A Find that Rocks

Ultrahigh temperature metamorphic rocks were not thought to have existed in the United States until now, geologists report in the December 13 issue of the journal Geology. Jay Ague, a professor at Yale University who specializes in metamorphic rocks, came upon the rocks on a stretch along Interstate 84 through northeast Connecticut toward the Massachusetts border. Ague walked over them for nearly 20 years without recognizing their unique history—which he believes stems from a mountain range much like the Himalayas that once towered across the Northeastern United States. The find is the first discovery of ultrahigh temperature metamorphic rocks in the United States.

“The fact that these rocks are there at all challenges all the existing models for mountain building in the area,” Ague, the lead author of the study, said. “These ultrahot (rocks) are becoming an important part of how we think mountain belts form,” he told OurAmazingPlanet.

By chance, Ague recently brought some of the Connecticut metamorphic rock—known as gneiss—back to his lab in Yale, where he and his colleagues examined them under a microscope. They discovered garnets in the gneiss had patterned inclusions of rutile, a mineral that also gives star sapphires their special appeal. “They are really quite beautiful,” Ague said. “Those features in garnets are really only associated with really extreme temperature or pressure conditions, so right there we knew there was something special about these rocks,” Ague said. “It was a completely serendipitous discovery.”

Based on minerals in the gneiss, they seem to have formed at a minimum depth of about 20 miles below the surface, with exposures to extreme 1,800 degree Fahrenheit temperatures—the maximum a rock can handle before it melts. To form that kind of heat at a 20-mile depth, an extremely radioactive crust may radiate excess heat, or a subduction zone may form hot magma to heat the rocks, or severely twisting rocks during mountain-building may produce heat, like how bending a paper clip back-and-forth makes it feel warm, Ague said. While Ague estimates that these rocks are roughly 400 million years old, their exact age—and what drove them to such extreme temperatures—remain uncertain.

“We can see back through all that history to unravel the earliest beginnings of how this rock formed,” Ague said. “These rocks began as basically muds on ancient ocean floors in very chilly conditions. Then they were buried and completely recrystallized up to 1,000 degrees [Celsius], and then subject to two more metamorphic and deformational events, then ultimately brought back to the surface so we can see them here today.”