Online Astronomy eText: Asteroids, Comets, and Interplanetary Debris
Meteor Craters
(also see Meteorites)
Barringer Crater (generally known as "Meteor Crater")
 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 Earth 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, that was formed by the explosion; but nearly thirty tons of iron meteorites and at least a hundred thousand tons of magnetic spheroidal pellets that 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 must have been killed or incinerated by the heat and blast effects accompanying the two megaton explosion which excavated the crater. (See The Barringer Meteorite Crater for a fascinating discussion of the crater and its history)
An aerial vew of Barringer Meteor Crater, near Winslow, Arizona
Barringer Meteor Crater, near Winslow, Arizona (D. Roddy (LPI), apod990711)

An aerial view of Barringer Meteor Crater, near Winslow, Arizona, with strong shadows emphasizing its relief
Another view of the crater, with strong shadows emphasizing its relief.
(Stan Celestian, Glendale Community College (Arizona))

A ground view of Barringer Crater, also known as Meteor Crater, in 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 that 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 from the 1940's onwards they were used to help sell small booklets, erroneously titled A Comet Strikes the Earth, which discussed meteors, meteorites and meteor craters. 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-1950's for 50 cents (a week's allowance at the time), and a closeup of the oxidized meteorite fragment glued to the back cover. (The booklets are now collector's items, generally selling for the best part of $100, and I wouldn't part with my copy for that.)

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.
The Manicouagan Crater, or more accurately the weathered remains of its basement rocks, as viewed from the Space Shuttle Columbia in 1983
Manicouagan Crater, photographed from Space Shuttle Columbia in 1983 (STS-9 Crew, NASA, apod050101)

Ries Crater
 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 numbers of greenish tektites resulting from the meltrock's return to Earth can be found 150 miles away, in the Moldau Valley in Czechoslovakia (as a result of which, those tektites are usually called moldavites.

A view of the Ries Crater from its interior, showing how weathering and erosion have almost completely removed any evidence of its origin
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 Ries Crater, showing how clouds near its rim outline its structure far more than the nearly level topography created by millions of years of weathering and erosion
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 postcard view of Steinheim, showing the meteoric depression and central rise. (A. Brugger, © Nördlingen)

The central uplift of Steinheim Crater
The central uplift of Steinheim crater (Peter Seidel)

Cross section of the Ries Crater and surrounding region, from a 1977 paper by Jean Pohl et al
Cross-section of the Ries Crater
(Image Credit J. Pohl, Institute for Geophysics, LMU München; no longer online due to retirement)

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 circular structure once mistaken for a meteor crater, but actually not an impact feature at all
The Richat Structure: a more obvious "crater" than most, but not an impact feature
(Image Credit NASA/GSFC/MITI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team Source)