Editing Canadian Shield (section)

== 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>