Riley Black

Science Correspondent

The nature of Earth’s deep past can often feel intangible. From our modern moment, eons billions of years in the past seem hard to touch. Among some of our planet’s rocks, however, are tatters and fragments from those distant times that can offer us a peek at what our planet was like when our ancestors were single-celled organisms. By studying some of these vestiges, geologists have been able to detect what was transpiring under the Earth’s crust over 2.5 billion years ago.

Below our feet—and our planet’s outer crust—Earth’s mantle makes up the vast majority of the planet’s volume. Different layers of the mantle are made up of different rock types, and one of the most common is an igneous rock high in silica content called peridotite. In the past, when geologists have compared samples of prehistoric peridotite from Earth’s mantle and their modern equivalents, they’ve found a significant discrepancy.

When rocks are exposed to oxygen, some of the rock changes through a process called oxidation. You might have seen this on your car or bathroom taps in your home, as rust is the result of oxygen interacting with iron. In the case of peridotite, geologists have found that very old remnants from Earth’s mantle are much less oxidized than those of the modern mantle. Something must have changed between the Archean Eon, more than 2.5 billion years ago, and Earth as we know it today.

Previously, geologists proposed that changes in Earth’s oxygen might explain the shift. Perhaps some significant influx of oxygen sometime in Earth’s history altered the chemistry of the rocks and led to greater oxidation. But geologist and chair of the National Museum of Natural History’s Department of Mineral Sciences Elizabeth Cottrell and colleagues found something different. As they report Wednesday in Nature, new clues suggest that the oxidation change is a sign that Earth’s mantle rocks were melted in extreme heat and then persisted through billions of years.

The telltale rocks were sampled from two spots on the seafloor where Earth’s mantle has been oozing up to create new crust. Some of the rocks are from the Southwest Indian Ridge between southern Africa and Antarctica, and others were collected from Gakkel Ridge near the North Pole. The rocks have been of special interest to geologists because the new crust is forming more slowly at these ridges, heightening the possibility of studying rocks from Earth’s mantle.

Gakkel Ridge rocks (14 mm)
Another look at the ancient rocks collected from Gakkel Ridge near the North Pole, photographed under a microscope and seen under cross-polarized light Elizabeth Cottrell / National Museum of Natural History

The new research got its start as a desire to understand the relationship between oxidation states in Earth’s oceanic crust and mantle, as well as oxygen-related variations in peridotites, says Cottrell. Previous work from the same researchers found that peridotites had extreme variations in how oxygen interacted with them, which led the researchers to the Gakkel Ridge samples. “We discovered peridotites from the Gakkel Ridge that were even more extreme in their chemistry,” Cottrell says, and so the geologists wanted to know what conditions could account for the difference.

Despite being sampled from ridges far distant from each other, the peridotites from both locales are less oxidized than modern mantle rocks and show signs that they had been melted to a much greater degree.

Back during the Archean Eon, between 2.5 billion and 4 billion years ago, Earth’s mantle was much hotter. Geologists have estimated that Earth’s interior was more than 360 degrees Fahrenheit (200 degrees Celsius) hotter than it is today. Such extreme temperatures could have certainly caused the melting seen within the peridotites, while hitting a sweet spot that allowed the rocks to continue to circulate in the mantle without changing further. The rocks formed during the Archean and persisted until they were squeezed out along the ocean ridges more than 2.5 billion years later. Looking at them is getting a peek at Earth when our species was only a distant evolutionary possibility.

Seafloor rock
An ancient rock dredged from the seafloor and studied by the research team Tom Kleindinst

“Peridotite rocks we recovered today, in modern times on the seafloor, record this earlier hot period in Earth’s history,” Cottrell says. It’s not just that the rocks preserve signs of what Earth was like more than 2.5 billion years ago. The study proposes that the samples truly are rocks that melted in the Archean and have been preserved through all the time since.

Ancient pieces of our planet’s mantle have been spotted by other researchers, too. “Such stagnant pieces of ancient mantle have been recognized already some years ago by another group using osmium isotopes,” says geologist Fabrice Gaillard of France’s University of Orléans, who was not involved in the new research. The new study offers another look at the conditions in Earth’s mantle during the Archean, Gaillard says, when there was not yet oxygen in the atmosphere and the way Earth’s rock moved was very different.

The findings alter Earth’s geological biography. Up until now, experts thought the differences in rock oxidation had something to do with oxygen shifts within the planet, such as some change in the way Earth’s core and mantle interacted, or old seafloor being shoved back into the Earth’s crust changed oxygen levels. But the new study suggests oxidation in Earth’s mantle has remained relatively steady over time. Instead, Cottrell and colleagues propose, peridotites and mantle rocks that show less sign of oxidation were formed in the deep past when the mantle was much hotter than it is now.

Chemical changes seen among ancient rocks may be attributable to changes in Earth’s temperature rather than alterations to oxygen and other elements. “It may be that cooling, rather than changes in the mantle’s bulk chemistry, is able to generate some of the chemical signatures we see in these rocks,” Cottrell says. Running hot had the unexpected consequence of preserving melted rocks within the Earth itself, a way for researchers to touch and study a chapter of Earth history far different from what we currently know.

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Riley Black

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.