Ancient Utah landslide provides insights into catastrophic collapse

Ars Technica : At 90 km long and 40 km wide, the Markagunt gravity slide in southwest Utah is a manifestation of the largest known landslide on Earth. During the event, which happened about 21 million to 22 million years ago, rocks up to 2.5 km² and 200 m thick were sent sprawling some 30 km or more. Because the debris field is so large, geologists had thought that it represented a series of individual landslides. In a recent paper published in Geology, however, a group of researchers propose that it was the result of a single, catastrophic event. Over time, they say, volcanic eruptions may have built up a thick wedge of material on top of the clay-rich sedimentary soil, which is structurally weaker. Pressure from the underlying magma then pushed the surface upward, which in turn put stress on the rocks and created faults and fractures. A violent eruption or earthquake could have then set the whole thing in motion. The researchers say the site could provide insights into the potential hazards of such catastrophic collapse.