The Coldwell Complex is a large circular gabbro and syenite intrusion within the Earth's crust. Located on the North Shore of Lake Superior in northwestern Ontario, Canada, it is the largest alkaline intrusion in North America with a diameter of 25 km (16 mi).
Formation
Like the steep cliffs of Lake Superior, the Coldwell Complex has its origins in the Midcontinent Rift System about 1,100 million years ago. It is an exposed magma chamber that formed beneath the surface causing surface swelling to occur. Eventually, this swelling turned into an active volcano, erupting basaltic magma. 600 million years ago, the volcano walls became so large and heavy, they caved in on the volcano, acting as a plug. The magma then becomes trapped in the magma chamber where it settles and cools with other minerals causing the initial phases of the complex. 66 million years ago, glaciers began to recede, stripping away the top layers of the volcano. Today, the once hot magma chamber is exposed surface rock on the northern shore of Lake Superior.