Last week, a sinkhole estimated to be 10 feet wide and 30 feet deep appeared on the Oregon coast at Cape Kiwanda. This is the second sinkhole to appear on the cape this year. Laura Gabel is the coastal field geologist for the Oregon Department of Geology and Mineral Industries. She joins us to share what exactly sinkholes are, how they happen and what could have led to the two that appeared on the coast.
Note: This transcript was computer generated and edited by a volunteer.
Dave Miller: From the Gert Boyle Studio at OPB this is Think Out Loud. I’m Dave Miller. A 10-foot-wide sinkhole appeared at Cape Kiwanda on Oregon’s North coast last week. It was the second sinkhole on the cape this year. So what is happening? Laura Gabel joins us to answer that question and more. She is a geologist for DOGAMI, Oregon’s Department of Geology and Mineral Industries. It’s good to have you on the show.
Laura Gabel: Thanks for having me.
Miller: So let’s start with the basics here. I understand that there are a couple of different kinds of sinkholes. But what unifies them? What characteristics do they all share?
Gabel: Well, the technical definition of a sinkhole is a depression in the ground that has no external surface drainage, meaning that when it rains, the water stays inside, ponds up, and/or drains down into the subsurface.
Miller: And then what happens? I mean, when it ponds up and then drains into the subsurface, why? How does that become a problem? How does that become a sinkhole?
Gabel: The reason that this happens when it is a sinkhole is because there’s a couple different ways that it can happen. But the basic idea is that some sort of cavity or cavern or cave is developing under the surface of the ground. And eventually, if that cavity becomes too large to support its roof, that roof will collapse, which is what’s creating the sinkhole. And so that’s obviously a problem if we are assuming that the ground level is at a certain spot and then it rapidly or even slowly starts to sink because there is no support from underneath.
Miller: What are the particular geologies of places that are more susceptible to this, that are more likely to form sinkholes?
Gabel: The most common source for a sinkhole is what’s called “karst terrain”, which is basically the bedrock that is underlying the surface is limestone or some other rock type that can be dissolved by groundwater. So for limestone, these are carbonate minerals that are in the rock and when the acids that are dissolved in our groundwater interact with that rock and dissolve it slowly, as it’s hanging out down there below the ground. So that’s the most common source for sinkholes. And so that’s not really a problem in this part of the world, the Pacific Northwest. But other parts of the United States and the world are predominantly limestone and dolomite. And there are some other rock formations that also are susceptible to sinkholes that evaporate, like gypsum and salt deposits can be really susceptible to this type of cavity formation due to interactions with groundwater.
Miller: But you said that’s not the case in Oregon. So what is happening on the Oregon Coast? What’s the geology at Cape Kiwanda?
Gabel: Cape Kiwanda is a headland on the Oregon Coast and most of our headlands are actually made of basalt, which is a very strong rock type. It’s an igneous rock coming out of volcanoes and it’s quite resistant to erosion of any kind. But Cape Kiwanda is actually made of sandstone. It’s called the Astoria Formation. And it is experiencing attacks from the Pacific Ocean, from our very high energy wave climate at all times. And you can see its effect on Cape Kiwanda just by a quick look. You can see how undulating the shape of the headland is and that is from waves constantly chewing away at the margins of this headland and very slowly, usually piece by piece, pulling pieces of that away and breaking it back down into the sand that it once was.
Miller: Oh basically turning sand stone back into sand?
Gabel: Yeah, pretty much. But so as far as the sinkholes themselves, the reason that we think we’re seeing these sinkholes here is the same premise as there being a hole underneath the surface and eventually that hole is large enough that it cannot support the overburden, or the weight, of the roof of the rock that’s in on top of that cavity.
So what we think is happening on Cape Kiwanda is that these sinkholes are lithologically controlled, meaning there are weaknesses in the sandstone and those weaknesses are like in the form of a plane or a surface. Like a linear surface. And the wave action is preferentially eroding at these weaknesses in the rock. So basically thinking about unzipping, anywhere where there’s already a crack, the waves are going to get in there and through a variety of specific mechanisms that’s going to continue to work at that crack. And so basically, just from the water line, we actually see a small sea cave that’s forming just seaward of these two sinkholes.
