Billions of years ago, parts of Greenland were a molten sea, a sharp contrast to the pale-blue ice seen there today. Researchers have now found isotope signatures of that burbling, primordial Earth in basalt rocks near Nuuk.
It’s generally accepted that early in its history, Earth had a big magma ocean; it’s a common step in the early evolution of planets. But thanks to the planet’s tectonics, little evidence remains of what that version of Earth looked like.
The research, published today in the journal Science Advances, describes iron and tungsten isotope signatures from 3.7-billion-year-old rock that are indicative of elements of Earth’s mantle in the first half-billion years of our planet’s formation.
“The driving question that motivated me was, if we think the magma ocean stage was important to the Earth’s history, why is there no geological evidence for it?” said Helen Williams, a geologist at the University of Cambridge and lead author of the paper, in a video call. “What if we actually tried to directly hunt for it?”
Earth is constantly cleaning the tectonic slate with its geological activity; the site in Greenland is the first to indicate such a giant, fiery sea. In fact, the research site at Isua is the same outcrop that caused a buzz a few years back when possible stromatolites—fossils of bacteria, the earliest known life on Earth—were identified there. The magmatic activity recorded in the rocks would’ve occurred hundreds of millions of years before such microbes ever set up shop, though, when Earth and the Moon had only recently become distinct objects. (In fact, it’s easier to find information about the magma ocean by looking on the Moon.) When the magma ocean finally cooled, some of its deeper components crystallized at very high pressures over 400 miles beneath Earth’s surface, Williams said.
So how do you hunt down an ancient magma ocean, long since solidified and cooled? You find geochemical signatures that indicate the circumstances in which they formed.
Williams said that there were positive correlations between the isotopes they detected and an indicator for high-pressure conditions, which gave the team “a lot of confidence” in their result.
Much more work will be needed to find other geosignatures that indicate what the early magma situation was like, oceanic or otherwise. Due to Earth’s natural tendency to clear its history, through tectonic shifts over huge amounts of time, we don’t know the exact time frame of the theorized magma ocean, nor how much of the planet it would have covered.
“It has been really unclear exactly how uniform the interior of the Earth is, how the interior may have changed over time, and how any anomalous regions may have come about,” Stephanie Brown Krein, a geologist at the Massachusetts Institute of Technology who isn’t affiliated with the recent paper, said in an email. “Most work, but not all, has tended to focus on plate tectonics processes in creating anomalous regions, so I’m excited to see new work on trying to find evidence of other processes, e.g., magma oceans, that we think could have occurred on Earth very early.”
In the future, it’d be worthwhile to look at other slow-forming rock, like Hawaii’s plume basalt, Williams said. Though the trail has long since grown cold, geologists just got a glimpse at something hot.