Digging into the Science of Sand

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HOST: This is the NOAA Ocean Podcast. I’m Marissa Anderson. Whether you’re strolling along a shoreline or gazing at a picturesque tropical island wallpaper on your personal electronic device, you may not be giving much thought to the sand. But there is more here than meets the eye. Today, we’re joined by Matt Kendall, marine biologist with NOAA’s National Centers for Coastal Ocean Science. Matt imparts his knowledge on how sand is formed, why it comes in different colors, and how it benefits ecosystems and coastal communities. Let’s dig right into our discussion.

KENDALL: Sand. It's basically tiny rocks. But it's not just tiny rocks; it's rocks of a certain size. So they have to be a certain, we call it a “size class.” And there's this guy, Chester Wentworth. About 100 years ago, Chester Wentworth published an article in a science journal, the Journal of Geology, that was called a scale of grade and class terms for clastic sediments. And here's where he defined sand.

HOST: Matt is referring to the chart called “Wentworth grain size classification.” You can view the chart in our show notes.

KENDALL: This classification thing that he came up with, it covers basically loose material, could be sand, could be mud, could be rocks and boulders, but he classified it all based on a few attributes, and the main one was size. So it starts at the top of this chart with boulders, and those are things that, picture a boulder in your mind. It's about a meter or a yard in diameter, maybe. So that's how big boulders are, and then you move your way down smaller and smaller things like cobbles. So maybe fist-sized pieces of rock; pebbles are going to be maybe pea-sized, that sort of thing. And then, we get to the middle of the chart, which is our sand. Sand is defined, according to Wentworth, as anything from one millimeter in diameter, and you can see that with your naked eye. Think about maybe a poppy seed or something like that, and then it goes all the way down to coarse sand, medium sand, fine sand, and he calls very fine sand 0.6 millimeters. And that's going to be like sugar, granulated sugar, but not powdered sugar. If you go further down in the chart that Wentworth's made up, you've got your silt and your clay, and those are super fine particles that are really, really small.

HOST: So my next question is, how is sand formed? Based on what you just shared, I'm assuming it's created a number of different ways.

KENDALL: Yeah, that's exactly right. The main way, but not the only way, that it is formed is through a process called weathering. And that can be through those waves, where if you get a storm event, it can pick up those cobbles, for example, and crash them together, cause little bits to break off, cause the cobbles to break in half and become smaller. Maybe you break enough cobbles up, and then the next thing you know, you got yourself some gravel. Keep doing that and pulverize some more of that gravel, and then you get into smaller and smaller and smaller pieces, and that's where you get your sand. Can be waves, can be storms, but a lot of it comes from further up in a watershed. So places where you've got mountains, you're going to have events on the mountains that are causing things to run down streams. You get freeze-thaw, which is an interesting process by which rocks get broken apart, and places that have a winter that freezes you, a little bit of water creeps into a crack in a rock. And then that water freezes, and remember water expands when it freezes, and it can actually do that enough it'll actually break the rock up, and it breaks it up just over millennia, over large amounts of time. It eventually breaks it up into smaller and smaller pieces, and that stuff gets carried down streams and rivers and, ultimately, to the ocean. But it can also be formed by biological processes. There's a couple of other kinds of sand that are out there that are formed through plants or even animals. There's something called a foraminiferan, which is a little thing that's about the size of a small-ish grain of sand. And it's a shell made out of calcium carbonate. And inside that shell lives a little amoeba-like animal that sends out its little amoeba arms and legs and lives its life out there. But when it dies, it's essentially a piece of rock. It's calcium carbonate. And if you get enough of those foraminifera in the same spot, that can make a sand. Another example is there's something called, there's a plant called Halimeda. And there are other kinds of plants in the same family as Halidmeda. It's called a calcareous algae. And it gets that name because, yeah, it's an algae. It gets its energy from the sun; it has chlorophyll and all that good stuff. The Halimeda is made of calcium carbonate. And when it dies, the same thing happens. Halimeda looks green when it's alive; when it dies, it's white. And as it breaks apart, it forms something called Halimeda sand.

HOST: So I also understand that parrotfish have a unique role in the sand formation process. Could you tell us a little about that?

KENDALL: It is true. There is a way that parrotfish are involved that is surprising to a lot of people. And this actually falls into that category of weathering sort of that I was talking about it.

