The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Friday, May 9, 2014

May 9. A Devonian cratering event?



In the History of the Earth book, written back in 1994, I had a page for a possible Devonian cratering event when the earth-moon system might have undergone a greater-than-usual bombardment from meteorites. In 1994, Copernicus and other young craters on the moon were thought to have formed as recently as 350 to 400 million years ago, the Devonian. Now age dates put the formation of Copernicus at more like 800 million years ago, or even as old as a billion years ago – still young, as the moon goes, but back in the Precambrian in terms of earth’s time scales. 

Copernicus (NASA photo)
So was there an impact period during the Devonian on earth? It doesn’t look like it. There are a dozen or more known large features that represent Devonian impacts. Some, such as the Siljan Ring in Sweden, are pretty big – at 52 km or 32 miles across, it’s the largest known crater in Europe. Even after much erosion, the edge of the crater is still evident in the topography – a lake fills part of the crater rim. It formed about 377 million years ago, during the Late Devonian.

Some of the Devonian craters have fairly reliable age dates, ranging from about 396 million years ago to about 360 million years ago. Nine well-dated events in 36 million years. Given the error bars in the dating, three of the impacts, including the Siljan Ring, were about 380 million years ago, plus or minus five million years or so. So I don’t think we can call this anything like a “cratering event,” and we probably really can’t say the Devonian has a greater incidence of impacts than any other time in earth history.

We talked about one likely global impact event, 480 million years ago, during the Ordovician. That one was based on multiple fossil meteorites in many locations around the world and it seems reasonable to call that a specific event. 

There’s plenty of research going on to investigate the possibility that there may be some predictable periodicity to impacts on earth – obviously we have a vested interest in knowing that, even if it is on scales of many thousands of years or more. Check out a 2005 paper on this topic. This is challenging work in part because there’s a bias toward information about younger craters, simply because younger craters and associated information are better preserved and often better exposed than the old ones.

For now, I’m going to say that there was no Devonian cratering event, nor even any noteworthy increase in impacts then. But you have to realize two things – first, I’m not an expert on this, and even though I reviewed a lot of papers before making that statement, I could have missed something significant. The other thing to realize – to always realize – is the problem of sampling – the total number of impact craters known on earth is only a couple hundred. That’s really not enough to make any confident, far-reaching conclusions about periodicity or increased occurrence. And remember that the older you get, the more likely that all the evidence of an impact might be eroded away or subducted into the earth.
—Richard I. Gibson
Image credit: NASA

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