Every few hundred million years, give or take, our planet is pummeled by a kilometer-scale chunk of rock. These rare, but cataclysmic, impacts are known to have altered ecosystems and caused widespread extinctions. Now, researchers have shown that such events can also trigger earthquakes that persist for thousands of years. This discovery, made at the Vredefort impact structure near Johannesburg, South Africa, sheds light on how the planet’s crust reequilibrates after a massive impact, the team suggests.
A Look Beneath
“What we’re looking at now is what the structure looks like very deep below the impact.”
The Vredefort impact structure is the world’s largest extraterrestrial scar. But because of its pronounced age—2.02 billion years, to be precise—the original crater is no longer visible. That’s because the cumulative effects of erosion from water, wind, and ice over geologic time have stripped away roughly 10 kilometers’ worth of material, said Matthew S. Huber, a geologist at the University of the Western Cape in Cape Town, South Africa, and lead author of the study. “What we’re looking at now is what the structure looks like very deep below the impact.”
And that’s a unique vantage point, said Huber. “We don’t have any other large impact craters where we get to see what it looks like when you slice into it.”
Taking advantage of that perspective, Huber and his colleagues studied several of the impact melt dikes within the Vredefort impact structure. These vertically oriented slabs of rock crosscut through the region’s granites. They likely consist of impact melt, an amalgam of once molten material that pooled at the planet’s surface shortly after the cataclysm, previous studies have suggested. And because these dikes are still visible today despite extensive erosion, impact melt must have traveled far below the surface, the researchers surmised. “Why do we have these dikes deep in the subsurface? That’s the overarching research question,” said Huber.
Rebounding Crust
The opening of fractures in Earth’s crust—associated with earthquakes—could have plausibly allowed these dikes to extend to great depths, previous research has noted. Seismic activity makes sense because of the titanic forces at play in the wake of the impact, said Huber. The impact scraped away kilometers of sediments nearly instantaneously, and the planet’s crust would have rebounded in response. “This is the same as if you have a glacier that’s retreating—the land is rising up again after the glacier has moved away,” he said. “It’s the same type of isostatic rebound.”
But the duration of such ground shaking has never been constrained. And that’s an important quantity to understand, Huber and his collaborators note, because it gets at a fundamental question: Is an impact event a one-and-done affair, or do aftereffects continue to roil a region for some time?
“Crustal settling over geologic time has always been suspected around large impact basins, but the duration of that settling has been elusive,” said David Kring, a planetary geologist at the Lunar and Planetary Institute in Houston not involved in the research. “The current paper attempts to resolve that issue.”
A Striking Boundary
Huber and his colleagues studied five dikes. Three appeared to be homogeneous across their exposed faces. But the remaining two stood out, even at first glance: Their interiors were dark brown, and their peripheries tended to be lighter in color and flecked with beige. “The boundaries between the phases are quite sharp—you can put your finger on the precise contact between them,” Huber told Eos.
Back in the laboratory, the researchers found that the two visually striking dikes were, indeed, chemically inhomogeneous: Their interiors contained a higher fraction of iron and magnesium, and their peripheries tended to be dominated by silicon and potassium. This finding suggests that two chemically distinct pulses of impact melt poured downward to form these dikes, Huber and his colleagues surmised. And that’s possible only if the melt sheet chemically differentiated between the two pulses, the researchers concluded, which in turn implies that some interval of time separated episodes of ground shaking.
The Reign of Earthquakes
“Even tens of thousands of years after an impact, you would not want to be building a house on the periphery of a crater.”
The first downpouring of impact melt must have occurred before the melt sheet differentiated, and the second had to have taken place after differentiation but before the melt sheet solidified, the researchers reasoned. On the basis of previous estimates of the timing of those events made by other researchers, Huber and his colleagues concluded that earthquakes shook the region for at least tens of thousands of years.
This finding reveals the enduring nature of massive impact events, said Huber. “It is not simply a moment in time. Even tens of thousands of years after an impact, you would not want to be building a house on the periphery of a crater.”
—Katherine Kornei (@KatherineKornei), Science Writer