When a meteorite struck Earth before humans were around towatch, did it still make a "splat?" Although it's too late to witnessthe many pummelings our planet has already seen, scientists are still findingthe humongous holes left here by long ago impacting space rocks.
At last count, there were more than 170 known impactcraters on our planet, according to the Earth Impact Database maintained bythe University of New Brunswick in Canada. These puncture wounds are litteredover every continent, as well as the seafloor.
There would be countless more if it weren't for Earth'sconstant remodeling. Plates shift, mountains form, volcanoes erupt and erosionwashes over the planet's surface, continually hiding the evidence of mostcraters.
"If there was no erosion or tectonic activity, we wouldlook like themoon," said Lucy Thompson, a geologist at the University of NewBrunswick. "The moon is just pockmarked with impact craters."
Scientists think the Earth was bombarded more heavilyearlier in the solar system's history, when planets were still forming andbushels of debris were flying madly around. Luckily for us, things have quieteddown lately and meteorite impacts are few and far between.
One of Earth's most recently-formed holes is Arizona'sBarringer Meteor Crater, created around 50,000 years ago. Though this crater,one of the most famous, awes tourists with its roughly three-quarters of a mile(1.2 km) diameter, it is considered quite dinky on the geological scale.
"That's a nice, simple bowl-shaped crater,"Thompson said.
Geologists get really excited about complex craters, such asManicouagan in Quebec, Canada. Scientists estimate this crater is more than ahundred times wider than Barringer, and was made more than 200 million yearsago.
"With large impacts, you have complex craters forming,and instead of having a nice bowl shape, you get a central uplift,"Thompson told SPACE.com. "It's like if you drop something in water,you get rings forming, but the middle comes back up."
Scientists want to understand how the rock achieves thiswithout actually becoming liquid or shattering into pieces.
Big and bad
A major heavyweight is South Africa's Vredefort crater,which at 186 miles (300 km) wide, is said to be Earth's largest verified impactcrater. At more than 2 billion years old, it is also one of the most ancient.
Other contenders are the 155 mile-wide (250 km-wide) SudburyBasin in Ontario, Canada, and the roughly 110 mile-wide (180 km-wide) Chicxulubcrater, half submerged off the coast of the Yucat?n Peninsula in Mexico.
The latter can claim fame as the landing spot of the asteroidthat purportedly killed the dinosaurs, along with most life on Earth.
If it weren't for erosion and other geological processesthat erase evidence of craters, there would likely be hundreds of thousands ofimpact craters on the Earth, Thompson said. Scientists are still discoveringnew craters, especially in remote areas and on theseafloor where evidence of them is easily missed.
- Gallery: Earth's Meteor Craters
- Video: Newsreel - The Tunguska Expedition
- Top 10 Ways to Destroy Earth
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Clara Moskowitz is a science and space writer who joined the Space.com team in 2008 and served as Assistant Managing Editor from 2011 to 2013. Clara has a bachelor's degree in astronomy and physics from Wesleyan University, and a graduate certificate in science writing from the University of California, Santa Cruz. She covers everything from astronomy to human spaceflight and once aced a NASTAR suborbital spaceflight training program for space missions. Clara is currently Associate Editor of Scientific American. To see her latest project is, follow Clara on Twitter.