SOME 50 million years ago, a gigantic mass of rock – several kilometres thick, 10 to 20 km wide and more than 55 km long – slid tens of kilometres, making it the biggest rockslide on land ever discovered. Geologists have finally figured out how so much rock slid so far despite the extremely gentle slope of less than 2 degrees.
The Heart Mountain landslide occurred in what is now the north-western corner of Wyoming. Edward Beutner, a retired geologist from Franklin and Marshall College in Lancaster, Pennsylvania, and his colleague have found evidence that the landslide was lubricated.
When the rock started moving, the friction heated the limestone at the base of the sliding mass, breaking down calcium carbonate into calcium oxide and carbon dioxide. The high temperatures and pressures turned the carbon dioxide into a supercritical fluid, giving the rock a nearly friction-free ride. When the rockslide eventually slowed, the carbon dioxide cooled and recombined with the calcium oxide to form the carbonate cement now found in the fault zone (Geological Society of America Bulletin, vol 117, p 724).
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