View source for Canadian Shield

{{Short description|Geographic and geologic area of North America}}
{{Infobox rockunit
| name           = Canadian Shield
| type           = [[Shield (geology)|Shield]]
| age            = {{Long fossil range|4500|540|[[Precambrian]]}}
| region         = [[North America]]
| country        = Canada<br />United States<br />Greenland
| unitof         = [[North American craton]]
| subunits       = [[Laurentian Upland]]<br />[[Kazan Region|Kazan]]<ref name=ca-atlas/>
| coordinates    = {{coord|52|00|N|71|00|W|region:CA_type:landmark_source:kolossus-cswiki|display=title,inline}}
| underlies      =
| overlies       =
| thickness      =
| extent         =
| area           = 8,000,000 km<sup>2</sup><ref name=britannica/>
| map            = Canada geological map.JPG
| map_caption    = The Canadian Shield is a broad region of Precambrian rock (pictured in shades of red) that encircles [[Hudson Bay]]. It spans eastern, northeastern, and east-central [[Canada]] and the [[Upper Midwest|upper midwestern]] [[United States]].
| namedfor       =
}}

The '''Canadian Shield''' ({{langx|fr|'''Bouclier canadien'''}} {{IPA|fr|buklje kanadjɛ̃|}}), also called the '''Laurentian Shield''' or the  '''Laurentian Plateau''', is a [[Shield (geology)|geologic shield]], a large area of exposed [[Precambrian]] [[Igneous rock|igneous]] and high-grade [[metamorphic rock]]s. It forms the [[Laurentia|North American Craton (or Laurentia)]], the ancient geologic core of the [[North America]]n continent. [[Glacial period|Glaciation]] has left the area with only a thin layer of [[soil]], through which exposures of igneous [[bedrock]] resulting from its long volcanic history are frequently visible.<ref name =EG>{{cite book |last1=Marshak |first1=Stephen |title=Essentials of geology |date=2009 |publisher=W.W. Norton |location=New York |isbn=978-0393932386 |edition=3rd}}</ref> As a deep, common, joined bedrock region in eastern and central [[Canada]], the shield stretches north from the [[Great Lakes]] to the [[Arctic Ocean]], covering over half of Canada and most of [[Greenland]]; it also extends south into the northern reaches of the continental [[United States]].

== Geographical extent ==
The Canadian Shield is a [[Physiographic region|physiographic division]] comprising four smaller physiographic provinces: the [[Laurentian Upland]], [[Kazan Region]], Davis and James.<ref name=ca-atlas>{{Cite web| author-link =Atlas of Canada |title=Physiographic Regions Map|author=The Atlas of Canada |date=12 September 2016|publisher=Natural Resources Canada |url=https://atlas.gc.ca/phys/en/ |access-date=2019-11-15 }}</ref> The shield extends into the United States as the [[Adirondack Mountains]] (connected by the [[Frontenac Axis]]) and the [[Laurentian Upland#Superior Upland|Superior Upland]]. The Canadian Shield is a U-shaped subsection of the Laurentia [[craton]] signifying the area of greatest [[glacial]] impact (scraping down to bare rock) creating the thin soils. The age of the Canadian Shield is estimated to be 4.28 [[Gigaannus|Ga]] (4.28 billion years). The Canadian Shield once had jagged peaks, higher than any of today's mountains, but millions of years of erosion have changed these mountains to rolling hills.<ref name="Canadian Shield">{{cite web |author1=Alexander Murray |author2=Harrisson Panabaker |author3=Holy O'Rourke |author4=David Barrett |title=Canadian Shield |url=https://www.thecanadianencyclopedia.ca/en/article/shield |publisher=The Canadian Encyclopedia |access-date=29 May 2024 |date=7 February 2006 |quote=Formed over 3 billion years through processes such as plate tectonics, erosion and glaciation}}</ref>

