Causes
Several factors led to the Frank Slide. A study conducted by the GSC immediately following the slide concluded that the primary cause was the mountain's unstable anticline formation; a layer of limestone rested on top of softer materials that, after millions of years of erosion, resulted in a top-heavy, steep cliff. Cracks laced the eastern face of the mountain while underground fissures allowed water to flow into the mountain's core. Miners noticed the mountain had become increasingly unstable in the months preceding the slide; they felt small tremors and the superintendent reported a "general squeeze" in the mountain at depths between 1,100 metres (3,600 ft) and 1,500 metres (4,900 ft). They found that coal broke from its seam; it was said to practically mine itself.
An unusually warm winter, with warm days and cold nights, was also a factor. Water in the mountain's fissures froze and thawed repeatedly, further weakening the mountain's supports. Heavy snowfall in the region in March was followed by a warm April, causing the mountain snows to melt into the fissures. GSC geologists concluded that the weather conditions that night likely triggered the slide. The crew of the freight train that arrived at Frank shortly before the disaster said it was the coldest night of the winter, with overnight temperatures falling below −18 °C (−0 °F). Geologists speculated that the cold snap and rapid freezing resulted in expansion of the fissures, causing the limestone to break off and tumble down the mountain.
Though the GSC concluded that mining activities contributed to the slide, the facility's owners disagreed. Their engineers claimed that the mine bore no responsibility. Later studies suggested that the mountain had been at a point of "equilibrium"; even a small deformation such as that caused by the mine's existence would have helped trigger a slide. The mine was quickly re-opened, even though rock continued to tumble down the mountain. Coal production at Frank peaked in 1910, but the mine was permanently closed in 1917 after it became unprofitable.
The slide created two new peaks on the mountain; the south peak stands 2,200 metres (7,200 ft) high and the north peak 2,100 metres (6,900 ft). Geologists believe that another slide is inevitable, though not imminent. The south peak is considered the most likely to fall; it would likely create a slide about one-sixth the size of the 1903 slide. The mountain, continuously monitored for changes in stability, has been studied on numerous occasions. The Alberta Geological Survey operates a state of the art monitoring system utilized by researchers around the world. Over 80 monitoring stations have been placed on the face of the mountain to provide an early warning system for area residents in case of another slide.
Geologists have debated about what caused the slide debris to travel the distance it did. The "air cushion" theory, an early hypothesis, postulated that a layer of air was trapped between the mass of rock and the mountain, which caused the rock to move a greater distance than would otherwise be expected. "Acoustic fluidization" is another theory, which suggests that large masses of material create seismic energy that reduces friction and causes the debris to flow down the mountain as though it is a fluid. Geologists created the term "debris avalanche" to describe the Frank Slide.
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