How do rocks move on their own?
Rocks can rise from the ground in fields, and in various places around the world there are examples of rocks as heavy as 200kg apparently moving independently, even leaving tracks behind them. How can such heavy objects move unaided?
Rocks do not move unaided, of course, but are shifted by forces that have required investigation to identify.
The founding father of evolutionary theory, Charles Darwin, studied fieldstones in the 1870s. He concluded that when major rocks appear in fields, it is because of farmers, though not because they carry them there. Darwin proved that rocks normally move downwards because of the activity of worms and other digging creatures in the soil below. But when ploughing loosens the soil, subsequently rain causes the soil around a rock to collapse, carrying them higher through the soil layers.
However, farmers are not responsible for sailing stones. These exist in several places, but the most famously in the 9km2 seasonally-dry lake of Racetrack Playa in California, USA, where rocks leave tracks on the dry lake bed when they move. The rocks typically weigh around 3kg, but some are over 200kg. Scientists have tried to solve the mystery of sailing stones since the mid20th century, and they long suspected that wind must play a decisive role. In 2013, three American scientists finally solved the mystery by means of video recordings, weather observations, and sophisticated GPS equipment drilled into the stones. They found that when special conditions exist – perhaps only once a decade – the wind moves the stones aided by huge but shortlived sheets of ice. The movement will only happen on a sunny windy day following a cold night when, for a few days or weeks, the lake becomes covered in 5-10cm of meltwater from heavy snowfall in the mountains surrounding Racetrack Playa.