The crust expands at mid-ocean rifts. But how?
One of the central features of plate tectonics is the formation of new crust at mid-ocean ridges. Part of the spreading process that drives continents apart, it was arguably the discovery of these ridges that drove widespread acceptance of plate tectonics as a theory. Thanks to decades of exploration, we now have a good picture of what the crust that forms at the site of spreading looks like. But we still have an incomplete idea of how its features are actually produced.
In other words, we have a good idea of the outcome of the process, but not a detailed picture of the process itself.
That is starting to change. In 2024, a team of French scientists was able to remotely monitor a major event on the border between the Australian and Antarctic plates, only two months after they installed equipment on the ocean floor. Their data shows that most of the spreading occurred in a relatively short time window, and some key events happened without any obvious seismic activity.
Lots of changes
The site where the events took place is incredibly remote, about halfway between Australia and Madagascar, and far south of India. There’s a large seafloor feature called the Amsterdam–Saint Paul Plateau that rises above the deep ocean plane in the area, interpreted as a rise driven by the presence of a deep ocean hotspot. The rift between the Antarctic and Australian plates runs right through the middle of this plateau.
Despite the indications of a tectonic hotspot, only two volcanic islands are present in the area, Amsterdam and St. Paul. The islands have a long history of failed colonization attempts, accidental strandings, and regular visits by fishermen and sealers. Initially claimed by France, they ended up so useless and remote that France dropped claim to them only a decade later. Forty years after that, the crew of a French ship reclaimed them on behalf of a country that didn’t seem to be certain whether it wanted the honor.
Now, over a century after that, the French government maintains research stations on the islands and sporadically sends ships to maintain equipment, deliver and return scientists for seasonal fieldwork, and perform supply and maintenance duties. The team behind the new work took advantage of one of these ships to deploy a series of underwater monitoring stations along the spreading zone. These included hydrophones that could provide rough locations of seismic events, and transmitters that allowed them to track any changes in the distance between monitoring sites. Latter visits from the French supply ships performed three-dimensional mapping of the seafloor in the area to determine the outcome of any events that were detected.
Earlier study of the region had shown that the spreading in the area occurs at an average rate of a bit over 60 millimeters a year. That occurs along a very typical-looking site that has a roughly 2,000 meter depression at the location of spreading, flanked by a series of rugged ridges.
All of the hardware was in place when the fault it was on rumbled to life in April 2024. The first cluster of events occurred progressively farther south along the main spreading area, with the last of them over 8 kilometers south of the first. That was followed by a series of events that moved to the north, this time extending out over a distance of 9 kilometers. The researchers say that this sort of activity is typical of the formation of dykes, thin but long and tall structures formed by the intrusion of molten rock.
At the same time that was going on, sensors located in the valley at the center of the spreading region started experiencing a drop. As the dyke events continued, the drop accelerated until the sensors were sinking at a rate of about 5 centimeters a minute before slowing. But subsidence continued well after the initial events, with a total of 4.2 meters over a six-day period.
The researchers interpret this as a magma reservoir beneath the ridge draining. Consistent with that, the temperature of the water at the nearby instruments started rising at the same time, suggesting that magma was interacting with the seawater.
That’s already a lot, but the list of events doesn’t end there. While all of this was going on, instruments on opposite sides of the central valley started moving farther apart, in some cases by well over a meter.
Rare, sudden changes
Sometime after the site had returned to background levels of activity, the next visit from a French research vessel occurred, and new imaging of the site took place. The resolution is quite poor, but even so, there are some sites that were over 90 meters higher than they had been during the previous mapping, well beyond the potential errors in the instruments. One patch of material was over 4 kilometers long, and the researchers estimate the total amount of new material at about 150 million cubic meters.
The researchers performed modeling of the events to try to figure out how all of them might be connected. They randomized different configurations of magma source, dyke extent, and fault geometries and sampled 10 million different ones to see whether they could produce the sorts of changes the instruments picked up. Only 2,200 of them could, and they had a number of common features. These include a collapse of a deep reservoir of molten material called a sill, which is a bit like a horizontally oriented version of a dyke. Some of this magma went into a dyke it is connected to, expanding it. At the same time, faults in the area spread by anywhere from 2 to 4 meters.
The team estimates that the total extension is the equivalent of 38 years of activity at the site’s average rate of spreading. And they think that this might be normally the way that mid-ocean spreading occurs: a buildup of strain and material followed by a series of rapid events that actually produce the new seafloor that gradually moves away from the site of spreading. The other striking thing is that some of the events occurred without an obvious indication of tectonic signals picked up by the hydrophones present. That suggests if we were to attempt to build a picture of spreading activity using seismic data alone, we might miss the full picture of what’s going on as our planet renews its crust.
Nature, 2026. DOI: 10.1038/s41586-026-10785-0 (About DOIs).
John is Ars Technica’s science editor. He has a Bachelor of Arts in Biochemistry from Columbia University, and a Ph.D. in Molecular and Cell Biology from the University of California, Berkeley. When physically separated from his keyboard, he tends to seek out a bicycle, or a scenic location for communing with his hiking boots.
