Lake Nipigon - Geography and Geology

Geography and Geology

Lying 260 metres (853 ft) above sea level, the lake drains into the Nipigon River and thence into Nipigon Bay of Lake Superior. The lake and river are the largest tributaries of Lake Superior. It lies about 120 kilometres (75 mi) northeast of the city of Thunder Bay, Ontario.

Lake Nipigon has a total area (including islands within the lake) of 4,848 square kilometres (1,872 sq mi) — compared to 3,150 square kilometres (1,220 sq mi) for Lake of the Woods. The largest islands are Caribou Island, Geikie Island, Katatota Island, Kelvin Island, Logan Island, Murchison Island, Murray Island, and Shakespeare Island. Maximum depth is 165 metres (541 ft).

The lake is noted for its towering cliffs and unusual green-black sand beaches composed of the fine particles of a dark green mineral known as pyroxene. The lake basin provides an important habitat for woodland caribou.

Abstract mafic rocks at Lake Nipigon give evidence of rift-related continental basaltic magmatism during the Midcontinent Rift System event, estimated at 1,109 million years ago. Great sills up to 150 to 200 metres (490–660 ft) thick are also related with the rifting event, forming cliffs hundreds of meters high. The mafic and ultramafic intrusions centered on Lake Nipigon represent a failed arm of the main rift called the Nipigon Embayment.