I Think I Found a Meteor Crater

This is a good place for an initial examination of something that may figure in the future of the human race: asteroid impacts. This is also one of the most common queries that we receive over time.

Q:

I think I found a meteor crater? Is there a national listing?  It is below the ice age cap so erosion would be minimum, and below tree growth so not visible by plane. 
I could mail you a graph. It looks like moon landscape.
--J.W.D.


A:
Here are a few links that may be useful:
1. http://www.unb.ca/fredericton/science/research/passc/
2. http://volcanoes.usgs.gov/jwynn/1998SciAm-Wabar.pdf

The first link is probably the most complete database on meteor craters on Earth currently being maintained; it started as an effort by the Geological Survey of Canada, but was picked up by the University of New Brunswick when the GSC program was de-funded a number of years ago. The database currently has 182 verified impact structures in it - and this covers only the ~30% of the Earth not covered by water.

The second link is an article I published in Scientific American more than a decade ago that provides some of the identifying characteristics of a meteor crater.  These features separate impact craters from the vastly larger numbers of circular structures on the Earth's surface that have nothing to do with a meteor impact.

Ask-a-Geologist receives several inquiries a week from people who have looked at Google Earth or other satellite imagery and feel they have found a previously-undiscovered impact crater. While the Moon is covered with impact structures, the Earth is not. The reasons for this include plate tectonics, volcanism, and sedimentation (features being buried), and weathering (features being erased over time). The Earth has a very active crust, and as a result only very young or very large impact features are represented.

During his lifetime, my SciAm article co-author Gene Shoemaker spent every Austral winter he could with his wife and sweetheart Carolyn working and mapping in Australia. He did this for a very cold-blooded reason (no pun intended): Australia is a large continent, and of all the planet Earth the continent least covered by forest, vegetation, or ice. Gene figured that if he could complete a full and informed assessment of the impact structures of Australia, he could get a statistical handle on the true number of asteroid and comet impacts that our planet has suffered.

From that, one can calculate how OFTEN we are being bombed from space, and how BIG the bombs are. That's one way to predict our future. If someone is pointing a gun at you, there is some consolation in knowing how bad a shot the gunman might be.

The Earth's atmosphere is also a very powerful protective blanket. Only about 5% of the meteorites found on the Earth's surface are the Iron-Nickel bodies that penetrate the entire atmosphere with sufficient velocity to create a crater on impact. Nickel-iron meteorites represent just 2% of the stuff floating between Mars and Jupiter, but because they are more distinctive are more easily recognized when they land on the Earth than the far more common stony and chondrite meteorites.

The rest (stony meteorites, chondrites, comets, etc.) are generally broken up in the middle to upper atmosphere, such as the Tunguska object that blew up over Siberia in June of 1908. This explosion burned, then flattened a forest the size of Rhode Island - nearly 2,200 square kilometers - but left no crater. As a general rule, the cratering record on the Earth only "turns on" at features above about 5 kilometers in diameter - when the object coming in is big enough to penetrate the Earth's atmosphere. As another general rule, these craters are roughly 20 times the size of the object that caused them - more on this in a later chapter.

The 99.99% of other circular features on the earth are caused by volcanoes, sedimentary processes like sinkholes, and intrusive magmatic bodies. The distinctive characteristics of a true impact structure include shatter cones, shocked-quartz minerals like coesite and stishovite, upturned and overturned rims, and if large enough, a central rebound peak. The largest known such features are ~300 km in diameter, and in most cases are severely weathered, folded, or otherwise obscured. Only about 15 of the 182 known impact structures have any remnants of the original bolide present, which makes them "meteorite impact structures" as opposed to just "impact structures" - again, this is a distinctive characteristic of our planet, which has a potent protective atmospheric blanket, active plate tectonics and weathering going on all the time.

In my studies related to the Wabar event (1863, in the Empty Quarter of Saudi Arabia), I have discovered records that indicate there have been at least five asteroid impacts on the Earth's surface between 1863 and 1947. At least two of these have been nickel-iron objects, and all five would be classified as "city busters" by virtue of the kinetic energy they delivered to the Earth's surface. By an amazing coincidence, all five happened in extremely remote, unpopulated areas of the planet.

I'm reminded of the poem by T.S. Elliot, called "The Hollow Men" that ends
"This is the way the world ends: This is the way the world ends: This is the way the world ends: Not with a bang but a whimper." 


But maybe it WILL be with a bang.

~~~~~