The Canadian Shield is a collage of [[Archean]] plates and accreted juvenile arc [[terrane]]s and [[sedimentary basin]]s of the [[Proterozoic]] Eon that were progressively amalgamated during the interval 2.45–1.24 Ga, with the most substantial growth period occurring during the [[Trans-Hudson orogeny]], between c. 1.90–1.80 Ga.<ref name=Corrigan>{{cite web |author=Corrigan, D. |year=2008 |title=Metallogeny and Tectonic Evolution of the Trans-Hudson Orogen |url=http://www.pdac.ca/pdac/conv/2008/pdf-tech-session/ts-corrigan.pdf | access-date =2008-03-05 |archive-url = https://web.archive.org/web/20080409105436/http://www.pdac.ca/pdac/conv/2008/pdf-tech-session/ts-corrigan.pdf |archive-date = April 9, 2008}}</ref> The Canadian Shield was the first part of North America to be permanently elevated above sea level and has remained almost wholly untouched by successive encroachments of the sea upon the continent. It is the Earth's greatest area of exposed Archean rock. The metamorphic base rocks are mostly from the Precambrian (between 4.5 Ga and 540 Ma) and have been repeatedly uplifted and eroded. Today it consists largely of an area of low relief {{convert|300|–|610|m|ft|abbr=on}} above sea level with a few [[monadnock]]s and low mountain ranges (including the [[Laurentian Mountains]]) probably eroded from the plateau during the [[Cenozoic]] Era. During the [[Pleistocene]] Epoch, continental ice sheets depressed the land surface (creating [[Hudson Bay]]) but also tilted up its northeastern "rim" (the [[Torngat Mountains|Torngat]]), scooped out thousands of lake basins, and carried away much of the region's soil. The northeastern portion, however, became tilted up so that, in northern [[Labrador]] and [[Baffin Island]], the land rises to more than 1,500 metres (5,000 feet) above sea level.

When the [[Greenland]] section is included, the Canadian Shield is approximately circular, bounded on the northeast by the northeast edge of Greenland, with Hudson Bay in the middle. It covers much of Greenland, all of Labrador and the [[Great Northern Peninsula]] of [[Newfoundland (island)|Newfoundland]], most of [[Quebec]] north of the [[St. Lawrence River]], much of [[Ontario]] including northern sections of the [[Ontario Peninsula]], the Adirondack Mountains<ref>Peterson Field Guide to Geology of Eastern North America by Roberts, David & Roger Tory Peterson.</ref> of [[New York (state)|New York]], the northernmost part of [[Lower Peninsula of Michigan|Lower Michigan]] and all of [[Upper Peninsula of Michigan|Upper Michigan]], northern [[Wisconsin]], northeastern [[Minnesota]], the central and northern portions of [[Manitoba]], northern [[Saskatchewan]], a small portion of northeastern [[Alberta]],<ref>[http://www.abheritage.ca/abnature/shield/shield.htm Alberta Heritage - Alberta Online Encyclopedia] {{webarchive|url=https://wayback.archive-it.org/2217/20101208161019/http://www.abheritage.ca/abnature/shield/shield.htm |date=2010-12-08 }} - The Canadian Shield Region of Alberta</ref> mainland [[Northwest Territories]] to the east of a line extended north from the Saskatchewan-Alberta border, most of [[Nunavut|Nunavut's]] mainland and, of its [[Arctic Archipelago]], Baffin Island and significant bands through [[Somerset Island (Nunavut)|Somerset]], [[Southampton Island|Southampton]], [[Devon Island|Devon]] and [[Ellesmere Island|Ellesmere]] islands.<ref name=britannica>{{Cite encyclopedia|url=https://www.britannica.com/place/Canadian-Shield|encyclopedia=[[Encyclopædia Britannica]] |title=Canadian Shield|access-date=2009-02-10|url-status=live|archive-url=https://web.archive.org/web/20150624232123/https://www.britannica.com/place/Canadian-Shield|archive-date=2015-06-24}}</ref> In total, the exposed area of the shield covers approximately {{convert|8000000|km2|mi2|abbr=on}}. The true extent of the shield is greater still and stretches from the [[North American Cordillera|Western Cordillera]] in the west to the [[Appalachian Mountains|Appalachians]] in the east and as far south as [[Texas]], but these regions are overlaid with much younger rocks and sediment.{{wide image|Flin Flon Wilderness and Outcrop.jpg|1000px|Panorama of Canadian Shield geography in the [[Flin Flon]], [[Manitoba]], region. Big Island Lake is in the background.}}

