Rock on: Iowa’s Geologic Record

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From glaciers to inland seas, from Midcontinent Rift to a cataclysmic meteorite strike, the stuff that Iowa is made of is more than meets the eye. While the state doesn’t have Rocky Mountains like Colorado, or rock formations like Utah, or the Grand Canyon like Arizona, Iowa nevertheless has a fascinating and dynamic geologic past.

Going deep: Iowa’s earliest beginnings
All the cool stuff that happened in Iowa’s geologic past happened in the Precambrian, according to Jane Pedrick Dawson (’83 geology, MS ’86), ISU senior lecturer in geological and atmospheric sciences. But then, Dawson admits, her research specialization is the Precambrian. She gets really excited talking about basement rock and rift systems and gravitational pull.

“It’s really hard to study the Precambrian in Iowa because it’s just not exposed,” she says. “So our knowledge about it is pretty fuzzy. You can’t get to the ‘basement.’ It’s there, but we don’t have access to it.”

What geologists here study, she explains in lay terms, are the “first and second floors of the house.”

“The Precambrian basement here is billions of years old. Iowa is on a very stable area of continental crust.”

Actually, she says, maybe not ALL the exciting stuff happened in the Precambrian (which ended more than 500 million years ago), but much of it did.

“One really interesting exciting thing that happened in Iowa toward the end of the Precambrian – about 1.1 billion years ago – there was a massive continental rifting event. It went from Kansas up through Lake Superior, called the Midcontinent Rift. This was a very fast event with a massive, massive outpouring of lava and associated volcanic activity. We’re sitting on the shoulder of the rift here in Ames.

“Later compressional forces squeezed the rift valley in Iowa up into a mountain range, and sediment shed off into basins on the side,” she continues. “It got eroded and then buried under younger sediments after the Precambrian (the last 540-odd million years) – that’s about the last one-eighth of the Earth’s history. Precambrian is the first seven-eighths of Earth’s history. The development of multi-cellular life is in the last eighth.”

So, that’s the beginning – before Iowa was covered in a warm, shallow sea, before it was covered by glaciers, before prairies, before bison, before people. That’s how it all got started.

Touching the past
rockon2There’s something unique in the far northwest corner of Iowa: A piece of history more than a billion years old.

Sioux Quartzite is the oldest exposed rock in the state, and you can find it in Gitchie Manitou State Preserve, where the closest town is actually in South Dakota.

Jane Pedrick Dawson, ISU senior lecturer in geological and atmospheric sciences, is beside herself with happiness when she sees the wind-polished, pink-tinted rocks in their natural habitat for the first time.

“This is about 1.7 billion years old,” she says reverentially, touching the rock. “It’s from a unit of time in the Precambrian called Proterozoic; it’s middle Proterozoic in age. These quartzite bodies are showing us where the southern margin of North America used to be” before the continents shifted many millions of years ago.

Sioux Quartzite is resistant to weathering. Through millions of freeze-thaw cycles and a half-dozen continental glaciers, it has survived. Beginning in the late 1890s, for several years the rock was quarried here in Iowa’s northwest corner. It is currently quarried in South Dakota and Minnesota, and the crushed rock is used on roads and for railroad ballast.

“One of the things that just trips my trigger is when you think about the processes that went into making all this [Sioux Quartzite] and then it just SAT THERE for over a billion years and then we come along and find a use for it,” Dawson says. “When this gets put into asphalt, people are driving over pieces of what used to be the edge of our continent without even realizing it.”

Continent on the move: Plate tectonics
In high school Earth science, most of us learned that the top layer of planet Earth is a series of plates that are constantly shifting.

Jane Pedrick Dawson, ISU senior lecturer in geological and atmospheric sciences, describes this process a bit more colorfully: “All continents are put together like patchwork quilts,” she says. “You start with a few pieces and then you just keep adding more and more pieces around the outside, so all continents are amalgamations of pieces that may be locally derived or may have traveled a long way and then get smashed in and attached.’’

While Iowa’s landscape was transforming through the years, the North American continent itself was slowly making its way up from the south, explaining the fossil records of some tropical plants and animals in Iowa.

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Glaciers R Us
When you ask Iowa’s geologists, anthropologists, and historians what has most influenced the current Iowa landscape, they’ll all give you one word: glaciers.

“We have a beautiful glacial history in Iowa because of the way the glaciers have shaped our land masses,” Hannah Carroll, ISU PhD candidate in ecology/evolutionary biology and environmental science, says.

Glaciers are what made central Iowa’s landscape flat and fertile. They produced rivers and streams and “prairie potholes.” In the northwest, south, and northeast, glaciers created the Iowa Great Lakes and the rolling farmland made famous by artist Grant Wood.

The Upper Midwest probably experienced around 25 phases of glaciation in the last 2.5 or 3 million years. Scientists know there were multiple glaciations from the many layers of sediment (or till) laid down by the glaciers and from mud layers cored from the ocean bottom that changed their chemistry every time glaciers advanced.

Central Iowa was de-glaciated very recently relative to other parts of the state. The Des Moines Lobe – with its terminal moraine right around downtown Des Moines – retreated from Iowa only about 14,000 years ago.

“The interesting thing about the Iowa landscape is that most of Iowa – the southern part, the western part, and the eastern part – those parts of Iowa were last glaciated more than 300,000 to 400,000 years ago,” Neal Iverson (’83 geology), ISU professor of geological and atmospheric sciences, says. “So those glacial sediments have been subject to river erosion for all that time since then. That’s why the land outside the Des Moines Lobe is more rolling.”

