The Grand Canyon of the Colorado, which receives several million visitors a year, is also home to one of the most fascinating mysteries for geologists around the world. They call it the ‘Great Unconformity’ – a mysterious and lost time gap in the canyon’s rock record that spans hundreds of millions of years . Now, a study published in the journal Geology proposes a plausible explanation for this phenomenon.
“The cliffs of the Grand Canyon are like an Earth history textbook,” explains Barra Peak, lead author of the new study and a graduate student at the University of Colorado Boulder. “If you climb the rocky canyon walls, you can go back almost two billion years into the planet’s past. But that textbook is also missing pages: in some areas, more than a billion years of rocks have disappeared from the Grand Canyon without a trace . “
According to the team of researchers, a series of small but violent faults could have rocked the region during the disintegration of an ancient supercontinent called Rodinia. The resulting chaos would tear apart the land around the canyon, causing rocks and sediment to pour into the ocean. “We have new analytical methods in our lab that allow us to decipher the story in the missing window of time through the Great Unconformity,” says Rebecca Flowers, co-author of the new study and professor of geological sciences.
The lines that make up the Great Unconformity can be seen with the naked eye from the river, as Peak, who took a rafting trip through the Grand Canyon last spring, explains: “At the bottom, you can see very clearly that there are rocks. that have come together. Its layers are vertical. Then there is a boundary, and on top you have these beautiful horizontal layers that make up the hills and peaks that you associate with the Grand Canyon. ” The difference between these two types of rocks is significant. In the western part of the canyon towards Lake Mead, the basement stone is between 1.4 and 1.8 billion years old. However, the rocks at the top are only 520 million years old. Since this geological anomaly was first described more than a century ago, scientists have seen evidence of similar periods of lost time at sites around North America.
A continent is divided
To explore the transition, Peak and his colleagues used a method called “thermochronology,” which traces the history of heat in the stone. Peak explained that when geological formations are buried deep, the pressure that builds up on them can cause them to heat up. That heat, in turn, leaves a trace on the chemistry of the minerals in those formations.
Using this approach, the researchers conducted a study of rock samples collected throughout the Grand Canyon. They found that the history of this feature may be more complicated than scientists have assumed. In particular, the western half of the canyon and its eastern part (the part that tourists are most familiar with) may have suffered different geological contortions over time.
“It is not a single block with the same temperature history,” they explain. About 700 million years ago, the basement rock in the west would have risen to the surface. In the eastern half, however, that same stone was buried. under miles of sediment. The difference was likely due to the rupture of Rodinia, a gigantic land mass that began to separate at around the same time. The researchers’ results suggest that it may have torn apart in the eastern and western halves of the Grand Canyon. in different ways and at slightly different times, producing the Great Unconformity in the process.The team of scientists now intends to investigate other areas of the Great Unconformity in North America to see if their results can be extrapolated.
