The scenic North Shore of Lake Superior is home to evidence of a once active geological past. Due to a magma plume in the mantle, a rift developed and threatened the cohesiveness of Proterozoic North America. The rifting process would eventually fail, leaving Laurentia whole. The rift would be appropriately named the Midcontinent Rift or the Keweenawan Rift after the Keweenawa Pennisula in Michigan. Besides the volcanic rock along North Shore, the Midcontinent Rift would leave little surficial evidence. It was not until the mid-1900s when magnetic gravity mapping would show the scar of ancient rifting. Since the mid-1900s, scientists have been working to unlock the secrets of the failed rift.
Some of the clearest evidence of early continental rifting was discovered from magnetic gravity anomaly profiling of North America in the mid-1900s. This profile showed a strongly positive magnetic anomaly located in the upper Midwest (Ojakangas, 1982). The igneous rock of the Midcontinent Rift contains a large amount of iron creating the positive magnetic anomaly throughout the Midwest.
The Midcontinent Rift can be thought of as having two well developed arms. The western arm extends northeast from east-central Kansas through the southeast tip of Nebraska, central Iowa, and eastern Minnesota to Lake Superior. The eastern arm extends southeast from Lake Superior through Michigan and into parts of Ohio. The eastern arm of the Midcontinent Rift crops out due to uplift and orogenies related to the Grenville province. Evidence for a third arm to the north of Lake Superior is not nearly as defined as the two arms to the south (Van Schmus & Hinze, 1985).
This narrow band of igneous rock extending through the Midwest was not interpreted for many years due to the fact that most of the rock is buried and hidden by younger sedimentary rock (Van Schmus & Hinze, 1985). The Lake Superior region is one of the few locations where igneous rock outcrops from the Midcontinent Rift are exposed at the surface and can be easily studied.
The Midcontinent Rift occurred between two minor orogenies, both relating to the Grenville province in the east. The Elzevirian orogeny occurred between 1240 and 1160 Ma and the Ottawan orogeny occurred between 1090 and 1025 Ma (Miller, 2007). These two orogenies gave rise to the Grenville Mountains being formed from compression on the eastern coast of Laurentia. Between these two orogenies, continental extension on Laurentia was thought to occur to due to upwelling of a magmatic plume near present day Lake Superior. The plume, along with its diverging convective currents, would force the land apart causing the rift. There is evidence of four stages of magmatic activity along the Lake Superior shoreline (Miller, 2007):
Questions remain as to the southern extent of the rifting in both arms of the Midcontinent Rift. The western arm could extend as far south as Oklahoma, while the eastern arm could extend through Tennessee (Van Schmus & Hinze, 1985). Evidence of this is not well known due to the fact that the rifting is buried by younger sedimentary rock and data comes exclusively from the magnetic anomaly maps and sparse deep drill-hole data.
The Midcontinent Rift has its own unique characteristics. The rift itself has similarities to the present-day rifting of Iceland and the Great Rift Valley of Africa. Differences remain between the outcome, or failure, of the Midcontinent Rift and modern locations of rifting today. The failure of the Midcontinent Rift was caused by the compressional pushing in the direction of the northwest (Miller, 2007). The Grenville province in the southeast was likely responsible for compressional pushing on Laurentia and the associated reverse faulting. This compression would leave the western arm of the Midcontinent Rift much narrower than the eastern arm (Miller, 2007).
The Midcontinent Rift left behind various igneous rocks as evidence of crustal extension and the subsequent fissure eruptions. Two sets of rocks that are well studied in the Lake Superior region are the North Shore Volcanic Group and the Duluth Complex. Both the North Shore Volcanic Group and the Duluth Complex tend to be mafic in nature, but they differ in texture. Several different formations of sedimentary rock throughout the Midwest are associated with covering the rift basin.
The composition of the North Shore Volcanic Group is mostly basaltic. There are a few lava flows that have an intermediate or even rhyolitic composition within the North Shore Volcanic Group. The fissure eruptions likely originated in what is now the Lake Superior basin (Ojakangas, 2009). The viscosity of the lava was very low and created many flat lava lakes and ponds when the lava spread out in the low areas. Along the North Shore of Lake Superior, there are hundreds of individual lava flows that vary in size and thickness. These flows are not necessarily stacked on top of each other but vary in locations all along the North Shore.
The tops of the North Shore Volcanic Group lava flows can be determined by observing air pockets in the individual lava flow tops. These vesicles in tops of the North Shore Volcanic Group lava flows often contain amygdules. The well known Lake Superior agates developed in these amygdules. Native copper is also known to develop in some amygdules, especially in the Upper Peninsula of Michigan (Ojakangas & Mastch, 1982).
Covering much of the northeast corner of Minnesota, the Duluth Complex crystallized under some of the aforementioned North Shore Volcanic Group. The course grained mafic rock was thought to be one large gabbro intrusion, but upon further study was determined to be a variety of intrusive igneous rocks from a number of intrusions. Despite being found below the North Shore Volcanic Group, the Duluth Complex is younger than some of the lava flows in the North Shore Volcanic Group. Evidence of this age relationship can be seen where the Duluth Complex intrudes the North Shore Volcanic Group lava flows (Ojakangas, 2009).
Lying under present day Lake Superior is over 18,000 meters of lava flows (Ojakangas, 2009). The weight of these high-density lava flows has created a syncline that tilts toward the center of the lake. This syncline can be viewed in many of the gradual tilting lava flows and rock formations along both the north and south shores of Lake Superior.
Running water over the course of millions of years would allow the rift basin to be filled in with sedimentary rock. Three different sedimentary formations are found covering Minnesota’s Midcontinent Rift south of Lake Superior. The oldest formation is the Solar Church Formation which was found only by drilling deep holes during natural gas exploration (Ojakangas & Mastch, 1982). The two other sedimentary rock formations are much better known. The Fond du Lac Formation and the Hinckley Sandstone sit atop the Solar Church Formation. These two layers are fluvial in nature and said to be the youngest Precambrian rocks in Minnesota (Ojakangas, 2009).
The drive along the North Shore in northeast Minnesota allows one to travel up and down the scars of a once violent past. Lava flows are stacked on top of each other as evidence of Laurentia pulling apart. With the exception of the Lake Superior region, evidence of the Midcontinent Rift remains hidden from scientist’s vision. While there are several examples of continental rifting occurring today, these examples do not completely model the complexities of this ancient rift. Many scientists today are working hard to unlock the secrets of the failed Midcontinent Rift.