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Probing the Impacts of Early Earth
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Between 3 and 4 billion years ago, the planets of the inner Solar System were scarred by a number of large impacts. This period, known as the Late Heavy Bombardment, marks the final stage of planetary formation in the Solar System. But a new paper published in Geochemistry, Geophysics, Geosystems suggests that it was also the beginning of plate tectonics here on Earth.

The rocks exposed around Barberton, South Africa are old. These rocks, some of which are over 3.6 billion years old, are among the oldest exposed on Earth.  And, according to research by Norman Sleep and Donald Lowe (Stanford), they preserve evidence of a cataclysmic impact that took place 3.26 billion years ago. Eleven years ago, Lowe published a study of strange glass beads, or spherules, that were preserved in distinctive rock layers in the Barberton region. Chemical studies suggested that the spherules were formed by massive impacts at the time the rocks were deposited.

But aside from the presence of the spherules, there isn’t much else to go on. The crater left by the impact has probably been erased by the ravages of time, leaving us with no idea how large the impactor was. “We knew it was big, but we didn’t know how big,” said Lowe in a statement to the American Geophysical Union. Without a crater to constrain the impactor’s size, Sleep and Lowe took a different tack. They studied the aftermath of the impact and worked backwards to figure out how big the impact needed to be to cause what they saw.

And what they saw was astounding: Sleep and Lowe calculated that the bolide was somewhere between 37 and 53km (23 to 36 miles) wide, and slammed to Earth at a velocity of 20km (12.5 miles) per second. While not fast by impact standards (most meteor showers have entry velocities of 30-70km/s (19-43mi/s)), the sheer mass of the object wreaked havoc on the Earth. The impactor, nearly 4 times wider than the dinosaur-killing Chixulub impact, would have left a crater nearly 500km (300mi) across.

At the time of the impact, the Barberton area probably was an underwater plateau. Within 10 minutes, shock waves from the impact reached the plateau. The seismic waves had an estimated magnitude of 10.8, approximately 470 times more powerful than the March 2011 Tohoku earthquake. Massive fissures opened in the soft sediments that plastered the bottom of the ocean. For nearly half an hour, the seafloor heaved before the earthquake finally ended.

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This rock preserves evidence of an impact by a massive asteroid 3.26 billion years ago. (From Lowe et. al. 2003)

Those fissures preserved the smoking gun of the impact. As a cloud of vaporized rock spread away from the impact site, it began to cool and condense to form the spherules that had captured Lowe’s attention in 2003. While the spherules don’t contain much iridium, an element commonly associated with impacts, they contained large amounts of chromium. Chromium is more abundant on Earth than iridium, but it still isn’t abundant enough to explain the amount found in the spherules. The most likely source was a carbonaceous chondrite asteroid, which contains large amounts of chromium. The spherules rained out of the sky and into the ocean, where they sank to the seafloor.

Within hours, a series of massive tsunamis, each thousands of meters tall, rolled through the Barberton region, sealing the fissures with mud and debris ripped from the tortured seafloor. But even as the dust was settling, the worst was still to come. The impact generated an incredible amount of heat, which raised global temperatures past the boiling point of water. The upper few meters of the ocean boiled before the atmosphere cooled again.

The aftermath was most likely sufficient to temporarily sterilize the surface of the Earth, although life seems to have quickly rebounded from the shock. But the impact’s effect on life is not the only thing of interest to geologists. For years, geologists have been accumulating a pile of evidence that shows for the first billion years or so of Earth’s history, plate tectonics didn’t operate. Instead, what is known as a “stagnant lid” controlled geological activity on Earth.

The stagnant lid formed because the early Earth’s crust was too warm and stiff to be convected down into the mantle. Instead, heat escaping the Earth’s interior gradually built up underneath the crust, occasionally forcing it to overturn. Roughly 3 billion years ago, the crust’s stagnant lid began to break down and transition to plate tectonics. The cause for this breakdown has yet to be explained in detail, but Sleep and Lowe suggest that the shock of the impact was an important factor.

This is because the Barberton greenstone belt also records evidence of the lid’s breakdown. And, perhaps not coincidentally, that breakdown began within just a few million years of the impact. Sleep and Lowe offer the tantalizing suggestion that the impact helped spur the development of modern plate tectonics. They calculate that the earthquakes that devastated the Barberton region may have been strong enough to create massive thrust faults that were quickly converted into subduction zones.

An impact 3.26 billion years ago might have brought in the age of plate tectonics, or at least hastened its development. But, given the paucity of data from the time, that will probably have to remain only an educated guess.

Featured image credit: American Geophysical Union

Side image credit: Lowe et. al. 2003

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About The Author
Justin Cowart
Justin Cowart is a geologist interested in Earth and Solar System history. As a geologist, he spends hist time looking at the ground, but in his free time he looks to the skies as an amateur astronomer.

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