Lake Superior and Keweenawan Rift

Lake Superior and Keweenawan Rift

Kim Woodbury
Spring 2008
ES 767 World Tectonics Project

HistoryRifting Glaciations Climate Lake Superior Today ReferencesES 767


The Lake Superior Region is a tectonically complex area. The oldest rocks of the region are an ancient craton that grew by accreting terranes on its margins. A hot spot underneath the landmass fueled a rift zone which almost succeeded in tearing the North American continent apart. The rifting started about one billion years ago and continued for 20 million years. It carved out the basin which became Lake Superior and erupted tons of basalt onto the rift zone, which extends to present day Kansas. If the rift would have proceeded, Duluth could now be located on the shore of an ocean instead of a lake. The rift, however, did stop. Glaciers encroached and retreated upon the land several times in the last two billion years. These glaciers had a profound effect in shaping the land and developing the features we see today. The glaciers shaped Lake Superior into its current form. The lake is currently the largest fresh water body in the world by surface area.

Great Lakes Region Regions

Taken from GLIN web site:


The oldest rocks in the Superior Region are from 3.5 to 2.6 billion years old. These rocks accreted from terranes in the Archean eon. They primarily consist of greenstone, granite and gneiss. These formations acted as a base for further accretion of continental material (Davis, 1). From 2.7 to 2.5 billion years ago the Superior Region collided with a large gneissic micro-continent. This is know as the Algoman Orogeny. The Algoman Orogeny added landmass along a 1,200 kilometer boundary that stretches from present day South Dakota into the Lake Huron region. This boundary is known as the Great Lakes Tectonic Zone or GLTZ (Davis,1). The GLTZ is still slightly active today, and has produced five out of seven of the state's last earthquakes (, 1). Beginning at 2.5 billion years ago, this province began to rift. The area continued to rift until 1.9 billion years ago (Davis, 1).

During this time, the atmosphere was changing and oxygen levels were increasing. These high levels are thought to be produced from an exploding blue-green algae population. Damian Nance tells us that during times of increased erosion, (times following continental building, in this case the Algoman Orgeny) high levels of nutrients are produced and used by sea life. Therefore, organic productivity should be high during these periods. Interestingly, this is also the time that blue-green algae developed heterocyst, or the ability to fix nitrogen in the presence of oxygen. This made it possible for the organisms to survive in an oxygen rich environment that otherwise would have been poisonous to them. These organisms are therefore the predecessors of present day photosynthesis plants (Nance, 79). This oxygen rich atmosphere eventually produced the Banded Iron formations, including the Mesabi, Gunflint, and Cuyuna iron ranges (, 1). These formations have alternating layers of iron and chert. The area began to subside and the sea encroached upon the land. The area then began to fill with oceanic sediment (Davis, 2).

Beginning 1.85 billion years ago, new terranes were accreted to the Superior Province in manner that closely resembles the modern Wilson Cycle. Davis suggested this was because plate tectonics had evolved during this time to operate in a fashion similar to modern plate tectonics. The collision of two island arcs with the Superior Provinces produced the Penokean orogeny. The first island arc consisted mainly of volcanic and plutonic rock. The arc was accreted along the Niagra fault that runs through western Minnesota, Wisconsin, and into central upper Michigan (Davis, 2). The second collision involved a mature microcontinent that consisted of gneisses and plutonic rock. The microcontinent is thought to extend as far as Illinois, but is the subject of some debate because the suture zone has little exposure. From 1.8 to 1.3 billion years ago, volcanic and plutonic activity dominated the region (Davis, 2). As this activity ceased the Penokean mountains began to be eroded down, producing quartz sand grains which produced quartz sandstone which later metamorphosed into quartzite, and can still be seen today as the Sioux quartzite in Minnesota and South Dakota, as well as many formations in Wisconsin, 1).

