- Hey smart people, Joe here, but why is this here?
Sand dunes are basically, well, big piles of sand, but they're formed by the interaction of sand with wind, but that's kind of confusing, right?
I mean, if wind blows across sand, wouldn't you expect, well, something way different to happen?
When air blows across sand, maybe you'd expect it to spread that sand out into nothing special at all, but that isn't what happens.
This is.
At the smallest scale, delicately traced ripples and ridges no more than a centimeter high, and at the largest scale, oceans of wave-like dunes up to hundreds of meters high.
So how do they form?
(playful music) Sand dunes are one of nature's most incredible examples of order, and even beauty, arising out of chaos.
Beautiful sand dune landforms like this form in countless places around the world, and they all happen from the same few ingredients, wind, time, and trillions and trillions of pieces of this stuff, and all of that creates the beauty of physics on both the granular and the grand scale.
Now, to make some shape like this, what geologists call landforms, you have to take stuff from one place, move it, and put it somewhere else, or technically speaking, erosion, transport, and deposition.
The key to sand dunes is what's doing the moving and what's getting moved.
Sand is a paradox.
I mean, it's solid, I'm sitting on it, but it can also flow like a liquid, and sometimes even behave a little bit like a gas.
Sand is weird stuff, and that weirdness is what causes it to form these patterns and shapes.
But, like, what is it?
Sand is just a special kind of dirt, right?
Wrong, sand is so much more than that, it's a record of millions of years of wind and water that have carved their way across the surface of our planet, mountains, rivers, and deserts long gone, it's, it's, you know, that stuff.
The thing about sand is it isn't defined based on what it's made of, it's defined based on the size of the particles.
Anything between 1/16 of a millimeter and 2 millimeters, that's technically sand.
The sand many of us are used to is tiny pieces of quartz that have been broken down from bigger rocks, but volcanic ash can be sand too, or gypsum, like the famous white sands of New Mexico.
Some sand is really ground up coral or seashells, even the skeletons of plankton.
Small snow crystals can even technically be sand.
But maybe the strangest kind of sand on Earth comes from the beaches of Normandy, where the D-Day invasions of World War II took place.
They're covered in microscopic particles of shrapnel known as war sand.
So what is sand?
It's just really small stuff.
Not like the very smallest stuff, but still really small.
I think that pretty much covers it.
And that Goldilocks size, not too big, not too small, let's sand move in a way that nothing else really does.
Big stuff, like rocks, wind doesn't do much to them.
Slightly smaller stuff, like gravel, if enough wind hits it, might do this.
The very smallest stuff, like dust, gets swept into the wind and stays there, sometimes traveling thousands of miles before it lands again.
Sand is just small enough to be lifted and swept along by the wind, but big enough that it doesn't stay up for long.
This bouncing is what happens as sand is blown by wind across arid desert-like surfaces, though it's difficult to see with the naked eye.
Sit down near a sandy surface on a windy day, though, and you'll definitely feel it.
I'm being sandblasted.
(wind blustering) You guys probably can't even hear me, it's too windy.
Oh, it's in my teeth.
If you've ever looked at any sand-covered surface that's been shaped by wind, you've probably noticed these evenly spaced repeating ridges or ripples of sand, usually a few centimeters apart, and these are another paradox.
I mean, how does this turbulent cloud of bouncing, chaotic sand create order?
I think the coolest thing about these dunes, well, it isn't the dunes themselves, it's these patterns that form on the surface of the sand.
Well, you think if the wind were blowing across this sand, you'd just end up with a nice, smooth surface and just even everything out, but that's not what happens.
These beautiful ripples, they're almost like fingerprints of the sand dune.
I mean, it looks like somebody came out here with a rake and made these, but what is so amazing about these ripples is they form themselves, they organize themselves out of just wind and sand.
So how do these ridges form?
Well, once a grain of sand is lifted into the wind and dragged along, but when it hits the sand surface again, it almost splashes, dragging more grains of sand up into the wind.
But the distance of those hops isn't random.
It depends on how big and heavy the grains are and how fast the wind is moving, and this is likely the key to why these ripples form.
So this feedback loop of bouncing sand begins to form on the surface.
All it takes is a little random spot where a few more grains of sand land than others, and that creates a little hump, which causes even more sand to land there, and a little shadow behind that hump where less sand is landing.
And that pattern repeats itself all the way down the row, again, just based on the size of the grains of sand and how far they bounce in that wind.
Bouncing sand pulled along by the wind self-organizes to make this.
The size and distance between these ripples can be different in different conditions.
Different wind, different size sand, different ripples.
And if we speed up time, we can even watch those ripples move.
Much of what is known about how sand and wind interact to form dunes is thanks to this man, Ralph Bagnold.
An explorer and brigadier in the British Army, he was stationed in North Africa between World War I and World War II.
In 1929 he completed an expedition in search of a mythical oasis city called Zerzura, crossing the sand seas of the Libyan desert in a caravan of Ford Model A cars.
His partner on that expedition was a Hungarian adventurer named Laszlo Almasy, who was later made famous in the film "The English Patient", where he was played by Voldemort.
Bagnold never found that mythical oasis, but he did discover the foundations of aeolian processes, that's the technical term for how wind shapes land through moving sand and sediment, named for the Greek god of the wind, and his 1941 book, Bagnold's book, not the Greek god of the wind, it's still used today.
I mean, it's a bit dry, but what do you expect for a book about sand?
