Geotimes - October 2002 - Early Impact Evidence

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Earth history
Evidence for early impacts

A giant meteorite slammed into Earth 3.5 billion years ago — a collision two to five times larger than the one thought to have wiped out the dinosaurs.

Geologists have not found the crater the meteorite must have left, nor have they found any pieces of the meteorite. But fallout from the collision lies in layers of the Kaapvaal craton in South Africa and the Pilbara craton in Western Australia, providing a rare glimpse into Earth’s early meteorite history.

A team of geologists recently determined the age of the impact — the oldest impact for which there is evidence on Earth. As reported in the Aug. 23 Science, the meteorite crashed 3.47 billion, plus or minus 2 million, years ago. Earth was only a billion years old, and the only forms of life that may have existed were single-celled bacteria.

Stanford geologist Donald Lowe (left) and USGS geologist Joseph Wooden stand behind the SHRIMP RG instrument they and Gary Byerly (LSU) used to determine the age of the oldest known meteorite collision with Earth. L.A. Cicero/Stanford University

The study shows that a terrestrial record of impacts extends all the way back to the early Archean. Until now, geologists have relied on records from the Moon and nearby planets to infer Earth’s early meteorite history.

“We now have much clearer overlap between the terrestrial and lunar record,” says Bruce Simonson, an impact expert at Oberlin College.

“For a long time we were able to look at the Moon, Mars, and Venus and supposed that impacts were true on Earth too. We now have an impact history on Earth that we can begin to decipher,” says Donald Lowe, a Stanford University geologist and co-author of the Science paper.

The Kaapvaal and Pilbara cratons contain Earth’s oldest well-preserved sedimentary and volcanic rocks . They “rest on greatly thickened sections of crust and lithosphere, which have prevented their later subduction, rifting and other major tectonism,” says Gary Byerly, a Louisiana State University geologist and lead author of the study.

These unique locations have kept the formations from experiencing the depths that metamorphose rocks, obscuring their original sedimentary and volcanic features.

When a huge meteorite like the one described in the Science paper careens into Earth, it can pass through the atmosphere in less than a second. At this speed, the meteorite pushes away air that does not have time to fill back in, creating a vacuum. The meteorite instantly vaporizes upon impact, and the vacuum sucks the vaporized meteorite and target rock into the atmosphere. The vapor spreads throughout the atmosphere, cools and rains back down to the surface as pellets of glass called spherules.

As sedimentary rocks accrete, they incorporate the spherules into impact layers. The Kaapvaal and Pilbara cratons are the only known formations preserved well enough to retain clear impact layers from the collision 3.47 billion years ago.

The team of geologists analyzed zircons buried along with the spherules to determine the precise age of the collision. Zircon, a highly durable mineral, often contains radioactive uranium. Measuring how much uranium has decayed to lead indicates exactly when the zircons formed.

The impact 3.47 billion years ago vaporized the meteorite and target rock. The vapor condensed to form spherules like the one to the right, which is a millimeter-sized spherule with quartz replacing original radiating crystallites. (Crossed-polarized interference microscopy with 1st order accessory plate.) Courtesy of Byerly, LSU

The team used an instrument at Stanford University called the Sensitive High-Resolution Ion MicroProbe Reverse Geometry — or SHRIMP RG — to measure the uranium decay in less than 10 minutes; standard methods to dissolve and prepare single grains can take months.

The grains from the Barberton greenstone belt, in the Kaapvaal craton, and from the Pilbara craton are exactly the same age, indicating the impact layers chronicle the same event. The impact probably had to be huge to show up in both sedimentary records.

It is possible that the two cratons were close at the time of impact, decreasing the size the meteorite needed to be to generate spherule layers at both sites. But Byerly points to paleomagnetic evidence that places the two cratons far apart about 3 billion years ago.

The match in ages is “new and really the most important part” of the paper, Byerly says.

The thickness of the impact layers and geochemistry of the spherules suggest the meteorite was huge, 20 to 50 kilometers in diameter. An impact of this size would have dramatically altered the biological and physical environment.

Several thin layers of mud within the spherule layers at both sites suggest the meteorite generated a tsunami. The impact-generated wave would have traveled around the world in 30 hours, run into itself, and bounced back to meet itself again 30 hours later. The thin layers may represent periods of mild sedimentation between arrivals of the tsunami.

In the Pilbara craton, a layer just above the spherule layers indicates a major increase in erosion, suggesting the tsunami massively eroded nearby land and changed coastlines.

The team next plans to search for new terrestrial evidence of early Archean impacts and to explore how those impacts influenced the physical and biological evolution of early Earth.

These explorations may eventually help explain the origin and dynamics of plate tectonics. “The early impacts were so large that they could change tectonic regimes locally, if not regionally, and possibly even globally,” Lowe says.

Greg Peterson

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