Several plutons were emplaced 1.5 to 1.4 billion years ago. These plutons make up the Wolf River Batholith. This formation is comprised of five major and four minor petrochemically distinct plutons (Davis, 1). These plutons were emplaced during a time of tectonic quiescence, a time that lacked a heat source for generating the magma, and are said to be anorogenic. The source of the melt is thought to be a large portion of magma which was trapped 27 to 36 kilometers below the surface of the Earth. This magma generated the melts that rose to the surface (Davis, 1).


The Great Lakes began to rift about 1.1 billion years ago. Continents may rift due to a build up of heat below them, generated in the mantle by radioactive decay. Continental crust is only half as efficient at conducting heat as oceanic crust (Nance, 1). This may cause the continental crust to dome up and break apart, much like a pie cracks open in the middle. The heat can then dissipate through the oceanic crust that forms from a continental rift. Hotspots can cause uplift, helping the rift to spread by allowing the crust to slip apart (White, 9). A hotspot is thought to have formed below the Lake Superior craton, creating a dome under the entire region. Evidence for doming includes a lack of detritus from far off regions. This may be because the localized areas of subsidence were domed because of high heat flow (Hollings, 407).

Continental Rift

Taken from Nick Strobel's web site

The Great Lakes rift, or Keweenawan rift, first broke open along a 2,000 kilometer arcuate, or bow shaped, rift (Miller, 1). The two arms of the rift extended into present day Minnesota, and Wisconsin. A third minor limb shoots off to the north, at the top of present day Lake Superior. This limb creates the 120 degree formation, or triple junction, that is characteristic of a continental rift (Davis, 1). The rift opened at a high spreading rate. The basalts derived from the mantle plume show little interaction with the country rock. This is associated with a fast spreading rift, that allows the magma to ascend with little resistance from the surrounding rock (Hollings, 407). As these lavas were extruded onto the surface, they cooled to form the igneous rock that can still be seen today as the North Shore Volcanic Group. Some magma cooled in large overlapping pools to become the dark colored basalt that is seen on the North shore of Lake Superior (,1). Some magma slowly solidified in magma chambers to form the course grained igneous intrusions that can still be seen in the Duluth Complex (Miller, 1).

Basalt rocks on the North Shore of Lake Superior near Grand Marais

Taken by Kim Woodbury, June 2006

Rifting and volcanic activity continued for about 20 million years and then abruptly stopped (Miller, 1). The reason for the cessation in rifting is poorly understood. As the region's volcanic activity ceased, the area cooled and subsided (Miller, 1). The area then began to fill with sediments. Flowing water deposited large amount of sediments from the North Shore to the Twin Cities area, along the rift zone. These sediments were then cemented together to form sandstone and can still be seen throughout Minnesota today (,1).

Back To Top

Glaciation of the Superior Region

The glaciers that formed after the cessation of the volcanic activity of the Keweenawan rift play a fundamental role in shaping the landscape that we see in the Lake Superior region today. Several ice sheets advanced and retreated in the past two million years. The Wisconsin Glaciation was the most recent of these glaciations. The sheet was centered in present day Hudson Bay and extended and retreated from there. Several lobes developed including the Wadena, Rainy, Superior, and Des Moines (,1). The Wadena lobe deposited the Alexandria moraine. The Superior and Rainy lobes, which advanced from the Northeast are responsible for the basalts and gabbros deposited as far south as the Twin Cities. The Des Moines lobe deposited limestone, granite, and shale, and advanced from the Northwest (,1).

Glaciation of North America

Glaciation; Minnesota River Basin Data Center

The Lake Superior basin was formed by rifting but it was sculpted by glaciers. The latest glacier to retreat did so about 10,000 years ago. As the ice retreated, it blocked the eastern drainage, effectively damming the lake and creating glacial Lake Duluth (, 1). The wave cut cliffs above Duluth skyline drive are 183 meters (700 feet) above the current level of the lake, and mark the highest level of Glacial Lake Duluth (, 1).

