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By Michael Marshall

If falling rocks are big enough and hit the ground hard enough they can create a blast so intense that the rocks are pulverised into powder. Such extreme rockfalls are followed by a shockwave that can snap trees hundreds of metres away.

“They’re extremely weird phenomena, which have been somehow overlooked,” says Fabio De Blasio of the University of Milano-Bicocca in Italy.

The first known example took place in Yosemite National Park, California, on 10 July 1996. Two large masses of rock fell from Glacier Point and plummeted 665 metres. When they hit the ground they released a blast of air that snapped or toppled about 1000 trees, including some half a kilometre away. This was followed by a cloud of abrasive sand, which scoured the fallen trees.

The rockfall was described by researchers in 2000 but it remained a curiosity. Now De Blasio and his colleagues have identified 21 other “extremely energetic rockfalls” from the past two decades, mostly from the European Alps and the Dolomites in Italy. They argue that these extreme events are more common than had been thought.

Mother of all bombs

The crucial point is that the impact of the rocks on the ground is so violent that they are smashed into powder. This needs a big mass of rocks, on the order of 10,000 cubic metres, to pick up speed by falling several hundred metres. “Typically they will develop in areas where erosion has been quite fast,” says De Blasio. Yosemite is a steep gorge eroded by the Merced river, so it has lots of steep cliffs that are ideal.

Based on physical calculations, De Blasio and his colleagues estimate that just one of these extreme rockfalls can release more than 80 billion joules of energy: more than any non-nuclear bomb. This even includes the US Air Force’s notorious “Mother of all Bombs”, which is estimated to release about 46 billion joules – and was used for the first time in a strike on Afghanistan in April 2017.

Up to 18 per cent of the energy from an “extremely energetic rockfall” is spent disintegrating the rocks, and almost all the rest powers the air blast and dust cloud.

The dangers are obvious, says De Blasio. There is the initial shockwave of air and rush of dust and sand – which can travel at 100 metres per second, comparable to a pyroclastic flow from a volcano. Then the dust cloud can last hours or even days, because the particles are so small and light, impacting visibility on roads.

Journal reference: Journal of Geophysical Research: Earth Surface, DOI: https://doi.org/https://www.newscientist.com/article/2182017-falling-rocks-can-explode-so-hard-that-only-nuclear-weapons-beat-them/10.1029/2017JF004327