Rockhounding in Arkansas
Each month, KUAR hosts a radio segment called Science Cafe. This month’s guest was Michael DeAngelis, an assistant professor of Earth Science at the University of Arkansas at Little Rock. The professor talked about geology during the 25-minute show and about places in the state known for their rocks and minerals. He discussed the Ozark and Ouachita mountains, the many minerals that can be found at Magnet Cove, the famous quartz crystal of Mount Ida, the formations of Crowley’s Ridge and the diamonds of Crater of Diamonds State Park in Murfreesboro.
Dorothy Graves, Ph.D., of the University of Arkansas for Medical Sciences, served as the host of the program. The show airs on the fourth Tuesday of every month. This month’s discussion was quite interesting and below I have loosely transcribed some of the segments featured during it in case there might be an interest. It is not a complete transcript of what was discussed, just a few bits and pieces I wrote down.
It seems Arkansas is a great place to be a rockhound! Enjoy!
Graves: Tell us how Arkansas geology came to be what it is as we know it today.
DeAngelis: Our Arkansas geology and the geologic history is really complex and it has a very deep history. It has undergone a lot of changes over geologic time and that’s something that geologists do, we talk a lot about geologic time. It’s deep time so when we talk about this, these changes are happening over millions and millions of years. So the very basement rocks, the rocks that are underlying most of Arkansas, are 1.5 billion years old. Since then there’s all these things that have happened on top of those basement rocks.
Geologists divide Arkansas into five different geologic provinces. Most people are aware of these groupings. They’re the Ozarks to the northwest and then below that is the Arkansas Valley and then the Ouachita Mountains to the south and the Gulf Coastal Plain to the very southwest of Arkansas and then the whole eastern side of the state, we call the Mississippi Embayment and it’s mostly made of sediments related to the Mississippi Valley. So as you travel from one of these provinces to another the rocks vary quite a bit. In the Ozarks you’ll see a lot of carbonate rocks and you can go explore the caves and find lots of fossils. And then when you get into the Ouachitas you see really folded and faulted rocks that have built these mountains. There is a lot of sandstones and novaculite and unusual minerals and lots of things like that. So as you travel around from one place to another the types of rocks are very different.
Graves: Geologically, what’s your favorite area in the state?
DeAngelis: Well my training is in mineralogy, which is the study of minerals. And petrology which is the study of rocks. I specialize in igneous and metamorphic rocks and so my favorite rocks here are really the igneous rocks of Arkansas.
One of my favorite areas is the Magnet Cove area which is between Malvern and Hot Springs. It’s just really an unusual area. It contains rocks and minerals that are found almost nowhere else in the world. It’s a very unusual story of the formation of Magnet Cove and so the rocks and minerals that you find there are extremely unique. So that’s probably my favorite. I like it all, I’ll go in a cave, I like that too but it’s really the igneous rocks, the ones that crystallize from magma, that I love the most.
Graves: Can you give us the CliffsNotes version of how Magnet Cove came to be and to offer such unique minerals?
DeAngelis: If you can imagine a volcano where Magnet Cove is today, where Malvern is. About 95 or so million years ago dinosaurs are wandering around, it’s the Cretaceous Period. There is this volcanic activity and sometimes when you have magma some of the rock, some of the magma will actually come out of a volcano, we call that lava, and you get all sorts of lava rocks and things like that. Some of this magma gets trapped and stays underground below the volcano. And that’s what happened at Magnet Cove. These rocks are what we call sub volcanic rocks and they intruded into the rocks that were already there and then crystallized in place. And then over time the volcano in the rocks above had been eroded away and have exposed those rocks at depth.
Graves: Can people go down and explore Magnet Cove? Is that an area that people have access to?
DeAngelis: It’s one of the favorite places of rockhounders. People come from all over the country. There is a creek that runs through Magnet Cove called Cove Creek and people go into the creek all the time and you can pull out big chunks of pyrite, which is fool’s gold, and people love that. There’s obviously magnetite and brookite, which is a titanium mineral. And there’s really some unusual minerals, there’s a mineral called kimzeyite, which is a zarconium bearing garnet that was first discovered in Magnet Cove, it’s the first place in the world it was discovered, what is called the type locality for this mineral. It’s since been found in other places in the world but right here in Arkansas is the first time it was ever found. So there is lots of lots of great stuff there, lots of great minerals.
DeAngelis talked about other interesting geological features of the state such as Crowley’s Ridge. There was also a caller that asked an interesting question:
Caller on line: We have in Arkansas one of the best representations of quartz, which happens to be the state mineral. I wonder if you can comment on why our quartz crystals are so good?
DeAngelis: Some of reasons why people love the quartz crystals of Arkansas is there’s a lot of variety in the way they look. A lot of people really enjoy the kind of clear crystals but you can find all sorts of oxidized quartz and you can find sometimes rutilated quartz which has got blades of rutile traveling through the quartz. If you go into Magnet Cove you can find smoky quartz and this is quartz that has been irradiated somehow, you can get grayish in the black [color]. So I think that’s part of it. I think people really enjoy finding the variety of quartz crystals and it’s really something that’s accessible. It comes down to people can go out and find it for themselves. And I think to be able to go and collect something yourself, it’s a thrill. It’s what geologists love to do, we love going out[ to do this], but a lot of a lot of my friends who are non-geologists but are rockhounders, like it too. It’s like a treasure hunt every time and every time you find something different.
Arkansas is known for quartz.
There is a lot and if you head out towards Hot Springs and especially the Mount Ida area, that’s a place that’s really known for quartz collecting. If you go to a place like the Smithsonian [ the world’s largest museum complex, located in Washington, D.C.] and you look at their best quartz displays, you’re going to see Arkansas quartz. And there’s really beautiful quartz, all different sizes. I have seen some crystals of quartz that measure in feet. The Arkansas Geological Survey has a lot of really great quartz crystals in their displays over at over at their building. They have some fantastic quartz crystals that have come out of Arkansas.
Graves: Let’s talk about Crater of Diamonds.
DeAngelis: It’s very unusual. It’s what we call a lamproite. Diamonds are not normally stable at the surface of the earth so these things had to travel up from deep in the mantle at supersonic speeds to get up to the surface of the earth. So it was a very explosive event when it formed, at about the same time as Magnet Cove, in the Cretaceous Period.
It is as far as I know the only publicly available place to search for diamonds and they also have a fantastic museum there that explains the formation of Crater of Diamonds.
For those that haven’t been out there, it’s a big field that’s plowed occasionally by the staff at Crater of Diamonds State Park. And if you’re lucky you dig around in the dirt and you might find a diamond. It happens fairly often. I know I’ve had the question, you are a geologist you must know how to find the diamond right? And the answer is I’ve no better luck than anyone else. I’ve dug around out there and haven’t found anything yet. I think it just takes a lot of persistence and there are people who spend all summer out there every day looking for diamonds. And they find them, they do. So it’s really kind of a neat opportunity, anybody young or old can get out there and dig for diamonds.