The Earth, seemingly solid and stable to many (though maybe not us living around the Ring of Fire), is constantly changing when viewed from long time scales.  The collision of the Andean plate with the Pacific plate creates an upward buckle and the mighty mountain range, and even to relatively recent days, the local environment is still changing dramatically.

Once upon a time — some 30-40k years ago, long before our last ice age ended — a large saltwater lake (Lake Minchin) existed in the Andean highlands, about the size of present day Ireland.  As the terrain changed, the water eventually evaporated faster than it was filled up, and left behind vast plains of lithium-rich salts and what we know today as the Salar de Uyuni (salt-flats of Uyuni) in present Bolivia.

Satellite image of Uyuni Salt-flats by NASA. The central white area is the Uyuni salt-flats. With an area of ~10,000 sq. km., it's about ten times larger than all of Hong Kong, 20% larger than Crete, is half of Belize, or a third of Taiwan. The large, flat surface and clear skies make it useful to calibrate satellite altimeters, which is useful for determining otherwise difficult to measure parameters like tide heights (used in fitting and checking ocean models in climate change studies).

SaltFlat Island

A view of the salt-flats from the top of Incahuasi, what was previously an island in the lake and is now covered by centuries old cacti. Coral, indicative of ancient sea life, can be found on/near the island.

On our way from San Pedro to Uyuni, our party of 16 crossed these flats on an envoy of 4x4s.  Our crossing was in the dry season; boundless white salt extends in all directions, with occasional pre-historic islands like Incahuasi “floating” on the lake.  The drivers could let their hands off the wheel for tens of minutes; there is simply nothing there to crash into.  At the same time, navigation must be difficult, with no features to pin down for hours on end.  All the drivers have to rely on are the shapes of distance mountains, and several rocks they placed in strategic locations.

SaltFlats Boundless

The scenary for hours: blue skies, white land, and all-too-similar mountains in the horizons. Note the rough, grainy, random looking surface.

This difficulty was hidden from us passengers, until the same good driver who saved me with coca leaves decided to try out a new route, breaking away from the other vehicles.  Our paths diverged. After several hours of driving at 100km/h and not seeing anything or anyone, the driver started to point and gesture with his left hand; we passengers glanced at one another.  Soon he took his other hand away from the wheel as well, gesturing with both hands; we exchanged arched brows.  Then, still in the middle of nowhere, he stopped the vehicle, and climbed onto the roof to look around; we broke into nervous laughs.  I half expected that the 4×4 would break down, the five of us slowly roasted, and the carcass of the car one day become convenient lavatory for other travelers.  I didn’t tell the others why I was having a stupid grin.

SaltFlats Perspective

A perspective picture, with mini-me hanging onto bonita senorita gigantica.

At one point we entertained ourselves with perspective pictures.  The absence of identifying landmarks, and clear environment (little aerial perspective in play) means that it’s easy to produce them.

SaltFlats Chemistry 1

A thin point of the flats.

The salt flats, despite their looks and their ability to support jeeps, are not solid through and through.  At certain points the salt crust is only inches in thickness, underneath which is salt water.

SaltFlats Chemistry 3

Poking open the thin layer of salt.


SaltFlats Chemistry 6

Fishing inside the salt-water...

SaltFlats Chemistry 2

And voila! A cubic block of salt. Look closer and you'll see the clean stair-steps. These are macroscopic manifestation of the underlying cubic structure of (chloride) salts.

…underneath which you can fish out salt crystals with cubic symmetry reflective of the sodium/lithium chloride on the molecular level.  (Since it’s such a pretty crystal, I wondered about its composition.)

SaltFlats Chemistry 8

Long range hexagonal patterns.

The other tremendously interesting feature is the pattern on the flat’s surface.  Unlike the first picture I’ve shown you, most of the flats is not grainy, but instead takes on a polygonal pattern.  Specifically, most of them are hexagons of approximately the same size; this holds throughout the majority of the salt flats.  I was mystified when I first saw it, and then again over the several hours when it didn’t change.  Why should it be this way?  Can’t it be heptagons, pentagons, or squares?  Who drew the shapes, and how?  How can it be so even and consistent over the size of a small country?  What does it mean?  (Ah, the mind of a physical scientist, always pondering inconsequential things.)

