Editing Lake Superior (section)

==Geology==
[[File:Mid-continental Rift System.webp|thumb|Most of Lake Superior lies within the basin of the [[Midcontinent Rift System|Midcontinent Rift]].]]

The rocks of Lake Superior's northern shore date back to the early history of the earth. During the [[Precambrian]] (between 4.5&nbsp;billion and 540&nbsp;million years ago) [[magma]] forcing its way to the surface created the intrusive [[granite]]s of the [[Canadian Shield]].<ref>{{cite book |title = Geology of National Parks |edition = 2nd |first = Ann G. |last = Harris |publisher = Kendall Hunt |location = Dubuque, Iowa |year = 1977 |page = 200 }}</ref> These ancient granites can be seen on the [[North Shore (Lake Superior)|North Shore]] today. It was during the [[Penokean orogeny]], part of the process that created the [[Great Lakes tectonic zone]], that many valuable metals were deposited. The region surrounding the lake has proved to be rich in minerals, with [[copper]], [[iron]], [[silver]], [[gold]] and [[nickel]] the most frequently mined. Notable production includes gold from the [[Golden Giant Mine|Hemlo]] mine near [[Marathon, Ontario|Marathon]], copper from the [[Keweenaw Peninsula]] and the [[Mamainse Point Formation]], iron from the [[Gogebic Range]], silver at [[Silver Islet]], and uranium at Theano Point.

The mountains steadily eroded, depositing layers of [[sediment]]s that compacted and became [[limestone]], [[Dolomite (rock)|dolomite]], [[taconite]] and the [[shale]] at [[Kakabeka Falls]]. The continental crust was later [[rift|riven]], creating one of the deepest rifts in the world.<ref>{{cite web |url = http://www.law.umkc.edu/faculty/projects/ftrials/superior/superiorfacts.html |last = Linder |first = Douglas O. |year = 2006 |title = 'Simply Superior: The World's Greatest Lake' Lake Superior Facts |archive-url = https://web.archive.org/web/20051105005812/http://www.law.umkc.edu/faculty/projects/ftrials/superior/superiorfacts.html |archive-date = November 5, 2005 |work = Law.umkc.edu |access-date = November 17, 2015 }}</ref> The lake lies in this long-extinct [[Mesoproterozoic]] [[rift valley]], the [[Midcontinent Rift System|Midcontinent Rift]]. Magma was injected between layers of [[Rove region|sedimentary rock]], forming [[diabase]] [[Sill (geology)|sills]]. This hard diabase protects the layers of sedimentary rock below, forming the flat-topped mesas in the [[Thunder Bay]] area. [[Amethyst]] formed in some of the cavities created by the Midcontinent Rift, and there are several amethyst mines in the Thunder Bay area.<ref>{{cite web |url = http://www.mndm.gov.on.ca/mndm/mines/ims/amethyst/default_e.asp |title = Ontario Amethyst: Ontario's Mineral Emblem |archive-url = https://web.archive.org/web/20070812025204/http://www.mndm.gov.on.ca/mndm/mines/ims/amethyst/default_e.asp |archive-date = August 12, 2007 |publisher = Ontario Ministry of Northern Development and Mines |access-date = August 4, 2007 }}</ref>

[[File:Basaltic columns Lake Superior.png|thumb|[[Basalt#Columnar basalt|Basaltic columns]] along Lake Superior]]

Lava erupted from the rift and formed the black basalt rock of Michipicoten Island, Black Bay Peninsula, and [[St Ignace Island|St. Ignace Island]].{{citation needed|date=June 2021}}

In the most recent geological history, during the [[Wisconsin glaciation]] 10,000 years ago, ice covered the region at a thickness of {{convert|1.25|mi|km|0}}. The land contours familiar today were carved by the advance and retreat of the [[ice sheet]]. The retreat left gravel, sand, clay and boulder deposits. Glacial meltwaters gathered in the Superior basin creating [[Glacial Lake Minong|Lake Minong]], a precursor to Lake Superior.<ref>{{harvp|Chisholm|Gutsche|1998|p= xv}}.</ref> Without the immense weight of the ice, the land [[Post-glacial rebound|rebounded]], and a drainage outlet formed at Sault Ste. Marie, becoming today's St. Mary's River.