Earth Impacts 
It’s been almost a year since a small asteroid exploded above Siberia, shattering windows for miles around. The blast was like a warning shot fired across the bow of a ship — a reminder that a lot worse could follow.
In fact, a lot worse has happened to Earth many times over the eons. Asteroids and comets have slammed into our planet, devastating life and gouging big holes in the ground.
Most of those holes have disappeared without a trace — erased by wind and rain, volcanic activity, and the motions of Earth’s crust. And most of the ones that remain are so degraded that they’re hard to see.
Even so, geologists have identified about 175 confirmed impact craters on our planet, with hundreds more possible impact sites. Some still look like shallow bowls, but others are identified mainly by the rocks beneath them, which have been altered by powerful shock waves.
The craters range from about 50 feet in diameter all the way up to more than a hundred miles, and from a few hundred to a couple of billion years old. The most famous is the Barringer meteor crater in Arizona, which looks fresh and sharp. But perhaps the most important is Chicxulub, off the coast of Mexico. It formed when a mountain-sized asteroid hit Earth 65 million years ago. The impact and its lingering effects on the atmosphere are at least partly to blame for the demise of the dinosaurs — and the rise of mammals.
More about impact craters tomorrow.
Script by Damond Benningfield, Copyright 2013