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Where have all the craters gone? Why Earth’s surface is less pockmarked than might be expected

Where have all the craters gone? Why Earth’s surface is less pockmarked than might be expected

Jun 14th 2014 | From the print edition

SOME 66m years ago Earth was hit by a space rock reckoned to have been 10km (six miles) across. The resulting chaos did for the dinosaurs and many other species, opening the way for the age of mammals—and ultimately humans. It also left a big hole in what is now southern Mexico. That hole is one of only three known of similar dimensions (the other two are Vredefort in South Africa and Sudbury in Canada). And this is odd. For, during the billions of years that Earth has had a solid crust, many more than three big asteroids might have been expected to have hit it.

That thought led Brandon Johnson of the Massachusetts Institute of Technology and Timothy Bowling of Purdue University, in Indiana, to wonder how many other craters have vanished, either by erosion or by being swallowed into Earth’s interior as its crust moves around, and therefore whether it is likely that some have survived and been overlooked. They have just published their analysis in Geology.

Earth’s crust formed more than 4 billion years ago, but the oldest surviving blocks of it large enough to harbour craters date back only 3.5 billion years. What is known of the sizes and orbits of modern asteroids suggests that, if things have not changed over the aeons, about 14 big asteroids (defined as having a diameter of more than 7.4km, which would cause a crater at least 85km in diameter) should hit Earth every billion years. That means 49, give or take seven, over the past 3.5 billion years. Such impacts may have been more common in the past, when more big asteroids were around. Allowing for this, Earth would have been hit by 113, give or take 11, of them. Either way, a lot of craters are missing.

Dr Johnson and Mr Bowling first considered the fate of craters punched into the 70% of the planet’s surface made of oceanic crust. This forms as magma rises at mid-ocean ridges. It then solidifies and is pushed outwards until it slides down a trench and melts. Its average age is a mere 65m years and the most ancient of it is 280m old—8% of the age of the oldest continental crust. Most ocean-floor craters, then, will have vanished.

Continental crust, being lighter than oceanic crust, does not sink so easily back into Earth’s interior. Nearly half of what is around now—about 15% of the planet’s surface area—is more than 2 billion years old. But continental crust is subject to the weather. Work done by other geologists suggests it takes about 3 billion years for weathering to destroy the crater created by a big asteroid, so old continental craters are likely to have been eroded into obscurity but traces of younger ones should remain. Vredefort is about 2 billion years old; Sudbury 1.8 billion.

When Dr Johnson and Mr Bowling put these numbers through their crater-survival model they found that if the impact rate has been steady through the ages only eight, give or take three, of the big craters thus formed will have survived. If the rate was once much higher the figure is 11, again give or take three. Either way, the two researchers suggest, while there are probably a few more big craters lurking in the crust, geologists have not been particularly remiss in failing to find them. Rather, most of Earth’s astronomical pockmarks have simply vanished.

 

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About bambooinnovator
KB Kee is the Managing Editor of the Moat Report Asia (www.moatreport.com), a research service focused exclusively on highlighting undervalued wide-moat businesses in Asia; subscribers from North America, Europe, the Oceania and Asia include professional value investors with over $20 billion in asset under management in equities, some of the world’s biggest secretive global hedge fund giants, and savvy private individual investors who are lifelong learners in the art of value investing. KB has been rooted in the principles of value investing for over a decade as an analyst in Asian capital markets. He was head of research and fund manager at a Singapore-based value investment firm. As a member of the investment committee, he helped the firm’s Asia-focused equity funds significantly outperform the benchmark index. He was previously the portfolio manager for Asia-Pacific equities at Korea’s largest mutual fund company. KB has trained CEOs, entrepreneurs, CFOs, management executives in business strategy, value investing, macroeconomic and industry trends, and detecting accounting frauds in Singapore, HK and China. KB was a faculty (accounting) at SMU teaching accounting courses. KB is currently the Chief Investment Officer at an ASX-listed investment holdings company since September 2015, helping to manage the listed Asian equities investments in the Hidden Champions Fund. Disclaimer: This article is for discussion purposes only and does not constitute an offer, recommendation or solicitation to buy or sell any investments, securities, futures or options. All articles in the website reflect the personal opinions of the writer.

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