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?