TOEFL統合ライティング

📖 読解文章

The concept of 'sailing stones' in Death Valley, California, has intrigued scientists for decades. These rocks, some weighing up to 300 kilograms, leave long trails on the dry lakebed of Racetrack Playa, yet no one has ever observed them moving. One popular theory proposes that strong winds, reaching speeds of over 100 kilometers per hour, are sufficient to push the stones across the slippery, wet mud after rainfall. Supporters argue that the flat, smooth surface of the playa reduces friction, allowing even moderate gales to slide the rocks. A second hypothesis suggests that ice sheets, formed during freezing winter nights, lift the rocks and carry them along as the ice breaks up and drifts. This mechanism, known as ice rafting, has been observed in other polar and alpine environments, where stones embedded in ice move with the thawing flow. Finally, a third explanation involves seismic activity: micro-earthquakes in the region could generate vibrations that cause the rocks to inch forward gradually over time, a process similar to soil liquefaction. While each theory has its proponents, none has been confirmed by direct observation, and the exact cause remains a mystery.

🎧 リスニング講義

Professor: The mystery of the sailing stones has been debated for years, but recent research has provided compelling evidence that challenges these older theories. First, the wind theory is implausible because the playa’s surface, while dry, is not uniformly smooth. In fact, high-resolution GPS tracking has shown that the stones move in sudden, jerky bursts, often at speeds of less than one meter per second, which would require wind speeds far exceeding any recorded in the area. Moreover, the trails show sharp turns and even U-turns, which are impossible for a simple wind-driven push. Second, the ice rafting idea, though plausible in colder regions, fails here because Racetrack Playa experiences daytime temperatures that melt ice quickly, and the stones often move in the summer, not winter. Thermal imaging has revealed that the playa’s ground does not cool enough to form thick ice sheets capable of lifting large rocks. Third, seismic activity is too rare and weak in this tectonically stable region to produce the frequent, consistent movement observed. Instead, a 2014 study using motion-activated cameras and GPS sensors showed that the stones move during winter when thin, floating ice panels, formed overnight, are broken by morning sunlight and pushed by gentle breezes. These ice panels, only a few millimeters thick, act as sails, dragging the rocks across the wet mud at low speeds. This mechanism explains the sudden starts and stops, the curved trails, and the lack of high winds or earthquakes.