Natural Connections: Banded Iron Formation

"My friend and I breathed deeply, grateful for the oxygen, the beautiful rocks, and the scientists whose research uncovers the Earth’s stories."

Natural Connections: Banded Iron Formation

The old road angled steeply up the hillside. Thick drifts of autumn leaves concealed loose rocks and little ravines. Saplings and brush crowded in from the sides, threatening to scratch cheeks already rosy from the cold. After several minutes of tough hiking, the road leveled out, and big concrete structures loomed among the trees.

From 1922 to 1924, the Pioneer Mining Company operated an open pit iron mine on Mt. Whittlesey near Mellen, Wisconsin, although pit doesn’t seem quite accurate. There’s no big hole, the hillside merely looks a bit sliced off. Minning and quarrying, along with fur trapping, logging, and attempts at farming have sculpted the landscape of the Northwoods since the late 1800s. But that history is influenced by far older events.

Continuing past the concrete ruins, my friend and I followed the scar of the old road to the top of a cliff. Smooth, dark rock peeked out from beneath dry leaves and grass. Kneeling for a better look, we found stripes of red, black, and gray with smooth, waxy, and sparkling surfaces. Crustose lichens had found toeholds in each tiny crack, so the surface was also decorated with little blobs in shades of brown, white, and yellow.

These lichens may be much younger than the outcrop, but the rock itself is no stranger to photosynthesizing friends. In fact, iron formations like this one record a major milestone in the history of life on Earth. Back in the day, and by that, I mean 1.9 billion years ago, the atmosphere was filled with carbon dioxide and methane, and the first inklings of life had only just begun. Volcanic activity in the early oceans, and erosion off the few continents, enriched the water with iron and silica.

As algae and cyanobacteria began turning water and carbon dioxide into sugar using energy from sunlight, they also emitted oxygen into the ocean where they lived. The oxygen reacted easily with the dissolved iron and silica, causing them to precipitate out of the water into the minerals hematite, magnetite, and jaspilite. Those minerals accumulated on the bottom of a shallow sea who sloshed between the shores of two early continents. Over time, the mineral mud hardened into stone. This stone. I pressed my hand to rock.

The minerals didn’t precipitate homogeneously, though, and built the rock in a series of bands with different colors, textures, and thickness. Seasonal fluctuations in algae growth may have contributed to some of the variation. The rise and fall of landforms on the early continents, and the weathering and erosion of different rocks, may also have altered the chemicals that fed into the sea. An even wilder source of the variation is that the early life hadn’t evolved with oxygen, and if ever the dissolved iron didn’t immediately clean up the oxygen they pumped into the water, the algae would have poisoned themselves, causing population fluctuations.

I’m not a good enough geologist to tell you what conditions led to which bands, but I could see that some dark gray bands were smooth while others were rough and sparkling. And a few layers were a whimsical mash of red and gray polka dots in a darker matrix. The red dots were iron-stained quartz, sometimes called jasper or jaspilite. Geologists call this texture “granular iron formation,” and in this case, it represents sands broken out of slightly older iron formations that rolled back and forth in shallow waves before solidifying again.

A billion years after precipitation stopped and the layers became rocks, intense tectonic activity in this region (the Mid-Continent Rift!) upended everything. Iron formations are sedimentary rocks, and therefore form in horizonal beds, flattened by the force of gravity. The action of the rift caused the center of Lake Superior to drop and the edges to curve upward like the pages of a book bent in a U. Rocks that once covered the bottom of a shallow sea now form narrow ridges at the surface and then dip steeply underground.

My friend and I descended the irregular, stair-like face of the cliff, vacillating between admiring the rock, the colorful lichens who clung in cracks, and the lush green mosses who soaked up trickles of water. Knowing the history beneath me, I was impressed by the sheer mass of iron that the early algae and cyanobacteria caused to precipitate out of the seawater. While there are a few iron formations older than this one, and some younger ones, too, iron formations of this age are notably abundant, especially in Minnesota and Michigan.

One of the final, amazing chapters in this saga is that the algae and cyanobacteria eventually evolved enzymes that allowed them to live with oxygen. No longer at risk of poisoning themselves with the element, they proliferated wildly, their oxygen waste sweeping most of the iron and silica out of the ocean water for good. Then excess oxygen, no longer tied up with iron, escaped into the air, and began creating the atmosphere we enjoy today.

My friend and I breathed deeply, grateful for the oxygen, the beautiful rocks, and the scientists whose research uncovers the Earth’s stories.

Author’s Note: You may have noticed that I refer to the sea, rocks, lichens and mosses as “who.” This is a deliberate choice in using a “grammar of animacy” and recognizing that not just humans possess the quality of life. I find that using this language causes a positive shift in the way I think about the more-than-human world. For more on this topic, I highly recommend an essay called “Speaking of Nature” by Robin Wall Kimmerer, available easily through an internet search.

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Our exhibit: “The Northwoods ROCKS!” is open through mid-March. Our Fall Calendar of Events is ready for registration! Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.

Last Update: Dec 14, 2023 5:53 am CST

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