Geologists puzzle over a billion-year-old, once lava-filled crack in the Midwest
About five years ago, Doug Wiens, a seismologist from Washington University in St. Louis, went knocking on the doors of farmers in Minnesota and Wisconsin. He asked them if they would let him and his team of geologists place seismometers, devices that measure the earth’s movements, on their land.
The farmers were a little suspicious.
“‘Why are you doing this here? Are you exploring for oil? Nobody’s done anything like this before,’” Wiens recalled them asking. “And so, we would show them this gravity map and we’d say, ‘Well, there’s this really unusual feature here and we want to study that.’”
The farms happened to be sitting on top of a 2,000-mile-long geologic mystery called the Midcontinent Rift. It was formed when the continent began to pull apart, 1.1 billion years ago.
“If it continued to split open, we would’ve had an ocean between the eastern states of the U.S. and the western states,” Wiens said.
But at the very last moment, the spreading of the continental crust stopped before it was wide enough to create an ocean.
On a gravity map, the Midcontinent Rift looks like a dripping bell curve, with the west “arm” starting in Oklahoma, before heading north through Kansas, Iowa and Minnesota. The east arm slopes down through Michigan, Kentucky and Alabama. In recent years, scientists, like Doug Wiens, have used seismic data gathered through the National Science Foundation's EarthScope program to put together a three-dimensional image of the Midcontinent Rift.
That would allow geologists to understand what forces were behind it and gain insight into what the Earth looked like a billion years ago. They argue that studying the ancient formation is relevant to understanding some of the phenomena that’s happening on Earth today.
The splitting of North America most likely stalled because of another rift that occurred. Laurentia, or present-day North America, and Amazonia, present-day South America, were once a part of a supercontinent called Rodinia. As the Midcontinent Rift was occurring, Amazonia broke off of Rodinia and started a new rift.
“When Amazonia broke away, the tensional stresses that were acting across [Laurentia] now had a new place to be concentrated,” said Michael Wysession, a professor of geophysics at Washington University. “For whatever reason we don’t yet understand, rifting was more favorable and easier to occur than the Midcontinent Rift. But if there’s any rule in the universe, and I’m speaking really generally here, it’s that things occur the easiest way they can.”
The Midcontinent Rift is often labeled as a “failed rift.” While other failed rifts exist on the planet, none is as large it is or accessible enough to conduct research.
“It’s probably the best place in the world to study this kind of phenomenon,” Wiens said.
Some geologists have also observed a pattern in Earth’s history, in which a rift will fail while two simultaneously occurring rifts will succeed.
“When you get long rifts like the Midcontinent Rift, they’re usually a failed third long arm associated with a successful seafloor spreading system,” said Carol Stein, a geology professor at the University of Illinois in Chicago.
While there’s no clear explanation for why one out of the three tend to fail, the two rifts involved in separating Laurentia and Amazonia did proceed to create an ocean.
Scientists have also puzzled over another feature that defines the rift: its enormous deposits of basalt and other igneous rocks. That suggests an unusually high amount of volcanic activity once filled the area.
“A lot of people have tried to figure this out,” Wysession said. “Given the magnitude of the lava, on the order of two million cubic kilometers, we don’t have any other explanation than there being a hotspot there.”
A hotspot would be similar to what’s responsible for the volcanism under Hawaii and Yellowstone. At the time, the North American continent was located on another place on Earth that could have had hotspots.
As time passed, the volcanism stopped and the magma cooled, creating the large formations of volcanic rocks that can be seen from the shore of Lake Superior. The hot fluids from the event also formed tremendous copper reserves that led to a mining boom in the 1840s.
Today, there is a large active rift in eastern Africa that geologists say is very similar to the Midcontinent Rift. The East African Rift is home to many active and dormant volcanoes, including Mount Kilimanjaro. It is also a part of a three-pronged rift system, and some scientists think that the East African Rift will likely fail.
“If we came back a billion years from now, the East African Rift would be this failed rift in the middle of nowhere,” Stein said.
It also could help geologists imagine what the Midcontinent Rift might have looked like more than a billion years ago.
“We say in geology the present is the key to the past,” Stein said. “If we look at what’s happening today, we can better understand how things formed in the past, but if we look at how things formed in the past, we have more insight as to how things formed today.”
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