Northern Great Lakes: Mid Continental Rift
by David Beachler

Glacial History
Porcupine Mountains
Geology of Porcupines


The basins that contain the Great Lakes are primarily a result of glaciation from the Pleistocene period. Geologically speaking, the Great Lakes life-span will be short lived. Similar to all lakes, over a period of time vegetation will flourish within and around the basin. Sediment from erosion will pollute the waters, and incisions will drain the basins in a matter of a few hundred thousand years.

Through studying plate tectonics, it has been argued by some that a mountain range larger than the Himalayas existed across Ontario, Quebec and into Labrador; with a massive rift, 30 km deep, existed where Lake Superior is presently. It is the mid-continental rift that this presentation will focus on.




The climate along the Northern Great Lakes reigon is one of long, cold winters; with a growing season lasting less than 110 days. The Upper Peninsula of Michigan is one of the cloudiest regions of the United States. Nearly three-quarters of each year are cloudy, and one third of the annual precipitation falls in the form of frozen precipitation. The annual snowfall totals range from 130 inches, to greater than 200 inches.

(Photo Courtesy of Webshots of Michigans Upper Peninsula, Winter 2006)

Glacial History:

The latest ice age, from the Pleistocene epoch, ice sheets from multiple North American glacial stages moved across the now known Great Lakes. The final stage of the ice age period, Great Lakean glaciation, completed the advancement to just southeast of the Green Bay WI area. Melting of glacial ice within the Superior Basin resulted in large rivers, which deposited crumbled rock along the southern edge of the Superior Basin. It is believed that the Grand Sable Banks were created at this time. The earliest geologic time, for the Great Lakes region began nearly 4.6 billion years ago (bya). At present time, the southern section of the Superior Province in Minnesota developed shortly after the consolidation of the planet (4.2 to 2.5 bya). The development stage of the Superior Province took place during the Arvhean eon.  

The Great Lakes region contains greenstone-granite and gneiss terranes. Several belts make up this province, with the furthest north called the Wabigoon subprovince. This is a volcano-plutonic belt, comprising much of northwestern Minnesota. The southern portion of the Canadian Shield underwent a  rift, 1.1 (bya), known as the Keweenawan Rift. The region opened up and allowed nearly 15 km of lava to flow towards the surface. As the lava solidified it became basalt. 

The image to the right provides a more detailed timeline of the geological elements founds within the Great Lakes region. From the Great Lakes region to the southern edge of Greenland, it is believed that rift-related magnetism began some 1.3 (bya). This period of magnetism continued for over 200 million years, allowing lava from the mantle to flood the midcontinental rift region.

Porcupine Mountains

(Map Courtesy of Michigan Department of Natural Resources)

    The foundation of the Porcupine Mountains lies along the inflection point of the mid-continental rift, along Michigan's Upper Peninsula. Much of the rock found here belong to the Keweenawan Supergroup. Thickness exceeds 25km dating approximately 1.1 bya. The mid-continental rift began during the infant stages of the North American Continent. Beyond the Lake Superior basin, the rift is covered byFreda Sandstone.

    The oldest rocks, of the Porcupine Mountains, are the andesite and rhyolite flows. They are known as Porcupine Volcanics, and reside atop older continental flood basalts. The volcanics were deposited by a large stratovolcano, which is unique to rift systems. Along the exterior, outer apex of the rift system, lies Copper Harbor Congomerate. This is sediment that has been deposited by rivers, and is the marker for the end of the rift extension.

    Basalt flows known as Lakeshore Traps, create the border for the extended basalt-capped escarpment that overlook the Lake of the Clouds.
Copper Harbor, along the Keweenaw Peninsula, has Nonesuch Shale sediment. This shale has a black to gray appearnace, that are found within a shallow body of water near the post-rift basin or valley.

    The Keweenawan sediment contain large deposits of copper. Through secondary mineralization of copper, epidote, quartz, prehnite, and pumpellyite fill the voids and fissures. The copper deposits have been beneficial during the industrial revolution, as extraction began just prior in the mid 1800's. This was close to the United States first mining rush.
    During the recent ice age, Pleistocene, much of the Porcupine Mountains were developed. Some relics include Lake Superior, erosion of bedrock from the mountains, and ground moraine deposits are indicators of the last glacial period in this region. The

(Photo Courtesy of R.Weller/Cochise College)


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