Some things can be too big to notice, as our flat-Earth-believing ancestors can attest, having failed to work out that the surface of the Earth curves around a sphere. Or, as the saying goes, you can focus on the details of some fascinating trees and miss interesting facts about the forest as a whole.

In southwest Utah, geologists had noticed some pretty cool “trees.” The area had been volcanically active between 21 and 31 million years ago, building up a host of steep, volcanic peaks. A number of huge blocks of rock from these peaks, up to 2.5 square kilometers in area and 200 meters thick, are obviously out of place—they've been interpreted by geologists as the result of many landslides around the volcanoes. In a recent paper in Geology, David Hacker, Robert Biek, and Peter Rowley show that rather than being the result of many individual landslides, these are actually all part of one jaw-droppingly large event.

The deposit, called the Markagunt gravity slide, covers an area about 90 kilometers long and 40 kilometers wide and is hundreds of meters thick. During the event, all of this slid 30 kilometers or more. The scale puts run-of-the-mill landslides—as terrifying and deadly as they can be—to shame.

The slide is a huge sheet of various types of volcanic rock that broke loose and slipped along a clay-rich sedimentary layer beneath. As the lower portion slowed to a stop, the upper layers carried on, settling at the far end of the slide.

At the base, the signs of catastrophe are everywhere. There’s a jumble of broken up rock up to 4 meters thick, and even zones of completely pulverized rock. Rock was shattered, sheared, and injected into fractures. In places where the friction was especially intense, rock was even melted, cooling to form thin layers of glass. But that kind of damage was localized—most of the slide is composed of huge blocks, each covering several square kilometers. Other than some faults (some of which formed after the slide), they’re pretty much upright and intact.

Walking along the surface of the slide, the upper layers sit across a zone of ramp-like faults that cut upward through 150 meters of rock. This upper portion also contains some huge blocks, lots of fractures, and sheared zones. Mapping out the whole area, a consistent picture emerges—one of a single deposit that extends far beyond the area that had previously been described as containing many smaller slides.

Not a lot is known about what triggered the slide. The original tilt of the layers could only have been a few degrees, so a lot of rock must have started moving at the same instant in order to provide enough momentum to bring the whole thing down. They see four conditions that could lead to this kind of event. First, you build up a thick wedge of material ejected by the volcanoes (thickest close to the source). Second, you have some kind of weak layer below (the clay-rich sedimentary rock, in this case), which is tilted at least a little. Third, a swelling of magma beneath the field of volcanoes pushes the land surface upward in a dome, putting stress on the rocks around the sides. Finally, faults and fractures could form for several reasons, compromising the integrity of the rock. With all that in place, all you need is a violent eruption or earthquake to come along and set the slide in motion.

The Markagunt gravity slide is the largest known slide on land anywhere on Earth—tied with the Heart Mountain gravity slide in northwest Wyoming, that is. The similarity between the two goes beyond size. The Heart Mountain slide also occurred in an active volcanic field with the same characteristics.

That tells us that we might want to understand these events better, which the researchers say “constitutes a class of catastrophic collapse hazard not widely recognized within modern volcanic fields.”

Geology, 2014. DOI: 10.1130/G35896.1 (About DOIs).