== Geology ==
The Canadian Shield is among the oldest geologic areas on Earth, with regions dating from 2.5 to 4.2 billion years.<ref>Tsuyoshi Iizuka, at al., "Geology and Zircon Geochronology of the Acasta Gneiss Complex", ''Precambrian Research'', 153 (2007) pp. 179–208</ref> The multitude of rivers and lakes in the region is classical example of a [[deranged drainage system]], caused by the [[drainage basin|watersheds]] of the area being disturbed by glaciation and the effect of [[post-glacial rebound]].<ref name="physicalgeography.net">{{cite book |last=Pidwirny |first=M. |year=2006 |chapter=The Drainage Basin Concept |chapter-url = http://www.physicalgeography.net/fundamentals/10aa.html |title =Fundamentals of Physical Geography |edition=2nd}}</ref> The shield was originally an area of very large, very tall mountains (about {{convert|12000|m|disp=or||abbr=on}})<ref>
{{cite book
|last=Clark
|first=Bruce W.
|chapter=Geologic History
|pages=[https://archive.org/details/makingconnection00clar/page/95 95]
|title=Making Connections: Canada's geography
|year=1999
|publisher=Prentice Hall Ginn Canada
|location=Scarborough, Ontario
|isbn=978-0-13-012635-1
|chapter-url-access=registration
|chapter-url=https://archive.org/details/makingconnection00clar/page/95
}}</ref> with much volcanic activity, but the area was eroded to nearly its current topographic appearance of relatively low relief over 500 Ma.<ref>{{cite journal |last1=Ambrose |first1=J. W. |title=Exhumed paleoplains of the Precambrian Shield of North America |journal=American Journal of Science |date=1 September 1964 |volume=262 |issue=7 |pages=817–857 |doi=10.2475/ajs.262.7.817|bibcode=1964AmJS..262..817A |doi-access=free }}</ref><ref>{{cite journal |last1=Shilts |first1=William W. |last2=Aylsworth |first2=Janice M. |last3=Kaszycki |first3=Christine A. |last4=Klassen |first4=Rodney A. |title=Canadian Shield |journal=Geomorphic Systems of North America |date=1987 |pages=119–161 |doi=10.1130/DNAG-CENT-v2.119|isbn=0813753023 }}</ref> [[Erosion]] has exposed the roots of the mountains, which take the form of [[greenstone belt]]s in which belts of volcanic rock that have been altered by metamorphism are surrounded by granitic rock. These belts range in age from 3.6 to 2.7 Ga.<ref>{{cite journal |last1=Card |first1=K.D. |title=A review of the Superior Province of the Canadian Shield, a product of Archean accretion |journal=Precambrian Research |date=August 1990 |volume=48 |issue=1–2 |pages=99–156 |doi=10.1016/0301-9268(90)90059-Y|bibcode=1990PreR...48...99C }}</ref> Much of the granitic rock belongs to the distinctive [[Tonalite-trondhjemite-granodiorite|tonalite–trondhjemite–granodiorite]] family of rocks, which are characteristic of [[Archean]] [[continental crust]].<ref>{{cite journal |last1=Smithies |first1=R.H |title=The Archaean tonalite–trondhjemite–granodiorite (TTG) series is not an analogue of Cenozoic adakite |journal=Earth and Planetary Science Letters |date=15 October 2000 |volume=182 |issue=1 |pages=115–125 |doi=10.1016/S0012-821X(00)00236-3|bibcode=2000E&PSL.182..115S }}</ref> Many of Canada's major ore deposits are associated with greenstone belts.<ref>{{cite journal |last1=de Geoffroy |first1=J. |last2=Wignall |first2=T. K. |title=A probabilistic appraisal of mineral resources in a portion of the Grenville Province of the Canadian shield |journal=Economic Geology |date=1 May 1971 |volume=66 |issue=3 |pages=466–479 |doi=10.2113/gsecongeo.66.3.466|bibcode=1971EcGeo..66..466D }}</ref>

