JACKSON, Wyo. – Stepping back to when the Earth was just half as old as it is today, the tectonic plates that Jackson Hole now rests upon were located in a place that nobody yet knows.
The planet’s terra firma at the time would have looked indistinguishable from the mosaic of land masses and water of the present, and geophysicists are still trying to determine how the continents have drifted around since. What is now known, however, is that about 2.68 billion years ago there were two continents that collided on the tectonic plates that now underlie the Teton Range.
“We found something in the Tetons that we had never seen before,” University of Wyoming geophysicist Carol Frost said. “And that is, there were two really different-looking blocks of rock. One of them looked like it was really old and full of mud that had come off of a continent. The other one looked, relatively speaking, younger.”
Between the blocks were granite rock formations, an indication that two continental plates were stacked on top of each other.
“The plate underneath was a lot warmer,” Frost said. “You start to get the rocks melting, and these granites represent the melted crust.”
The evidence of an ancient continental collision was discovered in the northern, less-traveled reaches of the Tetons in the Moose Basin area. The once-heated rocks implicated in the find are Bitch Creek and Webb Canyon gneiss, which are forms of “leucogranites.”
The emergence of the Tetons about 9 million years ago carried with it rocks that, at one point, were about 25 miles beneath the surface of the Earth. Now exposed, those formations are what Frost and colleagues first came across in the mid-1980s. The professor was pointed in the direction of the northern Tetons by U.S. Geological Survey researcher John Reed.
“He told us that there’s some rocks up there in the high peaks in the Moose Basin area that are different from anything else,” Frost told the Jackson Hole News & Guide. “He was absolutely right.”
A National Science Foundation grant acquired about a decade ago enabled Frost and her students to pour their energy into studying the formation. One culmination of the work was an academic paper that published in the January edition of the journal Geochimica et Cosmochimica Acta.
The study compared the exposed Teton Range granites with similar rock in the Himalayas, which jut up to form the largest mountain range on Earth as a result of a continental collision. But the Himalayan collision is much, much younger.
Because of its extremely old age, the Moose Basin continental collision has broader significance in the field of plate tectonics. Its 2.68 billion-year-old age falls in the Archaen Eon, which is around the time when oxygen appeared on the 4.54 billion-year-old Earth. At that time — four “supercontinents” ago — scientists believe a land mass known as Kenorland was in the process of assembling.
“One of the big questions in geology is, ‘How far back in time did modern-style plate tectonics happen?'” Frost said. “So the real finding of this paper is that it now pushes modern plate tectonics back as far as 2.7 billion years ago.”
Previously, Frost said, the oldest well-established evidence of a continental collision was a 1.1 billion-year-old Grenville rock formation in eastern North America.
For identifying the oldest known continent-continent collision on Earth, Frost thanked Wyoming and its well-exposed and ancient geology. The underlying Wyoming Province, she said, contains the oldest rocks found in the United States.
“The state of Wyoming is a geologic treasure-house,” Frost said. “When it comes to really understanding the origins of the Earth and the early history of its continents, Wyoming is the easiest and best place to get on those rocks.
“I think our understanding of the early continental crust,” she said, “a lot of those details will emerge from the Wyoming Province.”