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Difficulty: MediumIdentifying Core Claims and Hypotheses

### Racetrack Playa Sailing Stones

Racetrack Playa is a dry lakebed in Death Valley where rocks, ranging in weight from a few ounces to hundreds of pounds, slide across the flat clay surface, leaving long tracks behind them. No one has observed the rocks in motion in person, leading to several hypotheses.

Scientist 1
The stones are moved by the direct action of strong winds on the stones themselves. During winter storms, brief periods of rain cover the playa with a thin layer of water, transforming the clay surface into an extremely slick mud. When wind speeds exceed 75 km/h75\text{ km/h}, the force of the wind overcoming the minimal friction of the wet clay is sufficient to push the stones forward. The direction and shape of the tracks are entirely determined by the changing directions of these high-velocity wind gusts.

Scientist 2
Wind alone cannot push heavy stones across wet clay, even with reduced friction. Instead, water on the playa must freeze into thin, floating ice sheets during cold winter nights. These ice sheets lock the stones in place. As the ice begins to melt and break up under sunny conditions, strong winds push the large sheets of ice. Because these ice sheets have a massive surface area, the wind exerts a force large enough to drag the ice sheets, along with the embedded stones, across the slippery mud below.

Scientist 3
The movements are initiated by localized atmospheric vortices, or dust devils, that form over the playa. These vortices create sudden, rotating low-pressure centers directly above the rocks. The drop in pressure reduces the normal force of the rock on the muddy playa surface, effectively making it lighter, while the high rotational wind speeds of the vortex spin and propel the rock forward. Once the initial friction is overcome, the rock slides along the slick clay, guided by the vortex's path.

Based on the hypotheses presented, which of the following statements best describes the core claim of Scientist 2 regarding how wind is able to move the stones?

  1. A
    Wind directly pushes the stones across the clay surface once rainwater has made the mud slick enough to minimize friction.
  2. B
    Wind forms localized low-pressure vortices that decrease the normal force on the stones and spin them forward.
  3. Wind pushes large, thin ice sheets that have frozen around the stones, utilizing the ice's large surface area to drag the embedded stones across the mud.Answer
  4. D
    Wind creates pressure differentials that cause underground ice reservoirs to melt and produce sliding mud flows.

Answer

Wind pushes large, thin ice sheets that have frozen around the stones, utilizing the ice's large surface area to drag the embedded stones across the mud.
Scientist 2 claims that wind alone cannot push the stones directly. Instead, Scientist 2 proposes that thin ice sheets freeze around the stones, and the wind pushes these large ice sheets. The large surface area of the ice sheets allows the wind to exert a force large enough to drag the ice sheets and the embedded stones across the slick mud. This aligns with the option describing wind pushing large, thin ice sheets to drag the stones.

Step-by-Step Solution

1
Locate the hypothesis of Scientist 2 in the passage.
Scientist 2 argues that wind alone cannot push the stones directly, but instead acts on thin, floating ice sheets that freeze around the stones.
This establishes the target viewpoint for the question.
2
Identify the primary mechanism described by Scientist 2.
Scientist 2 states that the wind pushes the large surface area of the ice sheets, which drags the embedded stones across the slippery mud below.
This matches the option stating that ice sheets are used to drag the stones.

Key Concept

Identifying Core Claims and Hypotheses
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Topics

Identifying Core Claims and Hypotheses

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