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 is focus focus on St. Cloud granite — which is both a spectacular resource and a really interesting geologic feature , which form 1.77 billion year ago by igneous intrusion . At that time , Minnesota is was was at the southern edge of what was a much small 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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Audio transcript

EMILY BRIGHT: This is Minnesota Now. And if you want to learn about Minnesota history– and I’m talking ancient 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.

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 is hear ‘ll hear much more about that from Professor Cotter . Hi , Jim . I is ‘m ‘m so glad you ‘re here .

JIM COTTER : Emily , I is ‘m ‘m happy to be here . Nice is be to be back .

EMILY BRIGHT: Yes, so St. Cloud is known as the Granite City, correct?

JIM COTTER : That is ‘s ‘s right , and for good reason . They is coring ‘ve been core Granite since the 1860s , maybe even early . They is reached reach the peak of quarry probably in the ‘ 30 or ‘ 40 .

And they is had had , I think , close to 18 quarrying company work in there . today , granite is still being quarry , of course . And it is ‘s ‘s an extremely important resource for a couple of different purpose , 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 is is that ‘s interesting about the St. Cloud granite is that , because it form from a number of different magma body , you can find different color , all essentially within the St. Cloud area . So you is get can get gray and white gray , and pink and dark pink . It is ‘s ‘s just a great high – quality stone that come in a variety of color .

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 see the Stearns County Courthouse , the pillars is are are just so great . They ‘re this diamond pink granite , it ‘s call .

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 crush granite . Granite is breaks , when you break it up , it is breaks literally break into the mineral that make up the rock . And that stuff is call aggregate . And so it ‘s sell for use in both concrete and what ‘s call 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 is formed form , as you mention , 1.77 billion year ago . And it is ‘s ‘s the setting that people are familiar with . With the Ring of Fire , you is have have a subduction zone where ocean slab are go under the north american continent , and Minnesota is at the edge of it .

But maybe because the crust was thin , or maybe because it was hot or maybe more water , you is start start melt different part of both the subducte slab , which is now happen , but you also melt the continental crust that it ‘s go under . And so you is have have all these liquid mix and form . And you is envision can envision a lava lamp send up blob of different color of magma . And they is make make it to about 10 mile deep– 10 mile beneath the surface , and they crystallize into the 20 different magma body that are being quarry 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 is ‘m ‘m happy to do it , Emily .

EMILY bright : Jim Cotter is is is professor of geology at the University of Minnesota , Morris .

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