Ice rafts not sails: Floating the rocks at Racetrack Playa
A rock on the regularly cracked floor of Racetrack Playa. The trail behind. The rock is about 15 cm across. Some faint streaking is visible on the playa surface to the upper right of the rock. It is believed that this streaking is due to smoothing by drifting ice sheets.
A picture from a small digital camera lofted on a kite at the south end of the playa, near the dolomite cliff (upper left), which is the source for many of the rocks. Note that many trails are parallel to each other. The kite string is faintly visible at the upper right, pointing toward the first author.
Schematic of the forces and dimensions of a solitary rock and a rock frozen into an ice raft.
Wind speed required to move a rock 20 cm wide and 20 cm high, immersed in water 10 cm deep as a function of ice raft diameter and ice thickness. For the 1 m raft, the thickest ice blocks some of the side of the rock. More generally, the buoyancy of the raft facilitates motion by reducing friction.
The same as in Fig. 4, but for a smaller rock.
Measuring the drag on a rock at Bonnie Claire Playa. Even though the mud felt extremely slippery underfoot, the weight of the rock causes it to sink through this lubricant and grip the rigid mud beneath, resulting in a measured coefficient of friction of 0.5 or more.
A tabletop experiment showing trail formation with rafted rocks. A roughly cubical 4 cm rock was frozen (a) in ice (the assembled raft had a mass of 950 g) and is floated, rock-downward in a baking tray (b) with sand and water. (c) It is easy to form a trail.
The sitzmark of a rock is believed to have moved during Winter 2008–2009. Note that the trail is much shallower than the original depression.
The rock that originated at the depression in Fig. 8. Note the very shallow striations, forming a trail narrower than the rock itself. The card is .
A view of the playa from the dolomite cliffs, looking north during a December morning. Note the shadow in the foreground, where the rock density is higher.
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