According to the passage, why did the idea that wind alone moved the rocks seem unlikely?
Read an Informational Passage · ELPAC Grades 6–8 reading. The correct answer, and the reasoning behind it.
A longer factual text, with questions about the main idea, supporting details and vocabulary.
Read the passage. Then answer the questions.
The Rocks That Move by Themselves
On a dry lakebed in Death Valley National Park called Racetrack Playa, hundreds of rocks sit at the ends of long, shallow trails. Some trails run straight for hundreds of feet; others curve or zigzag. Some of the rocks weigh more than a hundred pounds. For decades, nobody had ever seen one move.
People offered explanations of every kind. Some suggested powerful winds; others proposed that sheets of ice had dragged the rocks, or that the ground itself shifted. A few blamed pranksters. Each idea had a weakness. Winds strong enough to push a heavy rock across dry ground would have to be far stronger than any ever measured there, and no one could explain how pranksters would leave no footprints in the soft mud.
In the early 2010s, a team of researchers decided to stop guessing and start watching. They placed rocks fitted with GPS trackers on the playa, set up a weather station and time-lapse cameras, and waited. Then, one winter, it happened. A shallow layer of rainwater had collected on the playa and frozen overnight into ice only a few millimeters thick. As the morning sun warmed it, the ice broke into large, thin panels. A light breeze pushed the floating panels, and the panels pushed the rocks, which slid slowly through the mud beneath the water and carved new trails.
The answer turned out to combine pieces of several older ideas: ice, wind and water all played a part. But none of those ideas had been right on its own, and it took patient observation, not clever arguments, to show how the pieces fit together.
The question
Choose the best answer.
According to the passage, why did the idea that wind alone moved the rocks seem unlikely?
- Wind never blows across Racetrack Playa in winter.
- The trails curve, and wind can only push objects in straight lines.
- It would take winds far stronger than any ever measured there.Correct answer
- The rocks were too small to be pushed by the wind.
Why “It would take winds far stronger than any ever measured there.” is correct
This tests locating the specific weakness the author gives for one explanation. The second paragraph says winds strong enough "would have to be far stronger than any ever measured there." The passage lists several weaknesses for several ideas, so the student has to match the right weakness to the wind explanation.
Why the other answers are wrong
- Wind never blows across Racetrack Playa in winter. — A light breeze pushes the ice panels in winter, so wind does blow there.
- The trails curve, and wind can only push objects in straight lines. — The trails do curve, but the passage never uses that as an argument against wind.
- The rocks were too small to be pushed by the wind. — The rocks are described as heavy, some over a hundred pounds, which is the opposite problem.
Practice more of this
More reading practice for Grades 6–8, or see the ELPAC practice test for grades 6–8. For other grades, see free ELPAC practice tests by grade.
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