Scientist Figures Out How 'Sailing Stones' Move Themselves Across Death Valley | Hacker News Reader

Scientist Figures Out How 'Sailing Stones' Move Themselves Across Death Valley

core77.com

122 pointssethbannon31 comments

It's a good theory, and not the first time that ice has been proposed as playing the major role at the Racetrack Playa. I have a friend who is obsessed with taking photographs of these, and it was about five years ago that I first heard the ice theory (and about these wacky furrows in the Playa) - I'm sure people have had ideas like this for even longer.

I think it's a stretch to say that this has been "Figured Out" - I'll give that credit to the first person who records an observation of them moving.

Until then - just more theories.

[Edit - I like this 10+ year old theory, which suggests it's actually a case of moving ice sheets, with rocks embedded in them, that actually causes the furrows. http://skeptoid.com/episodes/4021]

There exist a Skeptoid episode about this: http://skeptoid.com/episodes/4021 , the 'Living Stones of Death Valley':

"When we reached the lake, we found to our great surprise that the entire lake was moving with the wind, at a speed we estimated at about one half of a mile per hour. The sun was on the lake by now and we could see a few very thin ice sheets that were now dissolving back into water. This whole procession was washing past many of the famous rocks. It's easy to imagine that if it were only few degrees colder when we were there — as it probably had been a couple of hours earlier — the whole surface would be great sheets of thin ice. Solid ice, moving with the surface of the lake and with the inertia of a whole surrounding ice sheet, would have no trouble pushing a rock along the slick muddy floor. Certainly a lot more horsepower than wind alone, as has been proposed. The wind was gusty and moved around some, and since the surface is not perfectly flat and with rocks and various obstructions, the water didn't flow straight; rather it swapped around as it moved generally forward. Ice sheets driven by the water would move in the same way, accounting for the turns and curves found in many of the rock trails."

> And if you're wondering why they don't just strap a GoPro camera onto a rock to see what's going on, scientists returning to the site over the years have calculated that the rocks move for short periods of time, just once every three years. That's a bit longer than your battery's likely to last.

Stick a solar panel on it to charge it up then. Christ >_<

It is, after all, rather sunny in Death Valley.

Cameras are so cheap these days. Why doesn't he set up a camera continuously recording for the next 5 years until he catches a glimpse of it moving?

Hell, can't they just get some grad students to camp out there for a few months with a motion detector and a siren? It would hardly be the most boring job ever given to grad students.. This really doesn't seem like the sort of thing that should go unobserved for so long.

I suppose I should be glad that this isn't as bad as the pitch drop.

Because during part of the year it's actually a shallow lake, and the rocks only move one year in three, which is a fairly low chance of it happening.

Like gp said, perfect for a grad student!

How much would it cost to keep the whole place under UAV surveillance for the winter?

Less than it takes to keep them flying in other countries. In Death Valley there would not be any reason to spend money on equipping them with missiles.

Reminds me of this film: http://www.chasingice.com/

The small-scale proof-of-concept mentioned seems a little flawed to me...the surface area => volume ratio of a small rock can't be compared to a boulder. The ice collar formed inside a tupperware container is also very idealized compared to the ice layer that would form in nature. I guess if the ice collar were thick enough, maybe it's possible, but that's quite an ice build-up you would need. It doesn't seem too hard to get a large-scale experiment going to confirm some of this...Myth Busters needs to get on this.

Until then, my theory is that this phenomenon is brought to you by the same kind of people that make crop circles :P

If they only move a little bit in just the right conditions during the winter, why do they maintain course for so long before making sharp turn and then maintaining that course for a long period of time? Wouldn't the wind have to be mostly blowing in the right direction for years during the winters and then suddenly start mostly blowing in another direction for years? Or is it maybe some subtlety in terrain variation that determines where they'll move?

> Lorenz realized that as moisture levels changed with the seasons, a collar of ice could form around the rock. With the rest of the lakebed covered in a thin, cold-but-not-frozen sheet of water, this ice collar would enable the rock it encased to float—slightly.

So "ice collars" are responsible for moving 700 pound rocks on a "thin sheet of water"; and all this in the hottest and driest place in America? I must be missing something....

The Racetrack is a dry lake bed. Which is to say, it's a lake but the water level is (usually) below ground level. Often only just. Along some parts of the road running along the edge of The Racetrack is a trench and there's often water visible in this, 6-12" below the lakebed level.

Moreover, the playa is at 3700 feet. The desert floor itself gets cold enough in the winter time, and the extra elevation only excacerbates this: adibiatic lapse accounts for about 3.5F cooling per 1000 feet, so Racetrack Playa would be 13F cooler than sea level.

Deserts, despite their reputation as hot places (and yes, Death Valley and the Racetrack can be quite hot in the summer months) are largely characterized by their exposure. Which means they can get quite cold in wintertime.

Many deserts can get quite cold at night, even in the summer time.

No cloud cover, low humidity -> the heat just radiates off into space.

The Romans supposedly used to make ice in the Middle East and North Africa by digging a deep pit and insulating it with straw. They'd open it up at night and cover it back up during the day.

During the winter months the temperature often drops below freezing: http://en.wikipedia.org/wiki/Death_Valley#Climate

Ice isn't actually that bad at supporting compressive loads. And, under pressure, it melts[0].

So if you put something heavy on top of an iceblock, it will be on a lubricated base.

[0] A fun way to show this is "welding" ice cubes together by pushing them against each other.

One interesting point though is that current consensus is not that ice is slippery because it melts under pressure, but because of how air interferes with the bonding of the molecules at the surface.

Desert temperatures at night can get surprisingly low.

This is actually due to the low amount of water - very little of the greenhouse gas water vapour to keep the heat from radiating into space.

It happens once every few years. Rain isn't as uncommon as yu think, and temperatures drop below 0 during winter frequently enough.

Maybe. 700-pound rocks 'floated' by ice, though? I'm inclined to think about ice build-up on the ground on the windward side. Note the pancake-sized hillocks with valleys between that can fill with water. When it freezes, the rock lifts. Hmmm. Anyhow, the interaction with that 'special' surface is more complex than just 'sand'.

Interesting. The original Am. J. Phys. paper[1] is from 2011.

[1] http://ajp.aapt.org.offcampus.lib.washington.edu/resource/1/...

"The resource you requested requires you to log in with your UW NetID and password"

http://ajp.aapt.org/resource/1/ajpias/v79/i1/p37_s1

So you create a doughnut of ice around the base of a rock. The base of the rock has friction reduced by the area of the doughnut while slightly scraping the bottom sand. How high does this layer of frost get?

Ice blown by wind can be dangerous:

https://www.youtube.com/watch?v=rjWq-bwhgZA&noredirect=1

Nice summary of theory. I hope we can get this on video soon.

And I am thinking can this table top experiment be used in Earth Moving tech? There is a lot of rock to be moved both in construction and road-building industry.

Or aliens.

Reply on news.ycombinator.com