SaltFlats Chemistry 7

Each edge of the hexagon is approximately the length of a 2L water bottle.

The hexagons are, of course, not man-made but a natural phenomena.  Hot air rises and cold air dives; the same happens in liquids.  Instead of disorderly, randomly jiggling up and down, however, a component of heat transfer happens by the collective movement of molecules in a circuit that brings hot fluid up as it brings cold fluid down; this is called convection, and is responsible for many sorts of phenomena, from geological and meteorological happenings to mixing in a cup of chocolate.

Illustration of convection cells; from Commons.

When it comes to the evaporation of a liquid, there is a 2-dimensional surface that constraints what could have been a 3-dimensional phenomena, and convection cells are formed.  An array of hexagons is one of the optimal way of heat transfer with a surface boundary (this is known as Rayleigh-Benard convection), but only when turbulence does not play a significant role.  The size of the hexagons depends on a number of parameters such as the depth and temperature gradient.  One fun way of seeing this for yourself is to dissolve a packet of hot chocolate in a thermal mug and let it sit: often you’ll get to see the hexagons forming on the surface (this, in fact, was how I learnt about the Rayleigh-Benard convection as a teenager during a cold Italian winter). It’s marvelous to see simple physics appearing when you least expect it.

Thus, what the presence of the hexagons can tell us is that, between the rainy season and the dry season, the salty water is for the most part allowed to evaporate quietly (e.g., without strong winds to interfere with convective heat transfer).  (That said, why there is the deposit of additional salt at the boundaries is still mysterious to me.)  The presence of similar-sized hexagons throughout the region tells us that the conditions of the entire area is similar.  Sadly, not being a chem engineer, that’s the extent of my speculation – I’m curious what else can be said.

In any case, I couldn’t help but wonder why no one is putting solar panels there.  It’s a large expanse of unused, high solar-intensity land, with no oppressive weather nor offending life of any form.  I can see power loss over distance as being a hamper, but there’s loads of water for electrolysis so theoretically the energy could be trapped as hydrogen gas.  If nothing else, it could be a tremendous boon to the isolated societies living in and around the salt-flats.  I’d love to see a proper analysis on the topic if one exists. </dork>

On the salt flats we’d end up staying in a hotel in which the bricks, mortar, floor, furniture and beds were largely built from locally mined salt.  It’s quite pretty, and surprisingly warm at night.  I was told that the walls were tarped up during the rainy season to keep it from melting dissolving.  I’m very curious about the composition and properties of the mortar. </dork, for real>

SaltHotel 4

Exterior of the salt hotel.

SaltHotel 2

The salt bricks and mortar. I was told that the salt solution is somehow worked into a paste, but I can't quite figure how it would work.

SaltHotel 3

The bed, and other furniture (like dining table and chairs), are all formed from the same material. The floor, likewise, is covered with scattered cubes of salt. The ground is not very friendly to bare-feet-friendly people like me.

SaltHotel 1

Hanging salt decorations

Part of the local economy relies on the mining of salt, which with its high lithium content, is useful for lithium ion batteries.  The mining is relatively simple in that piles of wet salt were dried in the sun, and trucked away:

SaltFlat mining

Drying salt piles for mining.

At some point we stopped at a (the?) gas station to fill up, and my (biodegradable) balloon-twisting kit came in handy when a bus of school children rolled in.  I wish I had enough time to make one for each of them.

SaltFlat TrainCemetery

Eventually we went past the Train Cemetery, where dozens of trains were abandoned, possibly from a century ago.

Nearby the train cemetery is a garbage cemetery.  It seems that “waste disposal” in the sparsely populated highlands consists of dumping everything in a field just outside the village.  I hopped over to have a look.  With the large volumes and mixture of material, it’s going to take a very long time for them to decompose.  Throughout the whole trip I can’t help but wonder if these are common problems that small, independent communities in different parts of the developing world face, and if there are common, simple know-hows that can improve their quality of life and sustainability.  If there is, I’d like to know; it may come in handy one day.