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SCIENCE

Standing at the roots To begin with, the researchers noticed bull’s-eye-looking
But even these uplifted sediments were buried far below features up to a few centimetres in diameter. These structures,
Earth’s surface at the time of the impact, said Huber. “This area called accretionary lapilli, form within clouds of ash. Much
has experienced at least 10 kilometres of erosion. We’re at the as hailstones grow via the addition of layers of ice, accretionary
deep structural roots.” lapilli grow spherically as successive layers of ash are deposited
Because of all that erosion, there’s no hope of finding on their outer surface. They’re associated with both volcanic
impact ejecta — sediments launched during an impact, which eruptions and meteorite impacts.
have often been altered by high temperatures and pressures “There’s no doubt that it is impact ejecta.”Huber and his
— within the impact structure itself, said Huber. “It’s all eroded colleagues also spotted parallel lines running through grains
away. It’s gone.” of quartz. These lines, known as planar deformation features,
However, nearby sites — located within a few radii of the represent broken atomic bonds in the quartz’s crystal lattice.
Vredefort impact structure — might still contain impact ejecta, Ordinary geologic processes like earthquakes or volcanic
Huber and his colleagues reasoned. (In previous studies, Huber eruptions are rarely powerful enough to create these features,
and his collaborators had found millimetre-sized Vredefort said Huber. “These grains were subjected to a shock wave.”
ejecta much farther afield, in Greenland and Russia.) Planar deformation features are “unequivocal evidence” of
To search for this so-called proximal ejecta, Huber and his impact material, said Elmar Buchner, a geologist at the Neu-Ulm
colleagues looked a few hundred kilometres to the west. They University of Applied Sciences in Neu-Ulm, Germany, not involved
focused on a swath of the Kaapvaal Craton, a geologic feature in the research. “There’s no doubt that it is impact ejecta.”
th
that, like other cratons around the world, preserves particularly These results were presented recently at the 84 Annual
ancient sediments. Meeting of the Meteoritical Society in Chicago.
There’s a lot more to learn from these ejecta, Huber and
A violent event, told through rocks his collaborators suggest. The team next plans to analyse their
The researchers collected material from a pair of sediment samples for “impact melt,” material preserved from the time of the
cores originally drilled by mining companies exploring the impact that’s sometimes a chemical amalgam of the impactor and
region for iron and manganese. Huber and his collaborators the surrounding target rocks. Such ejecta could help reveal the
honed in on sediments dated to be 1,9 billion to 2,2 billion composition of the object responsible for creating the Vredefort
years old and assembled several thin sections of the rocks to impact structure, the researchers suggest. “We are already
analyse. The sediments exhibited tell-tale signs of a violent planning our next analyses,” said Huber. “There is a lot of work to
event, the team found. be done.” n

After more than 2 billion
years of erosion, features
of the crater created by
the massive meteorite
that impacted what is now
Vredefort, South Africa,
are barely discernible.
Credit: NASA

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