Where did a billion years of geological history go?

Geologists look at rock formations as an encyclopedia or a record that explains things that have happened. But, they’ve noted that some pages of the encyclopedia are missing. Something that has been a source of confusion since the 1860s is the Great Unconformity. John Wesley Powell first discovered these missing pieces of the geological record in the Grand Canyon.

The missing pieces add up to a billion years of history in some places, and geologists want to know where it went. In a study published in the “Proceedings of the National Academy of Sciences,” researchers from the University of California Santa Barbara, the University of Colorado Boulder, and Colorado College think they may have eliminated a popular hypothesis about what happened to the missing material.

"There are unconformities all through the rock record," said geologist Francis Macdonald, a professor in the Department of Earth Science at the University of California Santa Barbara. "Unconformities are just gaps in time within the rock record. This one's called the Great Unconformity because it was thought to be a particularly large gap, maybe a global gap."

Using granite from just below the Great Unconformity at Pikes Peak in Colorado, researchers tested the hypothesis that glaciers scoured away the rock around 720 to 635 million years ago, using thermochronology. Scientists used the ratio of helium to thorium and uranium to track the movement of rock in the crust as it was buried and eroded over time. The analysis of samples revealed that several kilometers of rock had been eroded from above this granite between 1,000 and 720 million years ago.

Macdonald found that this large-scale erosion was driven by the formation and separation of supercontinents, but noted that the supercontinent processes don’t happen simultaneously.

"It's a messy process," Macdonald said. "There are differences, and now we have the ability to perhaps resolve those differences and pull that record out."