Geology Professor Jim Cotter schools us on Ely Greenstone

EMILY BRIGHT: So what do you think when I say greenstone? Do you think emerald, jade? Jim Cottter, Professor of Geology at the University of Minnesota, Morris, is more likely to think Ely and the region's greenstone belts. Every month on Minnesota Now, Professor Cotter tells the story of our state through geology. Hello, Professor Cotter.

JIM COTTER: Hi, Emily. How are you?

EMILY BRIGHT: I'm doing great. So what is Ely Greenstone?

JIM COTTER: Ely Greenstone is one of these great ancient rocks that we have so many of in Minnesota. It's over 2.7 billion years old. It's a volcanic rock. And it occurs in and around Ely.

EMILY BRIGHT: And-- oh, go ahead, please.

JIM COTTER: OK. So the unique thing about it, though, is even though it's ancient, it still has a lot of features in it. It hasn't been all that beat up by geologic history. So it's just a great rock to study.

EMILY BRIGHT: Oh, I love it. So what does Ely Greenstone look like?

JIM COTTER: Ely Greenstone is a lava flow, a lot like the lava flows in and around Gooseberry Falls. So it has that nature to it, but it's undergone some metamorphic processes. And that has transformed some of the minerals into a group of minerals that are more green-like. And so it has a kind of greenish-black tone to it. The minerals are chlorite and epidot.

It's not anything absolutely gorgeous, but it makes an interesting fireplace stone. It's kind of nice to have slabs of it on your patio and things like that. But it certainly isn't a gemstone.

There's a gemstone called Michigan Greenstone. And a lot of people get them confused. They're not the same thing. They're different ages, although they're both volcanic. The Michigan Greenstone is a kind of distinctive iteration of the mineral pumpellyite, specifically it's called chlorastrolite. The word means star in the sky. It has these white threads that kind of shoot through a green rock.

But that's not Ely Greenstone. Ely Greenstone is much older and geologically much more interesting.

EMILY BRIGHT: And so where can I find some examples of Ely Greenstone in the wild?

JIM COTTER: Yeah. Ely Greenstone shows up in a couple of places that people frequently visit. It certainly is in Ely. There's a great example of it that's called pillow rocks. And if you Google pillow rocks in Ely, Google Maps will bring you to it. It's on the edge of town.

It's a big, big, boulder that was moved by glaciers. And it's just a great example of the characteristics of Ely Greenstone. In fact, astronauts from Apollo 14 and 15 were sent to Ely to train by it because greenstone forms in water. And so they were looking-- on the moon, they were looking for evidence of water on the moon.

In 2016, there was an interesting community story in Ely where they wanted to move the rock to where more visitors might see it. But they decided not to in the end.

EMILY BRIGHT: I love this, how Ely is famous, that it brought astronauts to come study it. So I have to tell you, I was excited to talk with you today. My dad was a geology major. And my whole life, he's always pointed out neat rocks to me and told me what they were and how they were formed. And one of the things that I've come to love about geology is that they're Earth's history records, right? These rocks tell a story. So what stories do Ely Greenstone tell us?

JIM COTTER: Yeah, at the heart of it is that Ely Greenstone is a lava flow that was flowing into water. There's these great-- the rock is called a pillow rock is because when lava flows into water, it forms a kind of bulby shape. If you think of a little kid blowing a bubble with bubble gum, it's kind of this big, bulby thing with a pinch on the bottom of it. That's what basaltic pillows look like.

And so one of the things that you can start to tease out of Ely Greenstone is that this lava was flowing into water. The oldest greenstone belt is in Greenland. And that one dates back to almost 4 billion years. And so these pillow lavas show up in Greenland. They're also in Brazil and South Africa, Australia.

But it's an indication that there's water on the planet almost 4 billion years ago. And that's kind of interesting because the event that formed the moon, there's an impact that hits the Earth of a Mars-sized planet. It wasn't Mars, but it was a Mars-sized planet, hits the Earth and breaks into pieces and breaks off a big chunk of the Earth. And that becomes our moon.

That was 4.5 billion years ago. And pretty much everybody assumes that there was no water after that. And so over the course of about a half a billion years, the Earth gets oceans, deep water. So it's one of these things that you can learn from greenstone belts.

EMILY BRIGHT: So before I let you go, what kind of insights can we get about our planet from studying rocks that are this old?

JIM COTTER: Right, one of the things that's really interesting about the greenstone-- and they're called greenstone belts because there's granite that shows up with them, granite greenstone terrains. Is that, one, there's not very much land above water. When you look at greenstone belts, the volcanics, there doesn't seem to be any contribution from continental sediments.

And so the granite that's associated with greenstone belts and what makes granite greenstone terrain seems to be the earliest efforts of our planet to make granite, which is what continents are made of. So you can kind of envision this planet with just water and maybe a few islands of volcanic islands and no continents of significant size, maybe small ones like the size of a Minnesota county size, but no land processes going on.

EMILY BRIGHT: Well, I love this lesson that we're getting in truly ancient history. Thank you so much, Jim, for your time today.

JIM COTTER: Oh, I'm happy to do. it's nice having nice having the conversation, Emily.

EMILY BRIGHT: Absolutely. Jim Cotter is Professor of Geology at the University of Minnesota, Morris.