We call it, as scientists, “bioerosion.” And that means that some aspect of biology is breaking something apart physically. And if you look at an image of a parrotfish, if you look at their mouth, they have, it's almost a beak-like set of teeth. And those teeth are basically like little bone chisels. And when you watch a parrotfish swim around, one of the things that you'll see every once in a while behind them is this plume of white stuff that is parrotfish poop. That white stuff, that's sand. So how do they do that? How are they making that? Well, parrotfish scrape algae, and in some cases, live coral; they scrape algae off of coral, off of dead coral. I have a piece of coral in my hand, and I have some teeth from a parrotfish in my other hand. I'm just gonna rub the teeth from the parrotfish on the coral just so you can get a sense of what it sounds like. And this is what it sounds like underwater. As I'm doing that, I can actually see little bits of coral getting scraped off and falling in between the keys on my keyboard. So I'm going to stop doing that right now. That basically made a little bit of sand, and it got into my computer. But what parrotfish are doing is they're scraping this algae off the coral, and they happen to be eating a little bit of the coral at the same time. They can't digest this coral. All they want is the algae. So when they are done digesting it, they poop out the sand. It makes this; actually, it's very beautiful white sand. It is pure white sand. So when you're on one of those beaches in the tropics, where there's a place with a lot of parrotfish, and you're just loving it in the sun, you're laying on parrotfish poop.

HOST: I don't think I'll ever look at a beach quite the same way again. So why are there different colors of sand? How does that happen?

KENDALL: Yeah, so we talked about a lot of different ways that sand can form so far. Maybe it's broken-down rocks. Maybe it's coral that was scraped by a parrotfish. Maybe it's those, those foraminifera. And all of those things have different - they're different colors. Like the sand that is made from the parrotfish poop is made from the skeletons of coral, which is really white. And so that's why you get a white sand in that case. The foraminifera sand that I was talking about, I think there's a place, it's in the Bahamas where that particular type of foraminifera is very common, and those are pink sand just because the shells are pink, and that's why the sand is pink. But you can also get other colors. You can get black sand that's formed from places with maybe a lot of volcanic history to them. Made out of rocks that are of the basalt type, which is the same type of rock that the crust of the ocean, the earth's crust under the ocean is made of basalt and it's black, and if you have sand made from that, the sand is going to be black. Can even get green sand if it's a green type of rock. I think there's another place in Hawaii that has that particular kind of sand. Around on the east coast of the U.S., where I live, most of the sand is tan. It's mostly from rocks like quartz and feldspar. The rocks are tan, so the sand is tan. If you look at sand under a magnifying glass, you can see that oftentimes it's not all exactly the same color. There's often a bunch of different things going on in there. It's a mixture of things, especially at the beach, because things get pretty mixed up there.

HOST: So I'd like to give our listeners a greater appreciation of sand. Why would you say it's important from an environmental, ecological standpoint? What are the benefits?

KENDALL: As we've been discussing, it, it occurs in a lot of different places, and different animals use it as habitat for maybe a place that they live their entire life, if they’re something like a clam, or they only use it for a small part of their life, but maybe it's a really critical part. Think of organisms that lay their eggs in sand or on beaches, things like horseshoe crabs, things like sea turtles. They live their whole life out in the ocean, but then when they reproduce, they need to lay their eggs in sand, and they do that on beaches. So obviously, with no beach, you're not going to have a place for the sea turtles and things like that to lay their eggs. Grunion is a good example. It's a fish on the west coast that lays its eggs kind of in the beach swash zone of sand. Sand also forms a bunch of different kinds of habitats. We're kind of focusing on the beach, but a lot of the seafloor is essentially covered with sand, and a lot of organisms make their living on the sand or living inside the sand. Think of a flounder, that is a flat fish that likes to lay on the bottom, and it can actually change the color of its skin to match the color of the sand to do some camouflage so that it can ambush its prey and swim up off the sand to eat things. There's other specialized habitats that are formed out of sand as well. Sand and mud, things like river deltas where there's some of the most productive places on the planet as far as fish and marsh creatures. And all of that is formed on a base of sand that comes out of the river, the base of sand and mud that just gets piled up over the millennia.

HOST: Could you describe what the benefits of sand offers people?

KENDALL: There's a couple ways to think about it. You know, there's things that sand does for us just by sitting there and being sand in its natural spot, right? So the beach is obviously one example for recreation that we can all think about. But like we talked about earlier, it also provides habitat for a lot of fish, for example, that people like to catch and eat. So it provides habitat for those kinds of things. It also does things like provide shoreline protection. It's basically a buffer for storms that would otherwise cause a lot of coastal flooding in erosion. If you have a beach, a natural beach there, with all the things that go with a natural beach, like sand dunes, and the sand dunes are going to be anchored by beach vegetation, all that is this big buffer zone that's going to prevent a storm that comes ashore from actually damaging homes and streets and businesses and things like that that are on the coast. But if you get rid of that buffer, then all that storm wave energy can go right into the community and cause a lot of problems. So just leaving some sand is one thing, one benefit. Leaving sand where it is in its natural spot is one benefit; then, you know, this is something that benefits people.

HOST: We hope you enjoyed our episode and learned something new about sand. Check out our show notes for more information and make sure you are subscribed to our podcast so you never miss an episode.