Sunday, April 21, 2013

Cape Charles Impact Crater

The Cape Charles Impact Crater

A plaque on the board walk at Cape Charles commemorates the very reason for the Chesapeake Bay's existence, at least in the place and form we see. Thirty-five million years ago an enormous meteorite, 2 miles in diameter and moving 60,000 mph, plowed into an ancient seabed, creating a crater some 60 miles in diameter, and vaporizing or incinerating everything within hundreds of miles. There are no obvious surface signs of the crater, and examination of nautical charts reveals no abyss. A similarly sized meteorite struck Tom's Canyon on the continental shelf off New Jersey within the next million years, and a combination of ejected material and devastating tsunami erased all surface trace; actually three meteorites, including these two and Popigai in Siberia, fell in a straight line and may have been simultaneous, much like Shoemaker-Levy 9’s multiple impacts on Jupiter in 1994. Even now, millions of years later, the ground in the Hampton Roads/Norfolk area continues to subside as the rubble from the impact continues to consolidate, causing the most rapid sea level rise rates in North America. The buried Ice-Age channels of  the Susquehanna, Potomac, and Rappahannock originally crossed over the Delmarva on their way to the ocean, but were redirected to the south by this continuing subsidence as sea levels rose, forming a brackish water estuary unique in the world. Though the Chesapeake Bay in its current form is more recent, created only 10,000 to 18,000 years ago by rising sea level, the underlying geography is much older.
The discovery of the crater was somewhat fortuitous; it was the search for dramatically increased drinking water supplies needed to support the World War II build-up at the Norfolk and Newport News shipyards that began to reveal this secret. Sedimentary rock formations encountered while drilling test wells did not fit the established patterns of the Virginia coastal plain. Saltwater intrusion was found in places established coastal geology could not explain, including pockets higher in salinity than the surrounding ocean. It has been theorized that sea water was trapped soon after the time of impact and concentrated by an effect similar to reverse osmosis, with fresh water squeezed out through the rock, leaving behind a more concentrated brine. It was not until the early 1990s that technology and experience gained in researching other large impact craters allowed the identification of this area as an impact crater.
Although this was certainly a climate changing event, it has not been connected with any mass extinction. A larger meteorite, Chicxulub, created the devastating crater off the Yucat√°n Peninsula is generally credited with causing the extinction of over 60 percent of all life forms and initiating the transition from the Cretaceous to the Tertiary period, over 65 million years ago. Some five times smaller, perhaps the Chesapeake Bay meteor was simply not sufficiently massive to cause the widespread extinctions necessary to be obvious in the fragmented fossil record, or to change the course of evolution.  Perhaps life is more tenacious than we give credit.


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