What makes Montana's Ringing Rocks ring?
Austin Amestoy: Welcome to The Big Why, a series from Montana Public Radio where we find out what we can discover together. I'm your host, Austin Amestoy. This is a show about listener-powered reporting. We'll answer questions large or small about anything under the Big Sky. By Montanans for Montana. This is The Big Why.
For this week's episode, reporter John Hooks is back with us. Welcome, John.
John Hooks: Thrilled to be back, Austin.
Austin Amestoy: So, I understand to start things off, you've got a little trip to take us on. Where are we headed?
John Hooks: I want you to come along with me to Pipestone. It's on the east end of Homestake Pass, about halfway between Butte and Whitehall. You get off the interstate and you start winding your way along a really rocky dirt road up into the hills.
[John Hooks on the scene] Some helpful signs along the way are pointing that direction. Otherwise, it might be easy to get lost up here.
Austin Amestoy: So how gnarly of a road are we talking about, John?
John Hooks: Well, it's rough enough that I felt compelled to park my car and hoof it up for the last stretch.
[John Hooks on the scene] Oh, it's embarrassing to be this out of breath on the radio.
Austin Amestoy: And what did you encounter at the end of your arduous journey?
John Hooks on the scene: Well, I see a big pile of rocks.
Austin Amestoy: A big pile of rocks.
John Hooks: A big pile of rocks.
Austin Amestoy: Okay, so I'm assuming you didn't hike and drive all that way just for a regular old pile of rocks, right?
John Hooks: Oh, no, no, no. These are far from ordinary rocks. This pile is a roadside attraction people have been visiting for a long time called the Ringing Rocks. A big heap of boulders that give off a resonant bell-like ringing sound when struck with metal; at least theoretically.
[John Hooks on the scene] First strike, first try with the hammer. That was underwhelming.
Austin Amestoy: Yeah. I have to say, I agree with you there, John. I was expecting a stronger sound out of something called the Ringing Rocks.
John Hooks: Oh, me too. Me too. Eventually, I did get some better sounds. I think it's just about finding the right rock.
[John Hooks on the scene] That's better. That's good. That's what I was, that's what I want.
Austin Amestoy: So the obvious question that comes to mind here is why do these rocks make this sound?
John Hooks: That is the same question that a listener Max Maxwell, had. He's been up in Kalispell for about a decade.
Max Maxwell: The previous 38 years, I lived in Butte. And I'm just curious what is it that makes those ring when you hit them with metal?
Austin Amestoy: So, John, who out there knows the secret of the Ringing Rocks?
John Hooks: Well, conveniently for us, Butte is full of geologists, so I took a much shorter, easier trip over to Montana Tech and met up with Steve Quane, a geologist and the director of the Mineral Museum on campus.
Steve Quane: John, nice to meet you.
Austin Amestoy: Seems like the logical place to go. Did Steve have answers for you?
John Hooks: He said we have enough evidence for at least a generally accepted scientific hypothesis. So, there are two parts to the idea. The first deals with the mineralogy of the rocks. That's the internal composition formed in southwest Montana's volcanic heyday.
Steve Quane: 75, 80 million years ago, around there. Lots of eruptions in southwestern Montana, lots of lava flows, explosive eruptions.
John Hooks: At the location of the Ringing Rocks, two different kinds of very thick, viscous lava mixed together under the surface.
Steve Quane: Kind of like taffy as it moves. So you had two kinds of taffy mixing together.
John Hooks: As the lava mixture cooled, it formed this really compact, dense crystalline rock.
Austin Amestoy: And the compact makeup of the rock impacts the sound it makes?
John Hooks: Exactly. And Steve helpfully demonstrated why.
Steve Quane: I've just got a sort of a crystalline rock.
Austin Amestoy: Is that one of the ringing rocks from the pile?
John Hooks: No. It's actually illegal to take those from the pile, since it's a protected point of interest. But he got one for us with a similar make up.
Steve Quane: You know, you can kind of hit this rock. It gives you a nice, solid sound. But if you compare that to, I have here a piece of coal. And the coal is much lower density. And so when I hit the coal, you get thud. Those are pieces falling off as it breaks, but you just get a thud.
Austin Amestoy: And how does that density affect the sound it makes?
John Hooks: Well, it's all about sound waves and how much work they have to do.
Steve Quane: In the crystalline rock, the sound waves travel more efficiently through the denser rock. In the coal, they have to compress the air in between the grains. And so you get that thud.
Austin Amestoy: But you said there were two parts to this hypothesis. What's the other one?
John Hooks: So, here is where things get even more interesting because there are other ringing rocks around the world. But there is one quality to Montana's ringing rocks that, at least as far as I can figure out, is unique. Here's our listener, Max Maxwell, again.
Max Maxwell: And I'm also being told if the rocks are moved from there, that they don't work.
Austin Amestoy: Whoa, that's weird. So, if you take a rock out of the pile, it doesn't make a sound?
John Hooks: Well, it'll still make a sound, but it's kind of weak. It's not the same nice ringing tone you get when you hit the pile.
Austin Amestoy: Does Steve Quane have any ideas why that is?
Steve Quane: So it certainly has to be how they're suspended on one another with that space in between them.
John Hooks: The really dense rock that makes up the pile was much more resistant to weathering and erosion than the rest of the rock around it. So that stuff eventually all weathered away, leaving these huge towers of rock, until ...
Steve Quane: Probably freeze thaw weathering, a lot of it done during our last glaciation say, 10,000 years ago.
John Hooks: As the glaciers built up and receded during the last ice age, those towers eventually cracked and crumbled into the boulder pile that we have now. Wind and rain then flushed out all the small stuff like soil and gravel.
Steve Quane: So that's why you have all this void space.
Austin Amestoy: Oh, so that void space is kind of like the hollow body of a drum or an acoustic guitar, maybe. Kind of allowing the sound to amplify and escape.
John Hooks: Pretty much, yeah.
Steve Quane: So it seems that ringing rocks is this combination of the nice, tight crystalline rock with the spaces in between to let it out. Different spaces, different size of boulders, probably give you the different notes.
John Hooks: But I should also say this is just a hypothesis. It's Steve's and other scientists best guess at why the rocks sound this way.
Steve Quane: You know, sort of geologically, scientifically, nobody's really been able to say for sure these rocks ring for this reason.
Austin Amestoy: Wow. So, the mystery continues. But I'm struck by Steve bringing up different notes and, it has me wondering, John, has anyone tried to make music on these rocks?
John Hooks: All right, Austin, you're going to have to pardon the pun here, but rock music is not anything new. There's actually a term for a rock instrument. It's called a lithophone. A really remarkable example is a thing called the Great Stalacpipe Organ. It's a giant instrument built into the rocks at the Luray Caverns in Virginia. They actually pressed a record of it back in the day. And here at the Ringing Rocks, Steve Quane is actually working with the Butte Symphony to get some musicians and geologists up there together and see what they can do with it.
Austin Amestoy: Well, that just sounds incredibly cool. John, were you able to record any of that happening?
John Hooks: Unfortunately, that event is not going to happen until the fall. But in order to not leave everybody with an unsatisfying cliffhanger, I grabbed a couple of hammers and tried to bang out a little ditty for us.
[John Hooks on the scene] We're going to see if we can get a little melody or something going with my very rudimentary percussion knowledge.
Austin Amestoy: I have to say, John, you play a mean rock. Thanks for trekking out to that big pile of boulders and bringing us the story.
John Hooks: Oh, it was my pleasure. Thanks, Austin.
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