Iverson is Iowa State’s top expert on glacial science. “What makes this [central] part of Iowa unusual,” he says, “is that this glacier stepped out of here not that long ago – about 14,000 years ago – probably not too long before the first Native Americans were in the area.”

He says that at the time the Des Moines Lobe came into Iowa, the climate was getting warmer. “There was actually a forest,” Iverson says. “It wasn’t tundra; there were trees like hemlocks, spruce, boreal forest sorts of trees, and they got pushed over by the glacier. If you’re lucky you can find these old logs buried at the bottom of the Des Moines Lobe sediments and you can date those old logs using radiocarbon. They tend to have ages of about 16,500 to 17,000 calendar years.”

The Iowa Great Lakes were formed by the last glacial advance. Other lakes – Clear Lake and Wall Lake, for example – are also natural lakes that wouldn’t exist if it weren’t for the last glacial advance. Outside the boundaries of the Des Moines Lobe there are virtually no natural lakes, only reservoirs.

Iverson thinks it’s important for people to understand glacial movement. “The last time ice came into Iowa, it was likely surging due to dynamic reasons rather than advancing for climatic reasons, and I think that’s pretty important,” he said. “I think people think of glaciers as being these passive blobs of ice that just respond in a very slow way to climate change. That’s just not true; sometimes glaciers advance very rapidly for dynamic reasons unrelated to climate change. The base of the glacier effectively gets very, very slippery and the ice slides out across the landscape very rapidly. People think the Des Moines Lobe advanced at a speed of something like 2,000 meters a year, which is quite fast for a glacier.”

Making an impact: the Manson Crater
rockon5Around 74 million years ago, something really shocking happened near what is now the town of Manson, in southeast Pocahontas County, Iowa. A large asteroid hit the Earth, causing debris to fly to areas as far away as Nebraska and Dakota.

Nobody was hurt, because nobody was there. This was in the Cretaceous period, and it would be millions of years before humans arrived on the scene. But the meteorite, thought to be about 1.2 miles in diameter, created an impact structure 24 miles in diameter and likely caused a tsunami in the Western Interior Seaway.

The site at the time was the shore of a shallow inland sea. No surface evidence exists today due to relatively recent coverage by glacial till, and the site where the crater lies buried is now a flat area – the crater is essentially invisible to the naked eye due to Iowa’s shifting landscape.

Researchers first became interested in the site in 1912 when well water proved to be unusually soft when compared to other Iowa water. The Manson crater was first recognized as an impact structure in 1966.

The Manson impact structure is the largest intact onshore meteorite crater in the continental U.S. The town of Manson holds an annual Greater Crater Days.

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Traveling Iowa’s geological wonders
Nothing to see here? You just have to know where to look! Take a trip across Iowa to check out these unique landforms. We’ve also included some detailed recommendations from many of our Iowa State experts.

  • Loess Hills in western Iowa (pictured, above)
  • Gitchie Manitou State Preserve, for Sioux Quartzite, in northwest Iowa
  • Fossil & Prairie Park in Floyd County near Rockford, for its Devonian-aged rocks and marine fossils that you can collect and take home
  • Devonian Fossil Gorge near Iowa City, for marine fossils
  • Ledges State Park and Dolliver Memorial State Park, for snapshots of the Pennsylvanian time period
  • Maquoketa Caves State Park in eastern Iowa, for Silurian-aged rocks and examples of karst topography
  • Freida Haffner Kettle Hole State Preserve in northwest Iowa’s Dickinson County, for an example of a glacially created kettle feature

The experts weigh in:

  • NEAL IVERSON: “I think the most interesting landscapes in Iowa are the Loess Hills; the Ames area because it was so recently glaciated; the northeastern part of Iowa, whose landscape is dominated by karst processes; and southern Iowa, which is this old landscape that was glaciated 300,000- 400,000 or more years ago, multiple times.”
  • HANNAH CARROLL: “What we have, I think, is a really interesting geological history because of all the different glaciations that have gone on and all these different landforms. You go over to the Driftless Area; you’ve got this Paleozoic plateau that hasn’t been glaciated for a long time, and you’ve got these beautiful limestone bluffs (northeastern Iowa). Loess Hills is interesting; you’ve got that really thick layer of very fine loess, and that sandy soil gives you a different kind of prairie than you would have in the Des Moines Lobe.”
  • JANE PEDRICK DAWSON: “Drive up Hwy 52 [in northeast Iowa]. Come in to Guttenberg from the south, go down a great big hill – the Mississippi River is on your right, and there’s a road cut on your left. This is considered to be one of the most complete exposed sections of the Galena Group Middle Ordovician-aged sedimentary rocks in Iowa. These rocks were deposited in shallow seas that once covered the state. We just don’t have too many places in Iowa where we get to see big exposures like that.”
  • JEROME THOMPSON: “People have a perception of Iowa being flat. Any person that’s ridden RAGBRAI will tell you it’s not. If you want to see Iowa, avoid I-35 and I-80. Traveling up Interstate 35, you see corn field and bean field and corn field and bean field…the area that was heavily glaciated and flattened. Now, if a person were to drive along Hwy. 18 in northern Iowa, they would get a sampling of the Loess Hills, and they would also get across part of the Des Moines Lobe and then get into the karst area of northeast Iowa. You get a lot of landscape change along that particular route.”

 


This article was originally published in VISIONS magazine. To receive the full issue delivered to your mailbox four times per year, become a member of the ISU Alumni Association.

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