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Thursday, March 23 2017 @ 10:22 am EDT

Earth Impact Craters

Lunar & Planetary
Barringer Meteor Crater, Arizona, USA
When we look at Earth's Moon, as well as NASA photos of Mars, Mercury, the moons of the outer planets, and asteroids such as Eros, the most obvious surface features are impact craters.

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.
The most recent large bolide exploded over Tunguska, Siberia.

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.
Scientists did not go to Tunguska for another 19 years, apparently reluctant to travel to a site so swampy and remote. When they did finally arrive, they were rewarded with a stunning vision of utter devastation, with scorched trees lying in rows that stretched to the horizon. They searched for a crater but none was ever found. They searched for fragments of a meteorite — an asteroid or a chunk of one — but found nothing. All they found were eyewitnesses in neighboring villages who told of a fireball streaking through the sky, a horrifying noise, and a blast that knocked people off their feet. Clearly something unprecedented had occurred at Tunguska, but the trees were the only tangible proof that remained. If this had occurred over a populated area of the earth, I could not imagine the death and destruction that would have been the result.

Manicouagan Impact Crater
It is known now that approximately 65 million years ago, a Mount Everest-sized asteroid impacted in the area of the Mexican Yucatan Peninsula, resulting in the extinction of the dinosaurs and most other species of life on the earth. The large Chicxulub Crater (21°, 20' N; 89°, 20' W), from that event, is between 112 and 186 miles in diameter and is buried under may miles of sediment and ocean.

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.

Mistastin Lake
The Barringer Meteor Crater in the state of Arizona was the first crater to be identified as an impact crater. Between 20,000 to 50,000 years ago, a small asteroid about 80 feet in diameter impacted the Earth and formed the crater. The crater is the best preserved crater on Earth and measures ¾ mile in diameter. For many years, scientists had denied that there were any impact craters on Earth. The origin of this crater had been a source of controversy for many decades.

Clearwater Lakes
Usually, finding impact craters on Earth is difficult due to water, wind erosion, and changes on the surface of the Earth as a result of other geological events. The craters slowly fade way over time. Most impact craters are only detectable by the changes in the consistency of the surrounding surface. Many are visible, nonetheless.

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:

Vredefort Dome
Manicouagan Impact Crater (51°23'N, 68°42'W) in Quebec, Canada is obviously an impact crater, especially since the 1960s, when hydroelectric dams were built to form the Manicouagan Reservoir . This is a NASA photo taken from the space shuttle. This is one of the largest impact craters on earth that has been identified, measuring approximately 62 miles in diameter. The event occurred 212 million years ago.

View of Hudson Bay in Google Earth
Mistastin Lake (55°53'N, 63°18'W), Newfoundland/Labrador, Canada has been identified as an impact crater and is 17 ½ miles in diameter. This is a shuttle image showing a winter view of the Mistastin Crater, a heavily eroded complex structure. Eastward-moving glaciers have drastically reduced the surface expression of this structure, removing most of the impact melt sheet and exposing the crater floor. Glacial erosion has also imparted an eastward elongation to the crater that is particularly evident in the shape of the lake that occupies the central 6 miles of the structure. Horseshoe Island, in the center of the lake, is part of the central uplift as seen on lunar craters. The crater was formed approximately 40 million years ago.

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!!
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