If you walked into your classroom and the whole room was rearranged, you'd assume your teacher did it, right? But what if you asked, "Why did you rearrange the desks?" and the teacher said, "I didn't! I locked the door last night when I left, and today they were all moved!" You might find that mysterious!
The same kind of mystery faced scientists in Death Valley National Park, one of the driest and hottest locations on planet Earth. Heavy rocks break off the surrounding mountains and land on the valley called Racetrack Playa. They are huge, so they should stay where they land. But these big stones were leaving trails behind them showing they had traveled long distances.
For a very long time, no one actually saw the rocks move. There were no footprints or animal tracks. They were too heavy to be pushed by wind alone, and there hadn't been a flood to sweep them away. Scientists were stumped. Who or what was moving the "sailing stones" in Death Valley when no one was looking? 👀
Geologists went to work. They attached GPS sensors to some of the biggest movers and set up time-lapse photography to capture how far the rocks were moving each day. They took readings on precipitation, temperature, wind speed, and amount of sunlight (insolation). The answer was bizarre.
Death Valley is famous because it's the hottest place on Earth and the driest place in North America. It only receives an average of 2 inches of precipitation per year! So, imagine how surprised scientists were to discover the sailing stones were sliding around on ice! ⛸️❄️
When a little bit of rain falls during the coldest seasons, there is just enough moisture to form very thin ice sheets during the cold desert nights. Racetrack Playa is extremely flat, so the winds are strong. During the day, if the ice melts just a little, the surface becomes extra slippery, and the ice may break up. The wind piles up broken ice chunks behind each rock, then starts the whole pile of rock and ice sliding forward.
Do the rocks always travel the same way? No! Sometimes the rock tracks stay parallel, and other times they wander and cross. It turns out, the weather isn't the only variable that affects the paths. Rocks that are rough where they meet the ice tend to travel in a straight path, while rocks with smoother surfaces will travel in lots of directions.
The following graphs show the weather at Racetrack Playa during the coldest months of 2013. The dashed lines show "sailing" events where rocks moved.
Here are some questions I think of when I look at this graph:
💡What is the range of wind speeds in December 2013? (Range is the difference between the highest and lowest values.)
💡Look at the weather patterns before the stone started moving. Look one day ahead on wind speed, one day ahead on sunlight (insolation), and 1 - 10 days ahead on air temperature. What similarities do you see?
💡From what you know about the the two times when the stones sailed, is it likely that the stones would have started sailing after the last day on the graph? Give evidence for your answer.