Meteor Crater is a national landmark just east of Flagstaff, Arizona. For many years there were competing hypotheses as to how the crater formed, but unlike the craters just a little ways to the west, in the San Francisco volcanic field, this crater was formed by a meteor impact approximately 50,000 years ago.
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| Humphreys Peak is the tallest of the (dormant) volcanoes in the San Francisco Peaks, as well as the highest point in Arizona, at 12,633 ft (3,851 m). You can also see several smaller volcanoes in the field. |
In the early 20th century, despite the prevailing view that all craters were volcanic in origin, mining engineer Daniel Barringer became convinced that this crater was the result of a meteor impact, and he attempted to find and mine the meteoric iron. Unfortunately for him, the meteor was much smaller than he thought (approximately 50 m/160 ft in diameter) and almost all of it vaporized on impact, leaving only small fragments scattered around the crater.
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| The heat of the impact melted the surface of many of the rocks, leaving all sorts of interesting patterns. |
One of the indications of an impact that Barringer noticed was the inversion of rock layers. Here, the red sandstone is the most recently formed, which is usually on the surface (a principle known as the law of superposition in geology). Along the rim of the crater, however, the red sandstone is no longer at the top, but covered by lighter layers of dolomite, limestone, and another sandstone. At the time, Barringer's evidence did not convince scientists, but this inversion is still considered an important feature of an impact crater.
Several decades later, in the 1950s, Eugene Shoemaker studied the crater and, by comparing it with nuclear bomb test sites, demonstrated that there were specific characteristics that were most consistent with an impact. For example, he found certain minerals that are formed only under pressure much more intense than that of a volcanic eruption. Using what was learned at Meteor Crater, many other impact craters have since been identified around the world. (As a side note, Shoemaker, his wife Carolyn Shoemaker, and David Levy co-discovered the comet Shoemaker-Levy 9, which--in the first observed extraterrestrial collision--impacted Jupiter in 1994!)
At 1.2 kilometers (3,900 ft) in diameter and 170 meters (560 ft) deep, the crater is larger, deeper, and steeper than it looks in pictures. Immediately after the impact, the crater was actually 750 ft deep, but it has filled in with erosion from the crater walls and sediments from a lake that used to fill the bottom. At the end of the last ice age the region--and lake--dried up, and the fact that Meteor Crater is in a desert is actually part of why it is so well preserved (arguably the best preserved impact crater on Earth).
For a general sense of scale, here's a close up of the rightmost part of the circle (the remains of a lake) in the middle of the crater.
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| In addition to the remains of the exploratory mining operations, there is a cutout of an astronaut (6' tall) and American flag (3' x 5') on the fence. |
Another fun tidbit of Meteor Crater history is that in the 1960s and '70s, NASA astronauts trained in Meteor Crater to prepare for the Apollo missions to the moon.
As a visitor, one cannot climb down into the crater, but there are a few paths and lookouts that allow you to get a few different perspectives on it. Here's one more look at the steep edges of the crater.
Lacking the ability to become airborne, this last picture is not one that I took (it's from Google map's satellite view), but it provides one more perspective on the crater and shows on a much larger scale how the lighter and older rock was ejected from the crater and settled on the surrounding surface.
