The Self-Organizing Secret of Sand Dunes

Hey smart people, Joe here. But why is this here?

Sand dunes are basically big piles of sand. 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 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? Sand dunes are one of nature's most incredible

examples of order, and even beauty, arising out of, well, 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 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. It's solid. I mean, 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 1/16th of a mm and 2 mm?

That's technically sand.

The sand many of us are used to is tiny pieces of quartz that've 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 WWII 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. That pretty much covers it.

And that Goldilocks size (not too big, not too small) lets 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

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. How does this turbulent cloud of bouncing, chaotic sand

create… order? I think by far the coolest thing about the

dunes, it isn't the dunes themselves, it's these patterns that form on the surface of

the sand.

you'd think if the wind were blowing across this sand, you'd just end up with a nice

smooth surface, it'd 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 someone 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, when it hits the sand

surface again, it almost splashes, driving 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.

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 sized 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 WWI and WWII.

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 László Almásy, 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 is still used today. It's a bit dry,

but what do you expect for a book about sand?

Thank you, thank you.

It's pretty awesome that in a system that seems like random noise—bouncing sand, it's

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 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. Then something weird happens. You can't stack sand forever. Once the angle

of that stack gets to be 33 or 34 degrees, 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 shalloEwer angle, because they can't pack as tightly.

And you can see that right behind me. This line is the palace where avalanche after avalanche

have occurred, shaping this side of the dune.

Anybody have a protractor? Anybody? Should have brought one. Measure it.

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 that dunes can take.

Why Care? 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 spaceport of Mos Espa, in Tunisia, slowly getting swallowed by a

giant moving sand dune.

Ok, 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 sandworms, kids! Never know

when 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-grained 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 are beautiful. Because it feels good to understand why something

is the way it is, not just how it is. And because they are 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 wildflower,

hold infinity in the palm of your hand, and eternity

in an hour.

Stay curious.