What can two ancient rifts in the American heartland tell us about our current divides?

Rifts are important to all epic stories. The story of the North American continent and its predecessors is no exception.

Episodically, forces from Earth’s interior started to stretch the crust apart, threatening to split the continents and open up a new ocean basins. Our first example, the Midcontinent Rift, started 1.1 billion years ago and eventually spanned an arc from modern-day Nebraska north to Minnesota, and back down to Michigan.

The map shows zones of low, medium, and high gravitational field strength across the central and eastern US. A distinct band of high gravitational field strength follows the Midcontinent Rift.
The Midcontinent Rift appears as an arc of high gravitational field strength from Kansas north to Minnesota and south again through Michigan (Map source: G. Randy Keller, U. Oklahoma).

Our second example, the Reelfoot Rift, began around 600 million years ago. It includes portions of modern Arkansas, Tennessee, Missouri, and Kentucky. “Successful” rifts eventually open up ocean basins, but neither the Mid-Continent Rift nor the Reelfoot Rift made it that far. Geologists are biased for action, so these “failed” rifts get a special name: aulacogens.

So what can we learn from these buried schisms from hundreds of millions of years ago?

The forces of separation may be inevitable, driven from deep within. Yet what fills the space of our separation has lasting consequences. In the case of the Midcontinent Rift, tectonic forces threatened to break the continent apart — but lavas helped to fill the gap. These lavas persist today, 1.1 billion years later, along the shores of Lake Superior. The waters circulating through the volcanic rock concentrated copper ore that has supported trade for thousands of years. Whereas weaker sedimentary rocks were scoured away, the stronger volcanic rocks stood up to attack from ice sheets. So despite being surrounded by one of the flattest regions in the US, those ancient rocks support  towering cliffs along Lake Superior. That lake, cradled by low land above the rift, stores a whopping ten percent of the global of fresh surface water supply.  

 

Cliffs on the shores of Lake Superior from the Minnesota shore.
Cliffs of volcanic rock along the shore of Lake Superior in Minnesota (Wikipedia).

On the other hand, the legacy of the Reelfoot Rift suggests a more ominous set of consequences for deep divisions. Take a look at this forecast of earthquake damage risk for 2017, produced by the US Geological Survey:

The map shows relatively higher risks of damaging earthquakes for broad swathes of California, but also Oklahoma and a smaller area including western Tennessee.
Map showing the forecast for a damaging earthquake for the continental United States in 2017 (US Geological Survey).

As you might expect, California has some of the most widespread elevated risk of a damaging earthquake, given its tangle of active faults. The elevated risks in Oklahoma and Kansas are a recent addition to this map due to induced earthquakes from hydraulic fracturing (aka “fracking”). But you’ll find another zone of surprisingly high earthquake risk in the east-central US, right in the neighborhood of the crust weakened by the Reelfoot Rift.

Some of the largest earthquakes ever to strike the continental US occurred in this same region. From December 1811 to February 1812, three enormous earthquakes struck near New Madrid, Missouri. Shifting ground beneath the Mississippi River threw it into instant turmoil, even causing the river to temporarily flow backwards by some accounts. Spouts of liquefied sand burst forth from the ground. Look around in Google Earth, and you might see the relict sand deposits lying on the surface.

The image shows a historic aerial photograph from Arkansas, US. The image shows farm fields on top of sedimentary deposits from the Mississippi River. The younger sand deposits triggered by earthquakes are brighter than their surroundings.
Deposits left over from eruptions of liquified sand are bright in this 1964 air photo from Arkansas . The sands sit atop older “point bar deposits” laid down by the Mississippi River  (US Geological Survey).

The region was sparsely populated at the time of the earthquakes, but now hosts 11 to 12 million people in the vicinity of Memphis and St. Louis. So could a similar earthquake sequence strike again? The available geologic evidence, largely from dating sand eruptions, indicates that similar events occurred in the past, at intervals separated by 200 to 800 years.

Just as cultural observers are digging to unearth the fault lines hidden beneath the surface of American society, geologists are searching for new clues about the rifts in the literal bedrock of America. Many citizens are disoriented by the suddenly uncertain contours of our society.

If there’s one way that geology can guide us, it’s this: the consequences of division can last long a lot longer than you might think. What will fill our political gaps: a society strengthened by renewed civic engagement, or one weakened by fake news? Either way, Earth history has its eyes on you.

Reading a valley, Part 2: How can you retrace a river’s footsteps?

For geoscientists, lacking direct experience is a common quandary (and it should be noted, a happy one!) One great workaround: the thought experiment. Back in 1909, the eminent geographer William Morris Davis used his powers of imagination to visualize how the Connecticut River might have carved its valley and left terraces. Here’s a beautiful sketch […]