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The Racetrack Playa must first fill with water — deep enough to float massive sheets of ice, yet still shallow enough to leave the rocks exposed. Nighttime temperatures must then get cold enough to freeze the water, forming mammoth-size ice panes — thin enough to glide across the lake bed, yet thick enough to gain momentum and clear the stones in their way.
As the next day’s afternoon sun thaws the ice, these sheets break apart into chunks that, with any luck, a steady wind will propel across the playa pool. When the ice chunks hit the rocks — ranging from pebble to boulder-size pieces weighing up to 200 pounds – the stones are driven across the soft mud below, leaving behind their signature trails.
A necessary condition for the rock motion we observed is the existence of a playa pool deep enough to submerge the southern section of the playa, yet shallow enough to leave many rocks partly exposed at the pond surface. Other repeating features of rock movement events that we observed include the presence of floating ice, temperatures and sunlight sufficient to create melt pools in the ice, and light breezes that are steady enough to drive floating ice. Although the ice breaks up around rocks, even thin moving ice sheets can generate sufficient force to drive rocks across the pool. All observed rock movement events occurred near mid-day when sufficient ice melting had occurred to allow ice break-up. Creation of rock trails is difficult to observe because trails form below the ice-covered pool surface where they are often not evident until the ice has melted, and liquid water has been removed. In addition, rock movement is slow and relatively brief—our GPS instrumented stones traveled at speeds of 2–5 m/minute for up to 16 minutes—so casual observation is likely to miss rocks in motion. Weather station data show that the freezing temperatures necessary for ice formation, and winds in excess of 3–5 m/s are common phenomena at Racetrack Playa during the coldest few weeks of winter. Therefore, the extremely episodic occurrence of rock motion (years to decades) is likely due to the infrequency of rain or snow events sufficient to form winter ponds.