Millions of years ago, the sloping ground of southwest Utah was frequently covered with ash and debris, as a result of the intense volcanic activity taking place in the area. Those ash and debris layers were deposited on top of deeper deposits of hardened ash, which after eroding into slippery clay, set the ground for what now scientists believe was the world's largest landslide, covering an area nearly 39 times the size of Manhattan!
Between 20 to 30 million years ago, a series of active volcanoes were erupting in southwestern Utah, leaving behind lava, ash and debris, forming the Marysvale volcanic field. Within the thick veins of lava, ash and mud, masses of fractured rock, with size ranging from boulders to entire mountains, led scientists to believe that were the signs of many ancient landslides, occurring at different times.
Recent fieldwork performed by Robert Biek and his team, with the Utah Geological Survey, suggest that the above signs are part of one major massive landslide, also referred to as the Markagunt gravity slide. The fractured rock layer is encountered across 3400 square kilometers in the area, summing up to between 1700 and 2000 cubic kilometers of material. Regarding the time that this landslide occurred, Biek notes that the rock is sandwiched between volcanic ash layers that are well dated. The lower hardened ash layer is about 22 millions years old, while the younger ash layer is believed to have been deposited about 21.6 millions ago, setting the time of the landlside between those two dates.
Sources: news.sciencemag.org, geology.gsapubs.org, Smithsonianmag.com
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