The geology of Ontario is the study of rock formations in the most populated province in Canada, home to some of the oldest rock on Earth. The geology in Ontario consists of ancient Precambrian igneous and metamorphic rock which sits under younger, sedimentary rocks and soils. Around 61% of Ontario is covered by the Canadian Shield. The shield, as a whole, can further be divided into three sections- these are known as provinces. The northwestern parts of the Shield, located north and west of Sudbury, are known as the Superior province. This is the largest of the three sections, covering 70% of the Canadian Shield portion in Ontario. The Southern province is a narrow region from Sault Ste. Marie to Kirkland Lake. The South central part is dominated by the Grenville Province but flanked by two basins of Phanerozoic materials. Abundant mineral deposits are found here and as a result are mined extensively.
Canadian Shield About 61% of Ontario is covered by the Canadian Shield, mostly with Precambrian rock. The Canadian Shield spans much of northern Ontario and is subdivided into three main geological provinces; The Superior, Southern and Grenville.
Superior Province The Superior Craton or Superior Province is an Archean craton. It is a 260 kilometer or 160 mile thick section of stable continental crust formed beginning (4.031 billion years ago-present) which forms the core of the Canadian Shield lying north of Lake Superior for which it is named. It is located roughly north and west of Sudbury The craton extends from northwestern Quebec along the east side of Hudson Bay south through Northern Ontario to the north shores of Lake Huron and Lake Superior. It extends across Northwestern Ontario south of Hudson Bay and underlies most of southeastern Manitoba. It extends southward through eastern North and South Dakota and western Minnesota. The Superior province is the largest of the three sections, covering about 70% of the Shield portion in Ontario. The Superior Province has east–west running bands of volcanic, sedimentary and gneissic rocks. The northernmost parts of the Superior Province is mostly granite and gneiss rocks. The 2,677 million year old Abitibi greenstone belt in Ontario and Quebec is one of the largest Archean greenstone belts on Earth and one of the youngest parts of the Superior craton which sequentially forms part of the Canadian Shield. Ontario's metallic mineral wealth such as gold, copper and zinc comes from the Abitibi/Wawa subprovince.
Southern Province The Southern province is a narrow region from Sault Ste. Marie to Kirkland Lake, is made of rocks dating 1.8 to 2.4 billion years ago. The Hudson Bay lowlands, located north of the Canadian Shield, are mainly made of sedimentary rocks from the Silurian Period, although some parts date from the Ordovician and Devonian periods. This area covers 25% of the province. Most of the bedrock in the Hudson Bay lowlands is composed of limestone and carbonate-dominated sedimentary rock. Besides Ontario it also forms parts of the bedrock of Michigan and Minnesota. The Midcontinent Rift System formed 1.1 billion years ago when the Craton split open and formed the basin of Lake Superior. In the Lake Superior region, the up welling of this molten rock may have been the result of a hotspot which produced a triple junction.
Grenville Province The Grenville Province makes up about 20 percent of the exposed Canadian Shield in Ontario and located south of Sudbury is 1.0 to 1.6 billion years old and is dominated by sedimentary rocks and later metamorphized. These rocks were metamorphosed between 990 million years ago and 1.08 billion years ago. Geologists subdivide the Grenville Province into the Allochthon along the river itself and the more northern Parautochthon. The Allochthon juxtaposed on the Parautochthon during the Grenville orogeny cycle from 1.09 billion to 985 million years ago. The Allochthon is composed of Paleoproterozoic to Mesoproterozoic rocks. In the western part, it is mainly marble, quartzite, and pelite platform levels and Mesoproterozoic amphibolite-grade rocks. There are also charnockite and anorthosite intrusions that intersect metasediments and orthogneiss. In the center, migmatite, quartzo-feldspathic orthogneisses and mangerites predominate. In its eastern part, it consists mainly of gneissic rocks of varied composition and origin, metasedimentary rocks, granitoid intrusions, gabbro, gabbronorite and anorthosite. There are also several anorthositic intrusions scattered throughout the Allochthon belt. The Parautochthon is a band running parallel to the Grenville Front, which varies in width from Labrador to northeastern Georgian Bay on Lake Huron. Parautochthonous rocks are made up of ancient Archean and Proterozoic rocks that are highly deformed plutonic and metamorphosed supracrustal (metasedimentary and metavolcanic) rocks that reached greenschist to granulite facies on the sequence of metamorphic facies during the Grenville orogeny. These rocks correlate with the least deformed rocks north of the Grenville Front in the Superior Province and show signs of east–west lateral extension.
Sudbury basin
The Sudbury Basin formed as a result of an impact into the Nuna supercontinent from a bolide approximately 10–15 km (6.2–9.3 mi) in diameter that occurred 1,849 million years ago Sudbury Basin is the third-largest crater on Earth, after the 300 km (190 mi) Vredefort impact structure in South Africa, and the 150 km (93 mi) Chicxulub crater under Yucatán, Mexico. The Sudbury Igneous Complex is an impact melt that formed from this impact and the high pressures and temperatures melted the surrounding rock. NASA used the site to geologically train the Apollo astronauts in recognizing rocks formed as the result of a very large impact, such as breccias. Astronauts who would use this training on the Moon included Apollo 15's David Scott and James Irwin, Apollo 16's John Young and Charlie Duke, and Apollo 17's Gene Cernan and Jack Schmitt. Notable geologist instructors included William R. Muehlberger.
Dike swarms
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