For decades, geologists have noted the signs of ancient landslides in southwestern Utah. Although many parts of the landscape don’t look that odd at first glance, certain layers include jumbled masses of fractured rock sandwiched among thick veins of lava, ash, and mud. Now, new fieldwork suggests that many of those ancient debris flows are the result of one of Earth’s largest known landslides, which covered an area nearly 39 times the size of Manhattan.

Between 20 million and 30 million years ago, southwestern Utah was home to active volcanoes. It isn’t clear how many peaks were erupting during that time, but together they left behind lava, ash, and other material that geologists have dubbed the Marysvale volcanic field.

Previously, scientists had noted that large volumes of broken rock—including everything from boulder-sized bits to mountain-sized chunks—covered as much as 500 square kilometers of the region. Many of those masses were presumed to come from different sources at different times, says Robert Biek, a geologist with the Utah Geological Survey in Salt Lake City. But now, an analysis by Biek and his colleagues hints that the ancient slumps were all part of one massive, catastrophic collapse.

Material from that landslide is sandwiched between volcanic ash that has been well dated, Biek says. The slide debris sits on top of a now-hardened layer of ash that fell about 22 million years ago. The ash layer atop the masses of broken rock fell about 21.6 million years ago. So, the researchers contend, the immense landslide happened sometime between those two dates.

Altogether, the broken rock can be found across 3400 square kilometers of the region. The volume of rock involved, somewhere between 1700 and 2000 cubic kilometers of material, makes the ancient slide one of the largest known to have occurred on land anywhere in the world, the researchers report in the current issue of Geology.

The ancient event has been dubbed the Markagunt gravity slide, Biek says. (Markagunt, the name of a plateau in the region, means “highland of the trees” in the language of the Paiute Indians who are native to the area.) Geological evidence gives scientists an idea of how the event occurred. First, the terrain at the time of the slide sloped, in general, slightly toward the south. Second, there’s a relatively thin layer of heavily weathered volcanic ash on the lower edge of many flows, Biek says. After long exposure to the elements, some of that material had degraded into slick clay, which acted as a lubricant for the landslide once it began.

Eventually, the accumulation of massive volumes of volcanic material on top of the region’s slick, sloping terrain led to a slide of almost unimaginable proportions. About one-fourth of the volcanic plateau collapsed and gave way during the event, Biek says. The front end of the slide, where material is dozens of meters thick, spilled southward more than 30 kilometers. In some places, the friction generated by the surging slide produced so much heat that it melted the surfaces of rocks into 2- to 5-centimeter-thick layers of glass.

What specifically triggered the Markagunt gravity slide is still a mystery, Biek says. It’s possible that molten rock rising toward Earth’s surface pushed the flanks of the area’s volcanoes upward as well as sideways, fracturing their rocks and creating an increasingly unstable pile of material ripe for a landslide, he notes.

Geologists have long suspected that a similarly large slide occurred in northwestern Wyoming millions of years ago, says Gary Smith, a geologist at the University of New Mexico, Albuquerque. That event, often called the Heart Mountain slide, was previously considered the largest landslide known to have taken place on land. “So while these [large] events are not common, they’re not unique,” he notes.

Ongoing fieldwork suggests that the Markagunt slide is substantially larger than first estimated, Biek says. That would dethrone the Heart Mountain slide as the world’s largest, he notes.

But the size of the Markagunt gravity slide isn’t the only interesting thing about the new study, Smith says: “It never ceases to amaze me how geologists can go into an area we think we know well and find new things.”