A Mountain Slid 45 km Across Flat Ground in Half an Hour

The Heart Mountain landslide carried a slab of rock a kilometre thick across forty-five kilometres of ground sloping two degrees — flatter than most driveways — in roughly half an hour. On a slope that shallow, friction says the block should have locked within the first few hundred metres. It did not, and the few centimetres of pale powder it rode on are still there. The block broke away along a bedding plane inside dolomite: the strongest unit in the column, and the last place a fault is supposed to form. It crossed the Eocene floodplain of what is now the Bighorn Basin, came apart into roughly fifty pieces, and spread from about 1,100 square kilometres of source area to more than 3,400. Those pieces sit today on much younger basin mudstone, with nothing connecting them to the range they came from. How it moved is written in the basal layer: carbonate grains coated in a turbulent gas cloud, powder injected upward into the blocks above, calcite twins agreeing on one direction. Heat from sliding drove the dolomite to decompose and release carbon dioxide under a kilometre of rock, and the slab rode on its own exhaust. The slide was not overcoming friction. It was using it. The dating is stated plainly, because this site is heavily misquoted, and so is what remains contested: hot pressurised water instead of carbon dioxide, fluidisation from the volcanic field, frictional melting, and a minority case for slow emplacement with no half hour at all. Inside a Planet reconstructs single days in the deep past from the physical evidence they left behind. CHAPTERS 0:00 Two Degrees of Slope 2:05 Friction Says It Cannot Move 3:40 Eocene Wyoming, Ash and Dolomite 5:05 The Bedding Plane Parts 6:10 Fifty Blocks, Spreading 8:15 Grains That Formed in Gas 10:05 Ninety Kilometres an Hour 10:55 Friction as the Engine 13:35 Why It Stopped So Fast 14:40 Water, Volcanism or Melt 17:30 Dating the Slide Twice 18:20 Standing on Heart Mountain WHAT THIS IS BUILT ON Sixty years of published work on the detachment, and the disagreement inside it: Pierce, USGS, 1950s–1980s — the mapping that established the three-part geometry and the case for a transported block Beutner & Gerbi 2005, GSA Bulletin — the basal layer, and the argument for emplacement in under thirty minutes Aharonov & Anders 2006, Geology — the competing case for hot pressurised water Craddock, Malone, Magloughlin et al. 2009, GSA Bulletin — direction of travel and strain across the transported mass Goren, Aharonov & Anders 2010, JGR Solid Earth — the thermo-mechanical model: heating, pressurisation, carbonate breakdown Where these disagree, the episode says so rather than choosing for you. WATCH NEXT Every River on Earth Was Once Straight —    • Every River on Earth Was Once Straight   #HeartMountain #Landslide #Geology #EarthHistory