Weird Island
21. MINI EPISODE: What the heck is Cumberlandite?
Episode Summary
There’s a unique rock called Cumberlandite. And it’s called this because it’s so rare that the only place in the world where it can only be found in its natural state is on a 4-acre lot in Cumberland, Rhode Island. For this reason, it’s Rhode Island’s state rock. To Find It: Head to Elder Ballou Cemetery, located on Elder Ballou Meeting House Road in Cumberland, RI. When you see the cemetery, park across the street. The cumberlandite can be found behind the cemetery, and if you walk to the back of the cemetery you’ll see a path in the woods. Follow it to a rock covered clearing, and look down!
Episode Notes
Episode Source Material:
Episode Transcription
- Hi. I’m Sara, and you’re listening to Weird Island. If you’re new, welcome! And thanks for joining me as I tell you about the strangest stories I can dig up from my tiny, little state of Rhode Island. If you’ve been with me for a while, I’ve got a tiny update to the format in July and August. Instead of doing full length episodes each week, I’ll be alternating between full length and mini episodes on topics that are still really cool, weird, interesting stories but don’t necessarily justify me talking for an unbroken 20 minutes. They might be five minute episodes--they might be closer to ten minutes. I’m not totally sure yet. Sometimes I can’t help myself, and I get a little rambly. But, this will allow me to get out a little more and get some sun and find some weird stories to come back and share with you all! So with that, today’s story is about rocks. But I promise, they’re pretty cool.
- Roughly 500 Million years ago, there was a supercontinent that doesn’t exist today called Gondwana, and it was centered over the South Pole. And on the northern edge of Gondwana, was a volcanic arc--essentially a chain of volcanoes--called Avalonia. Sometime around 475 Million years ago, Avalonia broke off from Gondwana, and it set sail, becoming a drifting microcontinent. As it drifted away, it created an ocean, called the Rheic Ocean, behind it. Eventually, Avalonia met up with another, slightly larger continent called Baltica. Following that, Baltica and Avalonia collided with yet another continent called Laurentia. And eventually, Gondwana actually caught back up with Avalonia and closed the previously formed Rheic Ocean. Everything came together to become the supercontinent of Pangea, that you’re probably a little more familiar with, and Avalonia was landlocked in the middle.. But when Pangea broke up, about 175 million year ago, Avalonia was divided up, and it ended up partially under Great Britain and southern Ireland and partially under New England. And within New England, Rhode Island is the only state formed entirely on top of basement rock from this ancient, volcanic microcontinent. I’m telling you this, because it influenced the geology of the state today.
- I’m not someone who goes rockhounding, or hunting for different rocks or minerals. But I’ve been on a couple of amateur fossil hunting trips, in Florida and New Jersey, and it is so incredibly fun to be on a treasure hunt, that I’ve always kind of wanted to get into rockhounding. But I was intimated, because, well, fossils aren’t everywhere, and depending on what you’re hunting for, you’ll probably know a fossil when you see one. But rocks are everywhere, and I don’t know enough about rocks to know which ones are worth hunting for and why. But I dipped my toes in, and went on my first hunt - for a very specific Rhode Island rock - and it was the best starting place. Now I’m dying to know more. Whether you’re a rockhound or you’re like me and you’ve never gone hunting for rocks, you’re going to want to give it a go after you hear about this one.
- It’s called Cumberlandite - yes, Cumberlandite. As in, it’s named after the town of Cumberland, and that’s because this rock is so rare that the only place in the world where it can only be found in its natural state is on a 4-acre lot in Cumberland, Rhode Island. Loose pieces of the stone can also be found as far south as Narragansett and Block Island, but they’re not naturally occurring there. It’s believed that as glaciers passed over the cumberlandite site, portions of loose rock were picked up by the glacier and transported south as the glacier melted. So this one little spot, and this rock, they’re incredibly unique.
- Cumberlandite hasn’t been radiometrically dated, so no one knows exactly how old it is. But it’s considered to be from the Proterozoic age, back when stable continents appeared for the first time, and it could be anywhere between 500 Million and 1.5 Billion years old. I mean, that’s a pretty big span of time, but cumberlandite likely formed back when Rhode Island was Avalonia.
