We all experience moments of temporary amnesia, like blanking on the name of an acquaintance or forgetting why we walked into a certain room. But these mental slips are nothing compared to epic and mysterious lapses in Earth’s memory, which can span hundreds of millions of years.
These gaps in our planet’s geological record, known as “unconformities,” are inferred from rock layers that originate from vastly different time periods stacked directly on top of each other, sometimes separated by upwards of a billion years, suggesting that natural forces somehow prevented preservation of sediment from the intervening eras.
These unconformities are tantalizing because they hint at a lost past, perhaps filled with revelatory insights about the evolution of Earth and its lifeforms. But in addition to telling us what we don’t know, the gaps can help scientists learn how to read our planet’s memory.
“One big picture question is the degree to which the Earth’s sedimentary rock record is complete versus a fragmentary one with substantial sections removed by erosion,” noted Rebecca Flowers, a geologist at the University of Colorado Boulder, in an email. “Another is to better understand the links between surface processes (such as erosion), deep Earth processes, and long-term biological, climatic, and environmental change.”
Flowers is the lead author of a study published on Monday in Proceedings of the National Academy of Sciences that reveals new insights about one of the most famous of Earth’s blackouts, the Great Unconformity, which is seen (or rather, not seen) in rocks around the world.
This gap extends from roughly 550 million years ago—an era just before the emergence of complex life—to more than a billion years ago when simple microorganisms still had Earth to themselves.
Scientists have previously hypothesized that the missing time was deleted by erosion from the so-called “Snowball Earth” phases of our planet’s history, when glaciation iced out the globe in at least two cycles between 715 and 640 million years ago.
But Flowers and her colleagues propose that the gap was created by “regional tectonic features rather than a synchronous global phenomenon,” according to the new study. The team came to this conclusion by examining the unconformity in a granite outcrop at Pikes Peak in Colorado, though that is not the only location that contains hidden secrets about this deleted past.
“We are actively working on other sites in North America, including the Grand Canyon, where the iconic Great Unconformity is perhaps most famous,” Flowers said. “We then plan to target sites on other continents.”
“The goal of this additional work is to determine if there was a massive, globally synchronous erosion event as some have proposed that lead to a singular ‘Great Unconformity’ or if there are multiple ‘Great Unconformities’ that developed at different times, in different places, with different causes,” she added.
To constrain this problem, the team studied samples of minerals and crystals from the rocks, such as hematite and zircon, which can be used to reconstruct the thermal history of the sediment layers. The results revealed that the older “basement” rocks at Pikes Peak had eroded before the first Snowball Earth and “therefore cannot be a product of glacial erosion,” according to the study.
The study also casts doubt on a hypothesis that erosion associated with the Great Unconformity may have seeded Earth with nutrients that sparked the Cambrian explosion, an event that marked the sudden emergence of complex life about 541 million years ago.
“If major erosion occurred several hundred million years before the Cambrian explosion, then it suggests that these events (the Cambrian explosion and Great Unconformity erosion) are not linked,” Flowers explained. “Our results indicate that at Pikes Peak in Colorado, the Great Unconformity erosion surface formed several hundred million years before the Cambrian explosion.”
The team suggests that tectonic processes associated with the formation and breakup of Rodinia, a supercontinent that existed from about one billion years ago to the Snowball Earth era, may have caused the Pikes Peak memory wipe. But it will take more research to tease out all of the uncertainties of the Great Unconformity—or rather, Unconformities.