THURSDAY DECEMBER 26 - IMPACT CRATERS, PART II



In its approximately 4.5 billion-year history, Earth has been hit time and time again by large meteorites, forming craters on the surface. However, over time, processes such as erosion, plate tectonics, and volcanism have destroyed, buried, or masked nearly all of them. Still, about 160 impact craters have been identified on Earth. Each dot on this map marks the site of one of these craters.

Fifty-three of these craters are in North America. The most famous and well-preserved is the nearly one-mile-wide Meteor Crater in Arizona, formed about 50,000 years ago. Other more highly-eroded craters, such as these in Quebec, Canada, are much larger - on the order of tens to hundreds of miles wide - and much older, more than 200 million years in these cases.

Closer to home, there's a large impact crater beneath part of Chesapeake Bay. That structure is about 50 miles wide and about eight-tenths of a mile down, so understandably out of sight. It was formed 35 million years ago when an object that was probably 2 to 3 miles wide slammed into the earth. Tomorrow night: did an impact similar to that one end the Age of Dinosaurs?

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