The [[Sturgeon Lake Caldera]] in [[Kenora District]], Ontario, is one of the world's best preserved [[mineralization (geology)|mineralized]] [[Neoarchean]] [[caldera]] complexes, which is 2.7 Ga.<ref>[http://www.d.umn.edu/~rmorton/ronshome/Volcanoes/calderas.html Caldera Volcanoes] {{webarchive|url=https://web.archive.org/web/20120814221700/http://www.d.umn.edu/~rmorton/ronshome/Volcanoes/calderas.html |date=2012-08-14 }} Retrieved on 2007-07-20</ref> The Canadian Shield also contains the [[Mackenzie dike swarm]], which is the largest dike swarm known on Earth.<ref>{{cite journal|doi=10.1190/1.1444345|title=Removing varying directional trends in aeromagnetic data|journal=Geophysics|volume=63|issue=2|pages=446–453|year=1998|last1=Pilkington|first1=Mark|last2=Roest|first2=Walter R.|bibcode=1998Geop...63..446P}}</ref> The North American craton is the bedrock forming the heart of the North American continent, and the Canadian Shield is the largest exposed part of the craton's bedrock. The Canadian Shield is part of an ancient continent called [[Arctica]], which was formed about 2.5 Ga during the [[Neoarchean]] era.

Mountains have deep roots and float on the denser [[Mantle (geology)|mantle]] much like an [[iceberg]] at [[sea]]. As mountains erode, their roots rise and are eroded in turn. The rocks that now form the surface of the shield were once far below the Earth's surface. The high pressures and temperatures at those depths provided ideal conditions for mineralization. Although these mountains are now heavily eroded, many large mountains still exist in Canada's far north called the [[Arctic Cordillera]]. This is a vast, deeply dissected mountain range, stretching from northernmost Ellesmere Island to the northernmost tip of Labrador. The range's highest peak is Nunavut's [[Barbeau Peak]] at {{convert|2616|m}} above sea level.<ref>{{cite bivouac|261|Barbeau Peak}}</ref> Precambrian rock is the major component of the bedrock.

<gallery widths="200px" heights="200px">
Image:Temagami greenstone belt pillow lava.jpg|Weathered Precambrian [[pillow lava]] in the [[Temagami Greenstone Belt]]
File:Nature's Art created by thousands of years of erosion by ice.jpg|Folded Precambrian [[gneiss]] of the Canadian Shield in [[Georgian Bay]], Ontario
Image:Canadian Shield Ontario.jpg|Typical Canadian Shield landscape: spruce, lakes, bogs, and rock
</gallery>

== Ecology ==
<!-- Not quite ready to delete this yet, but it does not seem germane to geology. Definitely of use to a GEOGRAPHY article about the same area. -->
[[Image:BlackRiver1.JPG|thumb|Typical shield landscape in a southern Ontario region with very few old growth trees, due to a history of logging and fires. Black River, Queen Elizabeth II Wildlands Provincial Park.]]

The current [[geomorphology|surface expression]] of the shield is one of very thin soil lying on top of the bedrock, with many bare [[Outcrop|outcrops]]. This arrangement was caused by severe glaciation during the ice ages that covered the shield and scraped the rock clean. The lowlands of the Canadian Shield have a very dense soil that is not suitable for forestation; it also contains many marshes and bogs ([[muskeg]]s). The rest of the region has coarse soil that does not retain moisture well and is frozen with [[permafrost]] throughout the year. Forests are not as dense in the north.

