Talks

Quo vadis, stone?

 By Eniday Staff

There is a perfectly flat surface, four kilometres long and two wide, lying about 1,200 metres above sea level. In summer it forms the bed of a dry lake, covered in the cracks that form when mud loses all its humidity. In the middle of winter it is slimy and murky, little more than a sheen of water on compact silt. Not a blade of grass glints forth all the year round. Nothing animal or vegetable is to be found, or at least nothing large enough to be seen. It is, to all intents and purposes, a desert…

The vast area of California in whose southern reaches it lies is known unappealingly as Death Valley, and is one of the largest natural parks in the Americas, with a length of almost 180 kilometres, a width of 80 kilometres at some points, and a total area of 13,000 square kilometres. Although inhospitable and difficult to reach, it presents a unique and fascinating landscape. This is partly because it is probably the only place on earth where stones move about by themselves. The stones in question are neither huge rocks nor pebbles. They are 10 to 20 centimetres long and weigh a few kilograms, up to 20 in some cases. They love to sail about on the almost muddy surface of the Racetrack Playa, as this very unusual place is known. There is no one pushing or kicking them. Although the stones seem to float above the surface, they are resting on the ground. Geologists, geophysicists and volcanologists have racked their brains to unravel this mystery, and seem to have found the answer at last. A strange combination of wind and ice is responsible.

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An image of the trace left by a stone after a walk

Invisible forces

It takes a detailed look to understand the causes. The Racetrack Playa is a temporary lake, fed by no rivers, only by winter rain. This gives it the appearance of a lake, but not for long. The water soon evaporates in the heat of the day or is absorbed by the muddy bed, made of very fine sand. In the hottest period of the year, the lake remains completely dry. It is then that you can see the sailing stones. But just how is this possible? Are the stones very smooth underneath, meaning the wind can move them over a slippery silt surface? Calculations reveal that to move a mass in such conditions it would take a force 0.15 times its weight. For example, it would take a horizontal thrust of 15 newtons to shift a mass of 10 kilograms and leave a visible trail in its wake. If the same mass had vertical sides, this would require wind travelling at 35 metres a second, or more than 120 kilometres an hour. Speeds like that occur once in a blue moon in these parts, and yet stones travel for hundreds of metres before the coming of every summer.

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The curious view of the trails left by the numerous "walking" stones (plosone.org)

Against any physical law

A group of Czech geologists put forward a complex theory in recent years. On winter nights, the lake’s surface sometimes freezes. As the thermal conductivity of rock is far higher than that of water, and the level of water in the lake changes with the weather, the unusual phenomenon arises of stones floating on their own small rafts of ice. But there is no getting around Archimedes’ principle. For this theory to work in the case of a stone measuring 15 centimetres and weighing 10 kilograms, the slab of ice would have to be at least four centimetres thick. More importantly, it would have to have a surface area of a few hundred square metres. The Czechs’ theory therefore does not explain things, not least because the routes taken by the stones are never regular or parallel. They sometimes diverge and curve away from each other, as if following some innate directions that have nothing to do with the wind and even less to do with the fluctuating water levels on the muddy surface.

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Researchers try to explain the mystery by analyzing the ground (NASA, GSFC, LPSA intern, Leva McIntire)

An answer may have been found by a group of American geologists, although observations have yet to support it. They have noticed that stones almost always move on clear and sunny days, after very cold nights on which the temperature falls below zero. At midday the heat and wind break the extremely thin layer of ice that has formed in the night. A combination of the wind and the large surface area of these ice sheets, just a few millimetres thick, makes the stones move, crossing paths, drifting away from each other and even colliding. This would explain the fascinating pattern of trails left in the mud, which look like the result of synchronised movement. A unique system of ice sheets pushes dozens of stones about, leaving us with the sight of them skating about hand in hand.

Cover by: Scott Beckner, Flickr

READ MORE: The sea in the desert by Sara Sangermani

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Eniday Staff