- I’m sure you’re asking yourself, “Why is this rock so rare? Why does it only exist in that one spot?” Well, Cumberlandite is an igneous rock - and igneous rocks form when magma (or molten rock) cools and crystallizes, either at volcanoes on the surface of the earth, or while the melted rock is still inside the earth’s crust. And igneous rocks can have tons of different mineral compositions, depending on the magma they cool from and how it cools, so the unique circumstances under which a rock is formed can result in a rock that only occurred that one time in that one place.
- If it cools quickly, like after a volcanic eruption when the magma is exposed to cooler temperatures above ground, then the rock that results is likely to have a fine grain, because crystals don’t have much time to grow before the rock completely cools. Whereas, if it cools slowly, like if the magma were to cool underground where it’s insulted, then it results in a rock that is coarse grained, and likely will have crystals that are big enough to see with your naked eye.
- And that’s what cumberlandite is like--that second one. It’s considered a coarse to medium grain rock, but it has these chunks of super visible crystals, and that makes it the perfect rock to go hunting for if you’re a novice, because it’s actually pretty easy to identify. Cumberlandite contains around 24 different minerals, and it’s got this dark brown or black overall coloring, depending on whether it’s been exposed to weathering or not, and embedded in the black groundmass of the rock are small, greyish white feldspar crystals.
- And there’s one other thing that makes it super easy to identify. It’s a very dense, heavy rock and it’s slightly magnetic. So if you want to go hunting for it and be certain that you’ve found it, bring a magnet.
- Because of all of these unique factors, cumberlandite has frequently been mistaken for a meteorite. And I learned that in the rockhounding community, a rock that is mistaken for a meteorite can sometimes be called a meteorwrong. Which I love.
- Cumberlandite is located in an area of Cumberland known as Iron Mine Hill, and the rock was mined as early as 1703, when colonists mixed the iron-containing cumberlandite with iron from a mine in Cranston to construct cannons, weapons and tools. It’s said that ore extracted from Iron Mine Hill was manufactured into cannons used during the British victory over the French as Louisbourg in 1745 during the French and Indian Wars. But, cumberlandite contains a relatively low percentage of iron--around 30% total--and a high percentage of titanium, so it wasn’t ideal for this use, and the casts were prone to cracking.
- Later, in the early 1900s, the hill was used to quarry large quantities of trap rock for road building, and gravel from the site was supposedly used to create the old Narragansett Racetrack parking lot in Pawtucket. Because of these different mining and quarrying operations, Iron Mine Hill has been reduced to approximately half of its original size.
- In 1966, Rhode Island named Cumberlandite the state rock, because it’s so unique and it’s got this kind of interesting history. But, many people haven’t really heard of it. If you want to go and find the rock today, you can - very easily. There’s a cemetery in Cumberland, called the Elder Ballou Cemetery, and you can park across the street and walk up behind the cemetery, where you’ll see a path running behind it in the woods. Back there, you see big boulders and chunks of Cumberlandite everywhere. Just look at the ground!
- But here are my amateur rockhounding tips. It is not a long walk at all, but wear long pants and boots or sneakers. Because it was very wet when I went, and there were hundreds of biting ants that crawled up my bare legs and bit my ankles. And it actually hurt! I brought my boyfriend and my adventurous but anxiety prone chihuahua with me, and all of us got bit. When my boyfriend noticed the amount of ants and realized they were biters, he scooped my chihuahua up and ran with her in his arms so fast that she got the fear hiccups. But if you can move quickly on the ant-covered path, you’ll see a grassy area and Cumberlandite all over the ground. And it’s totally worth it. Just bring a magnet so you can confirm your finds.
- Thank you so much for listening. As always, all episodes are researched and written by me, Sara Corben, and all of my source material is linked in the show notes! If you liked this episode, share it with your friends and family! Or you can leave a rating or review. And if there’s a topic you’d like to hear about, let me know! You can email me or send me a recorded voice memo at weird rhode island @ gmail.com. See you next week as we dig up more stories about all things weird and wonderful in the little state of Rhode Island. Until next time!