The shield is covered in parts by vast [[Taiga|boreal forests]] in the south that support natural [[ecosystem]]s as well as a major [[logging]] industry. The boreal forest area gives way to the [[Eastern Canadian Shield taiga]] that covers northern Quebec and most of Labrador. The [[Midwestern Canadian Shield forests]] that run westwards from [[Northwestern Ontario]] have boreal forests that give way to taiga in the most northerly parts of Manitoba and Saskatchewan. [[Hydrology|Hydrologic]] drainage is generally poor, the soil compacting effects of glaciation being one of the many causes. [[Tundra]] typically prevails in the northern regions. 

Many mammals such as [[North_American_beaver|beaver]], [[caribou]], [[white-tailed deer]], [[moose]], [[Gray wolf|wolves]], [[wolverine]]s, [[weasel]]s, [[mink]], [[otter]]s, [[grizzly bear]], [[polar bear]]s and [[American black bear|black bears]] are present.<ref>{{NatGeo ecoregion|id=na0612|name=Northern Canadian Shield taiga}}</ref> In the case of polar bears ({{Lang|la|Ursus maritimus}}), the shield area contains many of their [[Maternity den|denning]] locations, such as the [[Wapusk National Park]].<ref>C. Michael Hogan (2008) [http://globaltwitcher.auderis.se/artspec_information.asp?thingid=36084 ''Polar Bear: Ursus maritimus'', Globaltwitcher.com, ed. Nicklas Stromberg] {{webarchive |url=https://web.archive.org/web/20081224205716/http://globaltwitcher.auderis.se/artspec_information.asp?thingid=36084 |date=December 24, 2008 }}</ref>
The many lakes and rivers on the shield contain a plentiful quantity of different sports fish species, including [[walleye]], [[northern pike]], [[lake trout]], [[yellow perch]], [[lake whitefish|whitefish]], [[brook trout]], [[arctic grayling]], and many types of baitfish.<ref>{{cite web | url=https://northernontario.travel/fishing/making-sense-shield-lakes-and-rivers#:~:text=big%20walleye%2C%20bass%2C%20lake%20trout%20and%20northern%20pike.&text=As%20a%20matter%20of%20fact,this%20shield%20lake%20or%20river. | title=Making Sense of Shield Lakes and Rivers &#124; Northern Ontario Travel | date=28 June 2016 }}</ref> The water surfaces are also home to many [[waterfowl]], most notably [[Canada geese]], [[loons]] and [[gulls]].<ref>{{cite web | url=http://www.ecozones.ca/english/zone/BorealShield/wildlife.html | title=Wildlife of the Boreal Shield Ecozone }}</ref> The vast forests contain a myriad population of other birds, including [[Raven|ravens]] and [[crows]], [[predatory birds]] and many [[songbirds]].

== Mining and economics ==
The Canadian Shield is one of the world's richest areas in terms of [[mineral]] [[ore]]s. It is filled with substantial deposits of [[nickel]], [[gold]], [[silver]], and [[copper]]. There are many mining towns extracting these minerals. The largest, and one of the best known, is [[Greater Sudbury|Sudbury]], Ontario. Sudbury is an exception to the normal process of forming minerals in the shield since the [[Sudbury Basin]] is an ancient [[meteorite]] [[impact crater]]. [[Ejecta]] from the meteorite impact was found in the [[Rove Formation]] in May 2007. The nearby but less-known [[Temagami Magnetic Anomaly]] has striking similarities to the Sudbury Basin. This suggests it could be a second metal-rich impact crater.<ref name="GH">[http://gdcinfo.agg.nrcan.gc.ca/app/3Dimaging/temagami_e.html 3-D Magnetic Imaging using Conjugate Gradients: Temagami anomaly] {{webarchive|url=https://web.archive.org/web/20090711184237/http://gdcinfo.agg.nrcan.gc.ca/app/3Dimaging/temagami_e.html |date=2009-07-11 }} Retrieved on 2008-03-12</ref> In northeastern Quebec, the giant [[Manicouagan Reservoir]] is the site of an extensive [[Hydroelectricity|hydroelectric]] project (Manic-cinq, or Manic-5). This is one of [[List of impact craters on Earth#Largest craters (10 Ma or more)|the largest-known meteorite impact craters on Earth]], though not as large as the Sudbury crater.

