New to the GIS map: Aeromagnetic & Gravity Anomaly map layers highlighting the Midcontinent Rift (among other things).

Recently, two layers were added to the Wisco Geology GIS map:

•  Aeromagnetic: This map layer is a composite made from aeromagnetic surveys in Wisconsin.  Maps like this are used for mineral exploration.  Source: USGS and others.

• Bouguer Gravity Anomaly: This map layer shows where variations in gravity occur due to changes in rock type, density or thickness.  Source: USGS and others.

The most striking feature of both layers--but especially the gravity anomaly map layer--is the swath of red color in western Wisconsin extending into Minnesota and Iowa. This area was flooded by huge amounts of lava about a billion years ago as the continent started to split and pull apart. Today, this is called the "Midcontinent Rift". The lava cooled to form basalt within the rift zone.  The basalt here could be many miles thick and it is generally more dense than the surrounding rock types. For these reasons, the gravity anomaly map layer shows the rift as an intense "gravity high" zone.

Aeromagnetic and gravity surveys became more extensive in the 1940s as remote sensing technologies were developed. Before this time, the existence of the Midcontinent Rift was not known. Geologists were aware of igneous rock outcrops including basalt at places like Big Manitou Falls and at the Dalles of the St. Croix River / Interstate Park, but the full extent and cause of these igneous rock formations were not understood until the 1960s when plate tectonics became more well understood and geologic mapping became more complete across the globe.

The Midcontinent Rift system shut down before the continent was torn completely apart.  If rifting had continued here, the geology of this area might have resembled the Great Rift Valley of Africa and would have eventually filled with sea water to form something like the Red Sea. You can see that Lake Superior sits within the rift zone. Most people believe the Great Lakes were formed by glaciers, and is largely true, but the underlying rift influenced the formation of Lake Superior. You can read more about it in this article by geologist Dr. John C. Green of the University of Minnesota-Duluth.

Click here to check out the Wisconsin Geology GIS map.  Once in, turn on the Aeromagnetic and Gravity Anomaly layers (these are both near the bottom of the layer list to the right of the map).

Note:  I georeferenced existing maps to add these to the Wisco geology app. If you study geology, you have probably seen these maps before.  What's interesting about viewing these in the Wisconsin Geology GIS app is that as overlays, you can view these in context with the other map layers such as outcrops.