Early in the history of the solar system, when space was cluttered with the materials of its formation, the planets and their moons were heavily bombarded by meteorites. Some of the members of the solar system (Mars, Mercury and our moon, for example) still show the residuals of the primordial rain of iron and stone. On our dynamic planet earth, erosion by weather, water and ice and the continuous reshuffling of crustal plates have erased most of the evidence of that early cratering.
The solar system, not yet completely clear of the cosmic ebris which was left over at its birth, continued to rain small meteorites down upon the planets, and occasionally the earth is struck by an object large enough to excavate a sizable hole. Dozens of meteorite impact craters have been recognized on the crust of the earth. In most cases, erosion has removed all but the shattered root zones of the craters. The most famous terrestrial impact crater is in the desert near Winslow, Arizona.
Origin of Meteor Crater What happens when an irresistible force meets an immovable object? Meteor Crater ! 50,000 years ago, a huge iron-nickel meteorite, hurtling at about 40,000 miles er hour, struck the rocky plain of Northern Arizona with an explosive force greater than 20 million tons of TNT. The meteorite estimated to have been about 150 feet across and weighing several hundred tons, in less than a few seconds, left a crater 700 feet deep and more than 4000 thousand feet across. Large blocks of limestone, some of them, the sizes of small houses were heaved onto the rim.
Flat-lying beds of rock in the crater walls were overturned in fractions of a second and uplifted permanently as much as 150 feet. Today, the crater is 550 feet deep, and 2. miles in circumference. Twenty football games could be played simultaneously on its floor, while more than two million spectators observed from its sloping sides. In 1902, Daniel Moreau Barringer, a Philadelphia mining engineer, became interested in the site as a potential source for mining iron. He later visited the crater and was convinced that it had been formed by the impact of a large iron meteorite.
He further assumed that this body was buried beneath the crater floor. Barringer was correct. The crater was formed by a meteorite impact, but what he did not know was that the eteorite underwent total disintegration during impact through vaporization, melting and fragmentation. In 1903, he formed the Standard Iron Company and had four placer mining claims filed with the federal Government, thus obtaining the patents and ownership of the two square miles containing the crater. Barringer spent the next 26 years attempting to find what he believed would be the giant iron meteorite.
Barringer never found what he was looking for, but he did eventually prove to the scientific community that the crater was the site of a meteorite impact. ATextbooks are concerned with presenting the facts of the case (whatever the case may be) as if there can be no disputing them, as if they are fixed & immutable. And still worse, there is usually no clue given as to who claimed these are the facts of the case, or how Ait@ discovered these facts (there being no he or she, I or we).
There is no sense of the frailty or ambiguity of human judgment, no hint of the possibilities of error. Knowledge is presented as a commodity to be acquired, never as human struggle to understand, to overcome falsity, to stumble toward the truth. -Neil Postman. The End of Education Grove Karl Gilbert, the first person to conduct a full scientific survey of the mysterious crater in the Arizona desert, was the most renown geologist of his generation, and has been described as Aperhaps the closest equivalent to a saint that American science has yet produced. Hoyt, p37) He was tolerant, generous, and fair-minded, with an intense dislike of controversy of any kind. As chief geologists of the U. S. Geological Survey, his prestigious demeanor was held in high esteem. such that none of his colleagues or successors ere willing to publicly question his conclusions-even when it became apparent that some of those conclusions had been wrong. In 1891, Gilbert became interested in reports of a large collection of nickel-iron meteorites found in the neighborhood of a gigantic circular crater in the Arizona desert.
Since he had already speculated on the possible consequences of al large meteorite striking the earth, he decided to visit the crater and try to determine had it been the result of such an impact. Gilbert=s methodical approach to research, considered two alternative hypotheses for the formation of the crater. First that it had been formed by a meteorite, second, that is was he result of a massive explosion of steam, produced by volcanic heat at a great distance below the surface.
The idea that it might be an actual volcanic crater was ruled out by the absence of any volcanic rocks at the site. In his expedition to the crater (then called Coon Butte or Coon Mountain) in October of 1891, he devised two tests of the impact hypothesis. First, he reasoned that if the crater had been produced by an explosion, he material ejected from it would be equal in olume to the crater=s hollow, If it had been produced by a meteorite, on the other hand, the meteorite would still be there.
Lacking our modern understanding the mechanics of impact at planetary speeds, Gilbert assumed that the size of the meteorite would be similar to the size of the crater, and that it would fill a substantial portion of the hollow, The volume of the hollow would thus be smaller than that of the ejected material on the rim. The second test involved the supposed magnetic effect of a large volume of buried iron. If a mass of iron arge enough to produce the crater was still present below the surface, its attraction would affect the direction of a compass needle, creating local anomalies.
Both tests turned up negative. The volume of material in the crater rim jut equaled the volume removed from the hole. A variety of experiments with magnets produced no indication of a large mass of buried iron. The idea of a volcanic steam explosion was thus, in Gilbert=s view, the Aonly surviving hypothesis. @ The presence of meteorites in the vicinity of the crater was simply a coincidence. Gilbert used his investigation of the crater, and his own abandonment of the impact hypothesis, in a series of 1895 lectures illustrating the application of the dispassionate scientific approach.
It was left to an inherently different personality to demonstrate the limitations of that approach. There are only three different types of rocks represented at Meteor Crater. Just under the topsoil is the dark reddish brown Moenkopi sandstone. Below the Moenkopi are the yellow and orange layers of the Kaibab limestone. For several hundred feet below the Kaibab limestone there is only Coconino andstone. It is this grayish rock which is seen making up most of the crater walls. It is pulverized Coconino which is white that was dug up in the first shafts .
Piles of this white silica are very obvious on the crater floor still today. The Kaibab limestone at the crater contain a variety of fossils. (See attached photos) The layers of rock all around the crater rim have been uplifted and tilted, however, at the south cliffs the rock are vertical. It has been suggested that the force exerted on the south cliffs 0was greater because of the distance to the focus of the explosion was shorter. If the body of the asteroid came to a stop and released its kinetic energy under the south cliffs they would be closest to the focus.
Barringer was convinced that the remaining body of metal was under the south rim. The drill on the south rim hit something meteoric. The rocks of the south cliffs and rim are also displaced much more than the rest of the rim. Weighing in at over 1,400 pounds, the Holsinger Fragment is the largest of the meteorites found at Meteor Crater. Named for Samuel Holsinger who had been Barringer Foreman and drill supervisor for many years it now rests in the museum on the crater rim. Like many of the Canyon Diablo meteorites, it has holes in it.
Some of these holes were probably graphite nodules burned away. Nodules of graphite are very common in Canyon Diablo meteorites. The topographical terrain of Meteor Crater so closely resembles that of the earth=s moon and other planets, NASA designated it as one of the official training sites for the Apollo Astronauts. The US Government deemed the crater a Natural Landmark in 1968. It is still privately owned by the Barringer family. They lease the crater to Meteor Crater Enterprises, who operate the visitors center at the crater.
