Craters formed by asteroid impacts are ubiquitous on rocky bodies, and our planet is no exception. Researchers believe they’ve pinpointed yet another impact crater on Earth, this one submerged beneath the North Sea. The structure, known as Silverpit Crater, was discovered roughly 2 decades ago, but its provenance has long been debated. With new subsurface imaging and rock samples, the team concluded that an impact produced Silverpit Crater roughly 45 million years ago. These results were published in Nature Communications.
“People simply didn’t believe it was an impact crater.”
Uisdean Nicholson, a geologist now at Heriot-Watt University in Edinburgh, Scotland, remembered the controversy that swirled around Silverpit Crater back in the late aughts. Was the circular feature lurking beneath the waters of the North Sea under roughly 700 meters of sediments caused by an asteroid impact, or something more plebeian like volcanism or subsidence?
Nicholson, a graduate student at the time, remembered the spirited discussion that ensued among scholars attending a Geological Society of London meeting in 2009. “It was a classic, old-school debate,” he said. The vote came out strongly in favor of a nonimpact origin.
“People simply didn’t believe it was an impact crater,” Nicholson said. It looked as though Silverpit Crater wasn’t destined to join the rarefied group of 200 or so confirmed impact structures on Earth.
Begging for Data
As Nicholson focused his research on other impact structures such as Nadir Crater, he kept thinking about Silverpit. One dataset in particular piqued his interest: a survey of the North Sea seafloor sediments collected in 2022. Those data, amassed on behalf of the Northern Endurance Partnership, a venture to explore carbon capture storage under the North Sea, afforded a close-up look at the 3-kilometer crater and its environs. Previous datasets had also imaged a similar area, but they were of lower resolution and did not cover the entire structure.
A colleague alerted Nicholson about the Northern Endurance Partnership data, and the researchers worked for several months to negotiate access to some of the proprietary observations. “I begged,” Nicholson said.
The researchers were ultimately successful in their quest, and the team pored over high-resolution seismic reflection data revealing faults and buried layers of sediments around Silverpit Crater. “The new data gives a far sharper set of images,” Nicholson said.
The fact that Silverpit Crater is so inaccessible is actually important scientifically, said Matthew S. Huber, a planetary scientist at the Planetary Science Institute in Tucson, Ariz., who was not involved in the research. “Because this crater formed in water and it was buried by sediments in the water immediately after it formed, the whole thing is preserved.”
Faults and Holes
The Northern Endurance Partnership data revealed faults consistent with rock being compacted to varying degrees, as would be expected in an impact. The observations also spotlighted several roughly 10-meter-deep and 250-meter-wide troughs near the rim of the crater. Such scarps could be features eroded by water rushing back into the crater after the impact, the team surmised.
In addition, Nicholson and his colleagues noticed a few pits located beyond the crater rim that were tens of meters deep and wide. “We see all these holes, essentially, around the crater for at least a crater diameter,” Nicholson said. The team thinks that such features are secondary craters, that is, structures formed by material lofted outward from the initial impact.
Secondary craters tend to be rare on Earth because they’re often rapidly erased by erosion after an impact. “We think this is the first really robust terrestrial evidence for secondary cratering,” Nicholson said.
Atomic Wrenching
In 1985, the company British Gas drilled an oil and gas well just a few kilometers northwest of Silverpit Crater. As part of the drilling process, debris excavated from the borehole was pumped to the surface, and some of it was retained for analysis. Nicholson and his colleagues obtained some of those sediments. On the basis of the appearance of tiny marine fossils in rocks from the same depth as Silverpit Crater, the team deduced that the feature formed roughly 43–46 million years ago.
Two mineral grains the team analyzed—one quartz and one feldspar, each roughly the diameter of a human hair—exhibited curious microscopic features. Both grains contained so-called planar deformation features, which are atomic rearrangements of the crystalline structure, Nicholson said. Such wrenching on an atomic scale is indicative of the extreme pressures associated with shock waves.
“This could wind up being a controversial paper within the impact community.”
A celestial object such as an asteroid or comet slamming into a rocky body can readily generate such pressures, but not much else can, Nicholson said. “It’s very difficult to form that any other way.”
The discovery of those shocked grains was a dead giveaway that Silverpit Crater formed from an impact, Nicholson and his colleagues proposed.
These results are convincing, Huber said, but a skeptic might rightfully have some questions. For instance, couldn’t the shocked grains have simply washed into the North Sea from another impact event? “They’ve only found one grain of quartz and one grain of feldspar,” Huber said. “This could wind up being a controversial paper within the impact community.”
—Katherine Kornei (@KatherineKornei), Science Writer