The [[Flin Flon greenstone belt]] in central Manitoba and east-central Saskatchewan "is one of the largest [[Paleoproterozoic]] volcanic-hosted massive [[sulfide]] ([[Volcanogenic massive sulfide ore deposit|VMS]]) districts in the world, containing 27 [[copper]]-[[zinc]]-([[gold]]) deposits from which more than 183 million tonnes of sulfide have been mined."<ref>Troymet Exploration, ''[https://web.archive.org/web/20080530010956/http://www.troymet.com/i/pdf/2007McClarty43-101.pdf Report on the 2007 Diamond Drilling Program, McClarty Lake Project, Manitoba: The Pas Mining District NTS 63-K-08; UTM ZONE 14 N 415938 E, 6038968 N; 54° 29′ 28″ N 100° 17′ 52″ W]'', by Jessica Norris & Tracy Hurley (Whitehorse, Yukon: Aurora Geosciences, 2007‑09‑24).</ref> The portion in the Northwest Territories has recently been the site of several major [[diamond]] discoveries. The [[kimberlite]] pipes in which the diamonds are found are closely associated with cratons, which provide the deep [[lithosphere|lithospheric]] [[Mantle (geology)|mantle]] required to stabilize diamond as a mineral. The kimberlite eruptions then bring the diamonds from over {{convert|150|km}} depth to the surface. The [[Ekati Diamond Mine|Ekati]] and [[Diavik Diamond Mine|Diavik]] mines are actively mining kimberlite diamonds.

== See also ==
{{Portal|Geography|Canada}}
* [[Baltic Shield]]
* [[Athabasca Basin]]
* [[Geology of Ontario]]
* [[Platform (geology)]]
* [[Basement (geology)]]
* [[Volcanology of Canada]]
* [[Wisconsin glaciation]]
* [[Glacial history of Minnesota]]
* [[Geography of Canada]]

== References ==
{{Reflist}}

== Further reading ==
{{Library resources box |by=no |onlinebooks=no |others=yes |about=yes |lcheading=Canadian Shield }}
* Schwartzenberger, Tina (2005), ''[https://books.google.com/books?id=f2_0dwVpQGkC&dq=Canadian%20Shield&pg=PP1 The Canadian Shield]'', Weigl Educational, {{ISBN|1-55388-141-9}}

{{Canada topics}}
{{Continental Glaciations}}
{{Physiographic regions}}
{{Authority control}}

[[Category:Shields (geology)]]
[[Category:Geology of North America]]
[[Category:Precambrian North America]]
[[Category:Regional geology]]
[[Category:Geology of Canada|Shield]]
[[Category:Geology of the United States]]
[[Category:Precambrian Canada]]
[[Category:Precambrian United States]]
[[Category:Stratigraphy of Canada|Shield]]
[[Category:Stratigraphy of the United States]]
[[Category:Geology of Ontario]]
[[Category:Geology of Michigan]]
[[Category:Geology of Quebec]]
[[Category:Geology of Minnesota]]
[[Category:Geology of Newfoundland and Labrador]]
[[Category:Geology of New York (state)]]
[[Category:Geology of Manitoba]]
[[Category:Geology of Saskatchewan]]
[[Category:Geology of Nunavut]]
[[Category:Geology of the Northwest Territories]]
[[Category:Geology of Alberta]]
[[Category:Geology of Greenland]]
[[Category:Physiographic regions of Canada|Shield]]
[[Category:Regions of the Subarctic]]