(audience applauds) Thank you, thank you.
It's pretty awesome that in a system that seems like random noise, I mean, bouncing sand is like the physical version of TV static, even there, patterns do, in fact, exist, creating order on small scales and large.
Sand dunes can range from a few meters high to towering sculpted mountains hundreds of meters tall, like these at Great Sand Dunes National Park in Colorado, where I decided to climb them.
Like ripples, dunes are also formed by the interaction of wind and moving particles.
But sand dunes happen for a completely different reason.
It's not just sand colliding with other sand, this is aerodynamics.
When wind flows over an obstacle, like a large bump of sand, it speeds up.
This is why winds are often much stronger on top of hills.
Dunes form when wind accelerates over an obstacle, eroding sand as it goes.
What's weird is the spot on the hill where the wind is pulling the hardest on the sand, where the most erosion occurs, it isn't here at the top, which is what you might expect.
The most erosion happens here on the windy side of the dune.
Those grains bounce up the hill, pulled by the wind, and get dumped at the crest of the growing pile, so the dune can grow.
Because the sand gets eroded the most here, and not at the top, that's the reason dunes exist instead of getting blown away by the wind.
So as the wind comes up the front side of the dune, eroding sand, it deposits it here at the crest, but then something weird happens.
So you cannot stack sand forever.
Once the angle of that stack gets to be 33 or 34 degrees, well, you get an avalanche.
You can try it yourself at home.
Look at this finely powdered sugar, it's almost like dust.
Because all those small particles can pack in with each other and be nice and comfy, it can be stacked to super steep angles without avalanching, nearly vertical, even.
But larger granular materials like this table sugar, which, in this demonstration, represents sand, they avalanche at a much shallower angle because they can't pack as tightly.
And you can see that right behind me, this line is the place where avalanche after avalanche has occurred, shaping this side of the dune.
Anybody have a protractor, anybody?
Some really interesting things happen right at the top too.
The air passing over the dune will try to follow the curve of the dune, the same way that liquid will flow down the side of a cup if it's poured too shallow.
But if the flow is faster and the surface peels away really sharply, like it does thanks to that avalanche angle, the air detaches and can't follow, just like how water poured from a spout pours into our cup and doesn't flow down the side.
The wind is clearly blowing sand off the top of the dune, but as long as the wind from upstream keeps depositing more sand at the top than gets blown away, the dune won't shrink.
As sand piles and avalanches, the whole dune may even move or migrate over months or years.
Dunes can grow to ridiculously large scales.
The ones I climbed were more than 700 feet above where I started.
Many dunes are even visible from space, and from that perspective, I think you get the clearest view that there's obviously another level of order arising from the chaos and turbulence of sand blowing in the wind, and that order comes in many different shapes.
The shape of a dune tells us about the wind that created it.
This is a transverse dune, a barchan dune, a longitudinal dune, and a star dune, just a few of the shapes the dunes can take.
Let's be real for a sec, some of you are probably saying, "Hey Joe, why should I care about wind blowing a bunch of sand around?
", and to that I say, knowing stuff is awesome, and also because moving sand impacts a lot of people's lives.
As temperatures rise on Earth and our population expands, urbanization, mining, farming and deforestation are degrading lands into dry, desert-like landscapes.
It's called desertification, and every year an area equal to half the European Union deteriorates into dusty, scorched earth.
Climate change will only speed that up.
30% of Earth's surface could end up drying out, affecting billions of people, and as erosion increases, dunes may threaten to bury whole towns.
Towns like the tiny space port of Mos Espa in Tunisia, slowly getting swallowed by a giant moving sand dune.
Okay, that's actually the set they built for "Star Wars Episode I", but it'll happen to real places too, in fact, it was the threat of sand dunes overtaking a coastal town in Oregon that inspired one of the greatest works of science fiction ever written.
A young newspaper reporter was sent to investigate how engineers and ecologists were fighting to keep migrating dunes from swallowing roads, bridges and houses.
The story he was sent to write was never written, but a few years later he wrote a book you may have heard of, "Dune".
Watch out for sand worms, kids, you never know you're gonna run into one of those things.
In that story, the complex interplay of life and sand on the desert world Arrakis is threatened with collapse at the hands of humans.
It's a lesson about appreciating the delicate balance and the forces of nature, a warning about our own planet, but also a hint that what happens here on Earth happens elsewhere.
You see, our planet isn't the only planet with dunes.
We find sand dunes on Mars, with unique ripples and hills shaped by the thin atmosphere and constant winds of the red planet.
Dunes on Saturn's moon Titan, made of frozen hydrocarbons, even dunes of frozen methane on Pluto, it's pretty likely that every solid thing in our solar system, or others, has some fine grain stuff on its surface, something sand-like, so wherever there's an atmosphere that can move that grainy stuff, patterns like ripples and dunes will also form there.
These patterns are fingerprints left by a planet's past, ours and others.
So why care about big piles of sand?
Well, because they're beautiful, because it feels good to understand why something is the way it is and not just how it is, and because they're an incredible example of self-organization and patterns formed by physics alone.
But also, because something as simple as wind and sand can inspire us to think about something a bit bigger.
I think I finally understand what the poet William Blake meant when he wrote, "To see the world in a grain of sand, and heaven in a wild flower, hold infinity in the palm of your hand, and eternity in an hour".
Stay curious.
Fear is the mind killer, fear is the mind killer, fear is the mind killer.
