St. Cloud's granite reveals the history of Minnesota
If you want to learn about Minnesota history, look no further than the rocks beneath your feet. Every month on Minnesota Now we get some insight in that area from Jim Cotter, professor of Geology at the University of Minnesota Morris.
Today, we focus on St. Cloud granite — which is both a spectacular resource and a really interesting geologic feature, which formed 1.77 billion years ago by igneous intrusion. At that time, Minnesota was at the southern edge of what was a much smaller North America.
MPR News guest host Emily Bright spoke with Cotter about it.
Use the audio player above to listen to the full conversation.
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And this one's for you, St. Cloud. We're talking St. Cloud granite, which is both a spectacular resource and a really interesting geologic feature which formed 1.77 billion years ago by igneous intrusion. At that time, Minnesota was at the southern edge of what was a much smaller North America.
But we'll hear much more about that from Professor Cotter. Hi, Jim. I'm so glad you're here.
JIM COTTER: Emily, I'm happy to be here. Nice to be back.
EMILY BRIGHT: Yes, so St. Cloud is known as the Granite City, correct?
JIM COTTER: That's right, and for good reason. They've been coring Granite since the 1860s, maybe even earlier. They reached the peak of quarrying probably in the '30s or '40s.
And they had, I think, close to 18 quarrying companies working in there. Today, granite is still being quarried, of course. And it's an extremely important resource for a couple of different purposes, actually.
EMILY BRIGHT: OK, tell me about them.
JIM COTTER: Yeah, the first one is for dimension stone. Granite is just a great dimension stone. It's versatile. You can use it indoors and outdoors.
St. Cloud granite in particular is really, really interesting, because within a single quarry, the mineral patterns are really consistent. So you can build something and have the rocks matching over a fairly wide construction site. And if something breaks, you can come back in a couple of years and still match it up.
But the other thing that's interesting about the St. Cloud granite is that, because it formed from a number of different magma bodies, you can find different colors, all essentially within the St. Cloud area. So you can get grays and whiter grays, and pinks and darker pinks. It's just a great high-quality stone that comes in a variety of colors.
And if you're in the dimension stone industry, it's a place right out of this world. It's just a dream come true. The stuff has been used all over the world.
Of course, in the St. Cloud area, it's used quite a bit. And if you've ever seen the Stearns County Courthouse, the pillars are just so great. They're this diamond pink granite, it's called.
But it's found throughout Minnesota. The GM headquarters in Golden Valley has it. Hennepin County government building has it. It's the steps of the Morris Post Office if you happen to visit.
But once you get your eye in, you see the stuff everywhere. A trip out to Mount Rushmore, the visitor center uses St. Cloud granite. In DC, the Martin Luther King monument, the FDR monument, and-- I just love the Korean War vet monument. That has St. Cloud granite, and it's just great stuff.
EMILY BRIGHT: Oh my. OK, so pillars, monuments, building design, that is the aspect I'm familiar with for granite. What's the other use?
JIM COTTER: Yeah, the other use is crushed granite. Granite, when you break it up, it literally breaks into the minerals that make up the rock. And that stuff is called aggregate. And so it's sold for use in both concrete and what's called stone mastic asphalt.
The minerals, because they're not rounded-- a lot of times you think of sand and gravel in concrete. But the minerals in granite remain angular. And so when you compact them in either asphalt or concrete, they interlock and they prevent deformation. It's a much stronger cement or asphalt than if you use another type of aggregate for it.
EMILY BRIGHT: So just briefly, I'm going to cut you off, but I want to hear how St. Cloud granite forms.
JIM COTTER: Yeah, St. Cloud granite formed, as you mentioned, 1.77 billion years ago. And it's the setting that people are familiar with. With the Ring of Fire, you have a subduction zone where ocean slabs are going under the North American continent, and Minnesota is at the edge of it.
But maybe because the crust was thinner, or maybe because it was hotter or maybe more water, you start melting different parts of both the subducting slab, which is now happening, but you also melt the continental crust that it's going under. And so you have all these liquids mixing and forming. And you can envision a lava lamp sending up blobs of different colors of magma. And they make it to about 10 miles deep-- 10 miles beneath the surface, and they crystallize into the 20 different magma bodies that are being quarried in St. Cloud.
EMILY BRIGHT: That is such a great description. I can absolutely picture it. We are out of time. But Professor, thank you so much for teaching us today.
JIM COTTER: I'm happy to do it, Emily.
EMILY BRIGHT: Jim Cotter is professor of geology at the University of Minnesota, Morris.
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