Figure 1.
Time lapse images of a moving rock.
Image acquired with a handheld digital camera on January 9, 2014. Image on the left shows the wide-angle view; interior black frame indicates the view in other frames. In close-up frames, blue arrows show stationary rocks and red arrow—a rock in motion (moving from left to right). Total movement lasted ∼18 seconds. Dark, flat areas on the pond are panels of ∼3 mm thick ice surrounded by rippled water several centimeters deep. Ice thickness estimated from inshore ice panels. Broken ice panels accumulated on the upstream side of the moving rock in the last two images. Images have been cropped but not otherwise edited.
Figure 2.
View from the ‘source hill’ on the south shore of Racetrack Playa.
View is looking north on December 20, 2013 at 3:15 pm. Steady, light wind, 4–5 m/s has blown water to the northeast exposing newly formed rock trails. Lower image shows overlay of lines to emphasize the congruent shape of adjacent rock trails as well as the proximity of rock trails to rocks that did not move. Image has not been enhanced.
Figure 3.
Map shows the locations of the weather station, GPS-carrying rocks, and approximate locations of the northern shoreline of the pool and Figures 1 and 2.
Figure 4.
GPS-instrumented rock with its rock trail.
The GPS unit with its battery pack is inserted into a cavity bored into the top of the rock. The GPS continuously logs its position after a switch is triggered by the stone moving away from a magnet set in the playa. The surface of the playa is frozen in this image, but the ice had melted or was floating when the trail formed. Image by Mike Hartmann.
Table 1.
Characteristics of GPS-instrumented rocks.
Figure 5.
Weather record from Racetrack Playa, Death Valley National Park.
Temperature (red line), 1 hour average wind velocity (black line), insolation (blue line), and rainfall (filled blue line) for November 20, 2013 to January 9, 2014. Wind gusts shown in Fig. 9. Red arrows indicate known rock movement events; there have been additional movement events not recorded by direct observations. Movement events 1 and 2 are recorded by instrumented rocks, and direct observations were made for events 2–4. Time lapse camera observation shows ∼20 cm of snow accumulation on November 23. Total precipitation is likely under-recorded because our weather station does not record precipitation due to snow. Data reported in Table S1.
Figure 6.
GPS-instrumented rock trajectories and velocity.
Top and middle panels are data collected by rocks (A3 and A6) on December 4, 2013; bottom panels are for a rock movement on December 20, 2013 (Rock A11). Times are UTC. Note the broadly similar trajectories and velocity histories for rocks on December 4. Velocity errors are shown in Fig. 7. Data reported in Table S2. Rocks A3 and A6 moved at least once after their GPS instrument batteries were depleted and had total trail lengths of 157.5 m and 162.4 m, respectively (Table 1).
Figure 7.
Error analysis of GPS-instrumented rock movement events.
Note that the uncertainty in rock velocity is generally less than 0.2–0.3 m/minute in comparison with the recorded 2–6 m/minute velocities (Fig. 6).
Figure 8.
Parts show: (a) thin windowpane ice over-riding a recently moved rock (January 9, 2014), (b) water creeping onto the low gradient northern shore of the pond during the December 20, 2013 rock movement (∼11:15 am), (c) ice windrows on the eastern shore of Racetrack Playa near the weather station (∼1 pm, December 20, 2013); ice panels are ∼2–3 mm thick and mixed with mud and rocks scoured from the pond bottom, (d) rock carving a wake through ice that is moving left to right; open rippled water in the foreground (January 9, 2014). Images have been cropped but not otherwise edited.
Figure 9.
Gusts and average wind velocities.
Comparison of average hourly wind velocity (black, as shown in Fig. 4) and hourly maximum wind velocity (green) against the daily temperature record (red line) and rock movement history (red arrows). Data reported in Table S1.