On a slab of sandstone 200 million years old with dinosaur paw prints, scientists have found traces of "floating stones" - the same as in Death Valley. What is this phenomenon and how is it explained.
Moving stones - the same age as dinosaurs
One of the reports at the December meeting of the American Geophysical Union was devoted to a topic that was not quite usual for this scientific forum. Paleontologist Paul Olsen of Columbia University in the USA said that on one of the early Jurassic sandstone slabs displayed at the Connecticut Dinosaur State Park, next to the paw prints of the sauropodomorphic dinosaur Otozoum moodii, a furrow is visible, which has been interpreted as a trace of an ancient "floating stone."
A stone slab with dinosaur paw prints and a footprint of a moving stone from Connecticut, USA.
Traveler stones
Floating, sliding, creeping stones - this is the name of a geological phenomenon that scientists first encountered at the beginning of the 20th century on the dried up Lake Racetrack Playa in Death Valley in the United States. Boulders, sometimes weighing up to 320 kilograms, rolling down to the flat bottom of the basin from the surrounding dolomite rocks, mysteriously, without any participation of people or animals, move hundreds of meters, leaving behind them distinct traces. Moreover, they are not always straight: sometimes the stones are turned - in an arc or at a right angle.
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The stones move only once every two or three years, the traces remain for three to four years. Boulders with a ribbed bottom surface leave more distinct, straight furrows, while those with a flat surface wander from side to side. Sometimes stones turn over.
A moving stone on the dried up Lake Racetrack Playa in Death Valley, USA.
How the secret was revealed
The media started talking about the intervention of supernatural forces. Scientists have suggested that it was all about strong winds.
However, the stones were clearly too heavy for any wind. In 1955, geologist George Stanley of the University of Michigan proposed the "ice raft" hypothesis. During the period of seasonal shallow flooding of the lake, ice floes form on the surface of the water during a cold snap. They move, and the stones frozen in them leave traces. In particular, Stanley referred to the fact that the trajectories of nearby stones are almost parallel and repeat the bends of each other.
This hypothesis was supported by another American geologist, John Noel Earl Weber. He described how stone marks formed in the shallow waters of Great Slave Lake in Canada.
In 1995, scientists from the Hampshire College of the United States under the leadership of Professor John Reid, summarizing the results of observations over two decades, showed that the furrows from stones, which appeared in the cold and wet winter of 1992-1993, are very similar to the traces of the late 1980s, when ice also formed on the surface of the lake. It was concluded that the stones were moving along with the entrained flows of water by the ice.
In 2011, American physicists, led by planetary scientist Ralph Lorenz from Johns Hopkins University, experimentally confirmed that even very thin ice, when simultaneously influenced by the wind, is capable of moving rather large stones. Boulders frozen into the ice actually rise above the surface, which makes them easier to move.
To put an end to the debate, scientists conducted detailed field studies in Death Valley using continuous cameras, parallel meteorological observations, and GPS tracking.
The experiment began in the winter of 2011. Fifteen test stones with GPS sensors attached to them were placed in the southern part of the lake basin - where the boulders that rolled down the mountainside usually begin their journey.
The first two years did not produce any results. And then the scientists got lucky. In late November 2013, a rare winter cyclone brought heavy rain and snow to the Racetrack Playa area. At night, the southern part of the lake was covered with an ice crust three to six millimeters thick, which split into separate ice floes, and the boulders moved along with them at a speed of about five meters per minute.
In December 2013 and January 2014, the stones covered 224 meters. The mobile episodes lasted from a few seconds to 16 minutes. All this was recorded by cameras. Thus, the secret of the creeping stones was revealed.
Trajectories of movement of test stones located in the form of adjacent groups. The maximum movement was recorded on December 20, 2013, when this picture was taken. North-east wind, four to five meters per second.
Ice rafts and microbial mats
For stones to move, a rare coincidence of natural conditions is required. First of all, it is necessary for a heavy rain to fall in Death Valley, the driest place on Earth. Then the air temperature must drop sharply so that the water freezes before it has time to evaporate. Finally, a sufficiently strong wind is needed to break the ice into ice floes and move them through the shallow water below them.
Moisture accumulates only in the southern part of the lake, which is four centimeters deeper than the northern one. And the ice floes move to the north, since the basin is surrounded by mountains on the other three sides.
Such conditions are possible only at dawn after a frosty night. In the morning, when the sun melts a thin crust of ice, the stones no longer move.
Curvature and sharp turns of the trajectory are the result of changes in wind speed and direction, as well as collisions of ice floes.
Scientists have also figured out where the grooves come from without stones at the ends. Previously, the park administration thought that the stones were taken by tourists as souvenirs. In fact, "ownerless" traces leave piles of ice floes that then melted.
Surprisingly, the mechanism of the capture and transfer of stones by ice, which is characteristic of the highlands and arctic coasts, operates on Racetrack Playa in a hot subtropical climate.
The authors of the experiment believe that the same is happening on other lakes where moving rocks have been found: Little Bonnie Clare Playa and Alcali Flat in Nevada, Bonneville Playa in Utah and Magdalene in South Africa.
But the footprints at the bottom of Lake Altillo Chica in central Spain are explained differently. According to the researchers, the main role is played by microbial mats - thin biogenic films that cover the bottom in shallow water. When a shallow body of water dries up, the wind-propelled stones slide easily over the smooth surface, leaving furrows.
Traces of moving stones on the shores of Lake Altillo Chica (Spain).
Volcanic winter
Paleontologists who have found traces of moving stones on an ancient stone slab lean towards the "ice raft" mechanism, since dinosaur paw prints would hardly have been preserved so well on microbial mats - even the texture of a reptile's skin is visible on them.
Digital photogrammetric modeling showed that the deepest longitudinal grooves of the stone track in the slab are split by small parallel cracks filled with dirt. This indicates that the bottom of the dried-up reservoir, on which the dinosaur walked, was covered with a layer of clay, and not an organic film.
But the “ice” hypothesis also has its disadvantages. The fact is that 200 million years ago the territory of the present state of Connecticut was almost at the equator, at about 18 degrees north latitude. Most of the plants and animals that lived here at that time, among which dinosaurs dominated, were not adapted to frost.
To resolve this contradiction, the authors speak of an outbreak of volcanism on the North American continent at the beginning of the Jurassic period or a little earlier. A huge amount of ash got into the atmosphere - a volcanic winter came.
Short-term frosts occurred in the tropics 200 million years ago. Then the stones moved. As a result of global climate change caused by volcanic winter, 76 percent of biological species, including dinosaurs, have disappeared from the face of the planet.
Vladislav Strekopytov