JACKSON, Wyo. — After a significant section of the Second Tower on the Grand Teton collapsed over the fourth of July weekend, geologist John Willott walked Buckrail through the science behind the incident.
Willott, vice president of Geologists of Jackson Hole, explained how natural processes continue to move hard, metamorphic rocks in the Grand Tetons, which formed approximately 10 million years ago when the Teton Fault began to move. This movement triggered a series of large earthquakes — up to a magnitude 7.5 — and tilted the mountain block upward, while down-dropping the valley block, according to the U.S. Geological Survey (USGS).
The heat and pressure from the collision of tectonic plates over time changed seafloor sediments and volcanic debris into a metamorphic rock called gneiss, per USGS. Magma infiltrated cracks and weak zones in the gneiss, cooled slowly and formed the igneous rock granite. Many of the highest peaks within the Teton Range are made of granite, including Grand Teton, Middle Teton, Mount Owen Teewinot and Mount Moran.
Willott said that hard rock formed in the valley approximately 40 to 60 miles below the earth’s surface, about 2.4 billion years ago.
“At that depth, it gets hot enough for [rock] to become plastic,” Willott said. “As rocks come back up to the surface, they cool and harden. As rocks get closer to the surface, they fracture a little bit because pressure has been relieved.”
Willott said that the Grand Tetons are the youngest mountains in the Lower 48, forming over the last 2 to 4 million years. He said that ancient fractures can be seen on and in rock faces throughout the range. These fractures can continue to expand over many seasons when water gets into cracks and freezes overnight, producing an outward pressure. Each thaw and freeze cycle causes rock to eventually fail until “down it comes.”
Willott said that the Second Tower’s most recent rockfall event was just a continuation from its earlier collapse in the fall of 2022.
“It’s an ongoing process,” Willott said. “Water is the most powerful force in nature.”
Earth has been emerging from the Ice Age over the last 15,000 years, according to Willott. He said that increasingly hot temperatures in the summer coupled with colder overnight temperatures can also cause stress on rock oximes (organic compounds). Willott recalled that temperatures were in the high 80s over the holiday weekend.
“We’ve been steadily getting hotter and hotter,” Willott said. “That stresses everything, from the wildlife, trees, to the rocks.”
Willott said that all rocks are “trying to get back down to lower levels,” but it will take hundreds of millions of years for the Grand Tetons to shrink down to the size of the Appalachian Mountains.
“If you think about the Appalachians back east, they were as tall as the Himalayas 350 million years ago,” Willott said. “Now they are bumps in the road.”
Willott projects that rockfall will continue “on a steady basis.” He says that as chunks continue to fall, new rocks will then be exposed to the water fracturing process. Willott advises climbers to exercise caution while in the mountains.
“What you thought was competent last time around can loosen,” Willott said. “The Tetons can loosen, quite frankly. I know mankind always thinks of rocks as being pretty hard, but everything is subject to erosion.”
According to Willott, the wide, flat top of Mount Moran might take longer to erode because it has fewer crevices for water to creep into, but he anticipates that all of the peaks in the range will continue to loosen over time.
