suggest that that the growth of the North American continent can
be broken down into five different stages. Each stage is characterized by a major
plate tectonics event, when ancestral North America either collided with or ripped
apart from other land masses.
-Wilson Cycle: process of repeated continental aggradation and breakup
-Wilson cycle results in the development of supercontinents at certain times in the
Earth's history
-Because of the above process, each of the Earth's continents has a broadly similar
geologic history. Therefore it would be possible to work on the geology of Australia or
South America and recognize the same tectonic events as those that affected parts
of North America.
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 are found in the slave province in the North West
Territories, and are thought to be around 3.96 and 4.05 billion years old.
www.notesolution.com

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Description
Introduction to Planet Earth - Chapter 20 -The construction of North America began about 4,000 million years ago with the formation of the Acasta Gneiss of the North west Territories, which now forms part of the Slave Province of the Canadian Shield. -The building of North America was essentially complete by 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. -The northern part of the continent, in Canada, is underlain by the exposed part of the ancient core of craton of North America; this exposed part is called the Canadian Shield and consists predominantly of very old, Archean and Proterozoic rocks. These rocks range in age from 4 - 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 units called subprovinces, which are fault-bounded units containing similar rock types, structures, and mineral deposits. -Provinces and subprovinces are now widely recognized to be terranes. -Terranes are discrete fragments of oceanic or continental material that have been add to a craton at an active margin by accertion. -These likely originated as small continents and remnants of ocean-floor crust, each with its own complex geological history. -Most of the North American craton was assembled between 1 and 4 billion years ago. www.notesolution.com-Full geographic extent of the craton is not immediately apparent from a map of geology of North America as its outermost margins are buried by layers of younger cover rocks that reach thickness of about 10km -North American craton is largest in the world extending from the Atlantic sea board, west beneath the Rocky Mountains, and as far south as Texas. Greenland is a detached part of the craton; detached 80 million years ago -The shield is the exposed part of the craton, the craton is the entire extension mentioned in the latter point. -Eduard Suess: introduced the term shield around 1912, because of its dome like shape resembling a warriors shield. -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 bevelling of the ancient rocks of the craton created this peneplain by about 800 million years ago. -The outer, gently sloping margins of the Shield are buried below younger sedimentary rocks, and the ancient peneplain surface now forms an unconformity between the craton below and younger rocks above. -Can be seen in the walls of the grand canyon in Arizona, where it separates metamorphic rocks of the craton from overlying Paleozoic cover rocks. This is the same surface that is exposed many hundreds of kilometers to the north as the Canadian Shield. -Many other continents on Earth show the same basic anatomy of a central, ancient craton created by th fusion of many separate geological provinces buried around its margins by younger sedimentary cover strata. -Rocks got younger as one traveled 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. -Sir William Logan: Was the first to write about of geological provinces in the early 1860s; the term is till used to describe areas of the Shield with distinctive geological characteristics. www.notesolution.com
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