The Coldwell Complex is a large circular

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ...

and

syenite Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proport ...

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

within the Earth's crust. Located on the

North Shore North Shore or Northshore may refer to: Geographic features Australia *North Shore (Sydney), a suburban region of Sydney **Electoral district of North Shore **North Shore railway line, Sydney *Noosa North Shore, Queensland * North Shore, New So ...

Lake Superior Lake Superior in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh w ...

Ontario Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central C ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tota ...

, it is the largest

alkaline In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...

in North America with a diameter of .Twenty Third Annual Meeting Thunder Bay Ontario: Institute on Lake Superior Geology

Formation

The Coldwell Complex has its origins in the Midcontinent Rift System about 1,100 million years ago. It is an exposed

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it up ...

that formed beneath the surface causing surface swelling to occur. This swelling turned into an active volcano, erupting

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...

. 600 million years ago, the volcano walls became so large and heavy, they caved in on the volcano, acting as a cap. The magma then became trapped in the magma chamber where it settled and cooled with other minerals causing the initial phases of the complex. Glacial erosion stripped away the top layers of the volcano. Today, the once-hot magma chamber is exposed to surface rock on the northern shore of

References

Volcanoes of Ontario Proterozoic volcanism Landforms of Thunder Bay District Igneous intrusions {{Canada-geology-stub

Formation

See also

References