Earth Impact Craters
Tuesday, November 13 2007 @ 03:10 AM CST
Contributed by: nspace01
|Barringer Meteor Crater, Arizona, USA|
From our backyards, even with the smallest of optics, we can readily observe many thousands of craters on the Moon's surface. These craters range from the size of city blocks to several hundred miles in diameter. The age of the Moon has been estimated to be approximately 4 billion years, based on the lunar samples brought back by the Apollo astronauts. Throughout the lunar life span the Moon has been a cosmic target by millions of rocks and pieces of ice — from a size smaller than a grain of sand to asteroids many miles in diameter.
But has the Earth been hit? Of course, the answer is YES.
It's well known that there is a constant fall of debris entering the atmosphere of our planet every day, with most particles as fine as dust, but some are as large as a basketball. From time to time, larger chunks enter our atmosphere and are large enough to survive the plunge. These actually strike the surface of the Earth.
Recently, one such meteorite made worldwide news, after it struck in northern South America. Luckily, there were no casualties, and the damage was limited to a small crater. There are almost monthly reports of meteorites, fire balls, or bolides. A bolide is a bright meteor that usually burns up in the atmosphere, but on rare occasions, they do strike the Earth's surface.
|Trees felled by the Tunguska blast. Photograph from Kulik's 1927 expedition.|
On June 30, 1908, a vast fireball raced through the dawn sky over Siberia, then exploded with the force of 1,000 Hiroshima bombs. The heat incinerated herds of reindeer and charred tens of thousands of evergreens across hundreds of square miles. For days, and for thousands of miles around, the sky remained bright with an eerie orange glow. As far away as Western Europe people were able to read newspapers at night without a lamp. The effect was much like that of a great volcanic eruption, yet there had been no eruption. The only objective indication of the extraordinary event was a quiver on seismographs in the Siberian city of Irkutsk, indicating a moderate quake some 1,000 miles north in a remote region called Tunguska.
|This shaded relief image of Mexico’s Yucatan Peninsula shows a subtle, but unmistakable, indication of the impact crater.|
|Manicouagan Impact Crater|
Many other “extinction” events are found in the geological record. 250 million years ago, over 95% of all life was destroyed by an event that the cause has yet to be determined, but is believed to be an even larger impact.
Are there any other physical signs of large impact events on the surface of the earth? Again the answer is YES. The much smaller Arizona Meteor Crater (35°, 01'N; 111°, 01'W) is a good example.
Google Earth is a freeware computer program that is available on the Internet at http://earth.google.com/. Google Earth is based on satellite and aircraft photography and is quite good. With this program, you can explore the earth's surface.
Many impact craters can be identified on the surface of the earth with Google Earth. Here are a few examples of visible impact craters located in North America:
|View of Hudson Bay in Google Earth|
Clearwater Lakes (56°13'N, 74°30'W) are located in Quebec, Canada. These twin circular lakes (20 & 14 miles in diameter) were formed simultaneously by the impact of an asteroid pair which slammed into the planet approximately 290 million years ago. Notice that the larger western structure contains a ring of islands with a diameter of about 10 kilometers that surrounds the center of the impact zone. They constitute a central uplifted area and are covered with impact melts.
The world's largest known earth meteor crater is Vredefort Dome (27°0'S, 27°27'E), located near Johannesburg, South Africa. The crater is approximately 200 miles in diameter and was formed about 2 billion years ago.
All of these craters and many, many more are visible with Google Earth.
Look at the south/east edge of Hudson Bay in Canada (56°43'N, 80°44'W).....It looks remarkably like Sinus Iridum in the north/east quadrant of the Moon. If it IS an impact crater, it would be approximate 300 to 350 miles in diameter...twice the size of the Chicxulub crater. Whew!!