Glaciers are also responsible for many of the formations we see today. One of the most significant of these formations in state of 10,000 lakes is, of course, the kettle lakes. These lakes form when large blocks of ice dislodge from the main body of ice and are covered over with sediment. When the ice block melts, it leaves behind a kettle shaped depression which can then fill with water, creating a lake. The majority of Minnesota's 10,000 lakes formed in this manner. On the margins of a glacier, wet sediment can collect, which can then slump and form uneven terrain. This formation can become a chain of lakes (, 1). As water melts it can create tunnels beneath a glacier. These carry debris and sediment which can accumulate in the tunnel. After the glacier melts these tunnels of sediment are left behind and are called eskers (, 1). Striations are formed when a glacier accumulates rock beneath it. As the glacier moves is scores the land. The marks are called striations. An end moraine is an accumulation of debris that is pushed up at the toe of a glacier. Erratics are large boulders that are carried far form their source by glacial movement. These typical features of the Lake Superior region are a legacy of its glacial past.

Back To Top


The average temperature for Lake Superior is 4 degrees Celsuis. The lake moderates the temperature, making the winters warmer and the summers cooler. The lake also creates 52 days of fog per year. This occurs when warm air condenses above the cooler lake (,2). The average maximum temperature for January is -8.7 degrees Celsius, and the average maximum temperature for July is 25 degrees Celsuis. The average minimum temperatures for January and July respectively are -19 degrees Celsius, and 12 degrees Celsuis. The area averages 195 centimeters of snowfall and 76 centimeters of rainfall per year (, 2). The lake is 40 to 95 percent covered in ice percent during the winter months. It can freeze completely over, but strong winds typically push the ice apart in the center of the lake (, 2).

Back To Top

Lake Superior Today

Lake Superior is the world's large fresh water lake by surface area, and the third largest by volume (, 2). The lake is 82,100 square kilometers, and its shore line is close to 3,000 kilometers long (, 1). Much of the lake is 200 meters deep or deeper. The deepest part of the lake is 406 meters deep, which is deep enough to fit Chigargo's Sears Tower with only a few floors emerging above the surface of the lake (,1). The lake is a big tourist draw for the area. Each year, 3.5 million people visit the area. Duluth supports one of the nationís busiest inland ports with over 1,000 vessels docking annually (,3).

Lake Superior is the clearest and coldest off all the Great Lakes. It is low in suspended sediment as well as nutrients. Subsequently the lake has a low production of aquatic life. Superior only produces ten percent of the fish that Lake Michigan, which is high in nutrients, produces. Scuba divers, however, flock to dive the lakeís 350 ship wrecks (,1.)

Frozen tribuary of Lake Superior Waves on the shore of Lake Superior

Taken by Kim Woodbury Janurary 2008 Taken by Kim Woodbury Janurary 2008


  • Nance, D., Worsley, T. R., Moody, J. R. (1988). The Super Continent Cycle, Scientific American, 259/1, 72-79
  • Davis, P. The Big Picture, Retrieved April 15, 2008, from
  • Strobel, N. When Plates Separate (June, 1 2007). Planet Interiors. Retrieved April 17, 2008 from
  • Miller, J. Lake Superior Duluth Retrieved on March 28, 2008 from
  • White, R.S., McKenzie, D.P., (1989) Volcanism at Rifts. Scientific American, 261/1, 62-71
  • Tarduno, J.A. (2008) Hot Spots. Scientific American, 298/1, 88-93
  • Hollings P., Fralick, P., Cousens, B. (2007) Early History of the Midcontinent Rift inferred from geochemistry and sedimentology of the Mesoproterozoic Osler Group, Northwestern Ontario. Retrieved April 17, 2008 from NRC Research Press Web site,
  • Minnesota Sea Grant. (2007) Retrieved on April 9th, 2008 from
  • Mezzina and Associates. Retrieved April 16th 2008 from
  • GLIN (2006). Great Lakes. Retrieved April 20, 2008 from
  • Glaciation; Minnesota River Basin Data Center (2004). Glaciation Retrieved April 25, 2008 from

    Back To Top