The best preserved impact crater on Earth, and the first one recognized as such (after a tremendous effort on the part of Daniel Barringer to prove it), was formed fifty thousand years ago by a one hundred fifty foot wide iron meteoroid, weighing the best part of three hundred thousand tons, which struck the ground at thirty or forty thousand miles an hour. Millions of tons of rock and metal were melted, vaporized and scattered across the surrounding countryside. Most of the debris was concentrated on the rim of the crater, nearly three quarters of a mile across and six hundred feet deep, formed by the explosion; but nearly thirty tons of iron meteorites, and at least a hundred thousand tons of magnetic spheroidal pellets which are condensation products from the portion of the impacting object which was melted by the impact are found all around the crater, at distances up to several miles. Anything within ten or twenty miles of the impact would certainly have been killed by the heat and blast effects accompanying the two megaton explosion which excavated the crater. (Refer to The Barringer Meteorite Crater for a fascinating discussion of the crater and its history)
Barringer Meteor Crater, near Winslow, Arizona (D. Roddy (LPI), apod990711)
Another view of the crater, with strong shadows emphasizing its relief.
(Stan Celestian, Glendale Community College (Arizona))
A ground view of Barringer Crater (Harald Stehlik, Earth Impact Database)
(Geoffrey Notkin, Aerolite Meteorites, Wikimedia Commons)
One of hundreds of thousands of fragments of the Canyon Diablo meteorite, as the object which created Meteor Crater is called. Native Americans collected the meteorites as sacred objects long before museums and collectors became interested in them.
The Canyon Diablo meteorite fragments were so numerous that in the 1950's and '60's, they were used to help sell small booklets, misleadingly titled A Comet Strikes the Earth, which discussed meteors and meteorites. Included in the sale price was a small sample of the Canyon Diablo meteorite, glued to the back cover and framed by a die-cut hole bored through the book. The images below show my copy, purchased at Griffith Observatory in the mid-50's for 50 cents (a week's allowance at the time), and a closeup of the oxidized meteorite fragment glued to the back cover.
Manicouagan Impact Crater
Manicouagan Crater in northern Canada is one of the oldest impact craters known, even though -- at 200 million years age -- it was formed 4300 million years after the Earth itself. The crater itself has been completely worn away, but the effects of its formation remain evident in the subsurface geology of the site, and in the 40-mile wide ring-shaped lake surrounding the central structure.
Manicouagan Crater, photographed from Space Shuttle Columbia in 1983 (STS-9 Crew, NASA, apod050101)
Just under fifteen million yars ago, an asteroid perhaps a mile in diameter struck the Earth at the location now occupied by the city of Nördlingen Ries in Bavaria, creating a double-ringed impact crater fifteen miles in diameter. At the same time, a smaller object struck about twenty miles to the southwest, creating a mile and a half crater with a ring and central peak, which is now occupied by the town of Steinheim. Thanks to weathering and erosion, the actual nature of these depressions -- now largely filled in -- was a matter of debate for centuries; but in 1960, Eugene Shoemaker showed that coesite, a mineral naturally found only in meteorite craters, was present in the area, proving their extraterrestrial origin. Additional investigation showed an abundance of suevite breccias, shattercones, and meltrock created by the impact. In fact, the Church of St George in Nördlingen is built almost entirely of suevite breccia, because its combination of glassy and rocky materials makes it a light, strong material, similar to concrete.
The creation of the Ries crater vaporized and melted a large amount of the surrounding region, and meltrock ejected through the hole in the atmosphere left by the passage of the asteroid followed a suborbital path which carried it to altitudes in excess of ten miles; and large amounts of greenish tektites resulting from the meltrock's return to Earth are found 150 miles away, in the Moldau Valley in Czechoslovakia.
The Ries Crater
Thanks to weathering and erosion, not much remains to show that this was ground zero for an impact and immense explosion, fifteen million years after the fact. (Bernd Haynold, Wikimedia Commons)
An aerial view of the Ries Crater.
The basin's shallow depression is emphasized by the clouds around its rim. This image opens an impressive and informative exhibition at the Ries Crater Museum (Rieskrater-Museum), in Nördlingen. (A.Brugger, © Nördlingen)
A postcard view of Steinheim, showing the meteoric depression and central rise. (A. Brugger, © Nördlingen)
The central uplift of Steinheim crater (Peter Seidel)
Cross-section of the Ries Crater (J. Pohl, Institute for Geophysics, LMU München)
Elevation model of Ries Crater, with selected geology emphasized (Earth Impact Database)
The Richat (Non-Impact) Structure
Less than a century ago, it was presumed that no Earth structures had been formed by impact. We now know of more than two hundred impact structures, despite weathering and erosion having erased all evidence of most of features created even during recent history. So it is tempting to suppose that any round, layered structure, such as the one shown below, may well be an impact feature; but that would be as incorrect as supposing that none of them were. Studies of the Richat Structure, in the Sahara Desert of Mauritania, show that it was caused by erosion of uplifted sediments. Why that uplift produced such a markedly circular structure is unknown; but no evidence has been found of any subsurface structure, high-pressure minerals, or shock features that would be associated with meteoric impact.
The Richat Structure -- a more obvious "crater" than most, but not an impact feature.
(NASA/GSFC/MITI/ERSDAC/JAROS, U.S./Japan ASTER Science Team, Earth Observatory)