January 26. The Mid-Continent Rift

By Richard I. Gibson

Here is the podcast:

Green is the area where there is basalt in the subsurface, more or less.

Transcript:
I think most Americans have a sense of what Iowa is like today – relatively flat, low hills, scenic river valleys, thick fertile soils supporting lots of farmland. A billion years ago it was quite a different place. More like East Africa, minus the plants and animals. A billion years ago, North America was trying to split apart, just as East Africa is today.

The split, called the mid-continent rift system, extends from central Oklahoma through eastern Kansas, then diagonally across Iowa, along the Wisconsin-Minnesota border and into Lake Superior. It swings around to head south beneath the lower peninsula of Michigan before it ends around Detroit.

Visualize fissures spewing basalt magma along most of that zone for 15 to 20 million years. Like Iceland’s volcanoes, filling a trough as much as 40 miles wide and more than a thousand miles long with basalt lava flows. The piles of lava flows added up to 2 to 10 miles of basalt and other volcanic rocks.

This rift was a zone like the mid-Atlantic ridge where heat moving up from the earth’s mantle splits the crust. The best modern analog for the mid continent rift system is East Africa, the Red Sea, and the Gulf of Aden – three branches that form what’s called a triple junction. The third arm of the system in North America headed north from Lake Superior beneath lake Nipigon in Ontario.

Magnetic map of Iowa

The mid-continent rift system failed, for reasons that are not completely clear, but we’ll talk about one possible factor in a couple days. So it never reached the stage of true ocean crust, like the Red Sea, but was always like the complex system of troughs, mountain chains, and volcanoes that mark the East African rift today.

Gravity map of Iowa

How do we know it’s down there? The best evidence comes from remote sensing – measurements of the earth’s gravity and magnetic fields. Those basalts in the trough are very dense and very magnetic compared to other rocks, so they produce a dramatic signature in gravity and magnetic maps.

The maps of Iowa show a long curving band of gravity and magnetic highs extending from near the southwest corner of the state to the northern boundary with Minnesota. And further north, along the continuation of the zone, the rocks actually crop out on the surface around Duluth Minnesota, and on Lake Superior’s Isle Royale and the Keweenaw Peninsula of Michigan.

This all happened over a geologically short period, 20 million years or so, about 1.1 billion years ago. The consequences are still evident today, in the presence of Lake Superior in the basin that sits on the rift, and in the mineral resources created by the volcanism. We’ll talk about that tomorrow.

There are a lot of good online resources about the mid-continent rift, so be sure to check the links below for more information. 

http://en.wikipedia.org/wiki/Midcontinent_Rift_System
http://crustal.usgs.gov/projects/midcontinent-rift-minerals/index.html
http://www.nature.com/news/geology-north-america-s-broken-heart-1.14281
http://www.gravmag.com/gmprimer.shtml
http://www.igsb.uiowa.edu/browse/rift/mrs.htm

All maps from USGS.