About 25 million years ago, active volcanoes occupied much of southwest Utah and, as ScienceNOW describes, their frequent belches covered the sloping ground with ash and debris. Underneath those layers laid a deeper deposit of hardened ash, eroding into slippery clay. When enough weight built up on top of that old ash, it began to slide.
According to recent research, this event triggered the largest known landslide in our planet's history—an event, according to ScienceNOW, "of almost unimaginable proportions." More than 1,300 square miles, the equivalent of 39 Manhattans, were affected by the landslide, which moved as many as 700 cubic miles of ashy, rocky debris, ScienceNOW says.
This massive movement of earth, which geologists call the Markagunt gravity slide, likely originated with a perfect storm of landslide-prone conditions. But it was not an isolated occurrence. A similarly large landslide, the Heart Mountain event, took place millions of years ago in Wyoming—these two now rank as the two largest known landslides history.
The team points out in the new paper that these events "constitute a class of catastrophic collapse hazard not widely recognized within modern volcanic fields." In other words, it wouldn't hurt to take a lesson from the past and keep our eye on sites that share a worryingly similar geology today.