We’ve been working with State Parks talking with them a lot about this, going back and forth. [They] flew some really fantastic drone footage that we’ve been looking at. So we see these sea caves that are forming just seaward of the sinkholes and we suspect that the caves actually go further in under the headline than we can really see. And we also see erosion kind of at another spot uphill from the sinkholes. And when you draw a straight line between these two locations, the sinkholes lie right on it between the sea cave that we see forming seaward and then some other erosion that’s kind of happening wrapped around from the next inlet to the north. And that line that you can draw that the sinkholes sit on actually parallels a narrow inlet 200 feet to the north where waves have already created a really narrow and deep little passage in.
And the fact that these two straight lines are parallel, one is real and one we’re drawing, connecting two features, reinforces the idea that there’s weak planes in the bedding of the sandstone itself. So when it was deposited, it was deposited as a flat seafloor sand. And so every episode of sand deposition that ultimately became sandstone creates a slight weakness in that rock. And so one of those, these parallel lines are just showing parallel lines of weakness in the rock that the waves are preferentially eroding.
Miller: And if I understand you correctly, it seems like based on the drone footage and the analyses that you’ve been doing that we can expect more sinkholes. If the places where you’ve already seen them, if they’re sort of mirrored in other areas that haven’t yet turned into sinkholes, is there any reason to believe that there won’t be more sinkholes at some point?
Gabel: It’s quite possible. You would really need to do a lot more work to say that definitively. This is a very large unit of rock with a lot of unique, not homogenous, rock types. There are a lot of fossils and cavities and all sorts of unique things within the Astoria Formation. That’s the name of the sandstone unit that Cape Kiwanda is made from. And so yes, generally speaking, I think there’s a lot of compelling evidence that we already have in hand to suggest that this could be the next chunk of the headland that will ultimately separate from the main body of the headland and either form one or two little mini sea stacks or just kind of collapse down into the waves themselves and relatively quickly be broken away. It is possible.
Miller: In other words, what you’re describing is actually a more profound change than one or two more sinkholes. You’re talking about the development of sort of just a rock island sticking up by itself?
Gabel: Well, as a geologist, I think in long periods of time and the reality is that we probably won’t necessarily see that in our lifetime. There are examples on the Oregon Coast of relatively small headlands that have changed over the course of tens to hundreds of years. So we have been able to witness it ourselves. But for the most part, this is happening on a much longer time scale. So will we see, if anyone were to go on Google Earth and take a look, you can very clearly see this inlet, this very straight inlet, that already exists just to the north of the sinkholes. And how long would it take for this spot to turn into something like that? It could be a very, very, very long time.
Miller: Is there anything that humans could do to prevent that? And would there be a reason to do so?
Gabel: In this location, there’s no reason to do so. This is of a highly dynamic environment and both on the surface as far as sands shifting around just active dune sand as well as just the wave action down on the bottom at the water line. Also there’s no development here and this is a spot where people come to recreate. A very small number of people will come walking all the way around to this kind of outer edge of Cape Kiwanda. And State Parks already has safety fencing up because they can very quickly become dangerous when these areas rapidly go from what feels like a pretty safe slope to something very, very steep that will drop down very quickly. So Parks has adjusted their fencing to reflect the sinkholes that exist now. And then they will consider where the zone of safety really should be and make the fencing reflect that.
So there is no reason to try to mitigate or repair these sinkholes now. And also the effort that it would take would be monumental, given the environment that it’s in and how far it is from development and roads and the ocean down to the bottom.
Miller: Laura Gabel, thanks very much for joining us.
Gabel: Thank you for having me.
Miller: Laura Gabel is a geologist with the Oregon Department of Geology and Mineral Industries. In fact, we did get a statement from Oregon State Parks that mentions some of the things that Laura just said to us. This is what they wrote:
“While any natural area carries risk, enjoying Cape Kiwanda safely requires visitors to pay special attention and to always respect the safety barriers throughout the park and all state parks. The sinkholes could change at any moment and others could appear. If you see something that concerns you, leave the area and report it to Cape Lookout State Park staff at (503) 842-4981. In an emergency, call 911.”
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