Meteor Crater
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The Meteor Crater, sometimes known as the Barringer Crater and formerly as the Canyon Diablo Crater, is an impact crater, located about 35 miles (55 km) east of Flagstaff, near Winslow in the northern Arizona desert (USA). Its elevation is 1744 m (5,723 ft). The crater is somewhat misnamed, as it was actually excavated by a meteorite, not a meteor.
The crater is about 1,200 m (4,000 ft) in diameter, and some 170 m deep (570 ft). It is surrounded by a 45 m (150 ft) high rim of rock, raised above the surrounding plains by the force of the impact. The bottom of the crater is filled with rubble from the impact, to a depth of 210-240 m (700-800 ft) below the crater's center.
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[edit] Formation of the crater
The crater was created about 50,000 years ago during the Pleistocene epoch when the local climate on the Colorado Plateau was much cooler and damper. At the time, the area was an open grassland dotted with woodlands inhabited by woolly mammoths, giant ground sloths, and camels. It was uninhabited by humans, the first of whom are thought to have reached North America only around 13,000 years ago.
The object which excavated the crater was a nickel-iron meteorite about 50 meters (54 yards) across, which impacted the plain at a speed of several kilometers per second. The speed of the impact has been a subject of some debate. Modelling initially suggested that the meteorite struck at a speed of up to 20 kilometers per second (45,000 mph), but more recent research suggests the impact was substantially slower, at 12.8 kilometers per second (28,600 mph). It is believed that about half of the impactor's 300,000 tonne (330,000 short tons) bulk was vaporized during its descent, before it hit the ground. [1]
The impact produced a massive explosion equivalent to at least 2.5 megatons of TNT – equivalent to a large thermonuclear explosion and about 150 times the yield of the atomic bombs used at Hiroshima and Nagasaki. The explosion dug out 175 million tons of rock. The shock of impact propagated as a hemispherical shock wave that blasted the rock down and outward from the point of impact, forming the crater. Much more impact energy, equivalent to an estimated 6.5 megatons, was released into the atmosphere and generated a devastating above-ground shockwave.
For a meteorite of its size, the impact melted surprisingly little rock, though it produced high enough temperatures and pressures to transform carbon minerals into diamonds and lonsdaleite, a form of diamond found near the crater in fragments of Arizona's Canyon Diablo meteorite. Limestone blocks as massive as 30 tons were tossed outside the crater's rim, and debris from the impact has been found over an area of 100 square miles (260 km²). The shock of the impact would have produced a localized earthquake of magnitude 5.5 or higher.
The blast and thermal energy released by the impact would certainly have been lethal to living creatures within a wide area. All life within a radius of three to four kilometers would have been killed immediately. The impact produced a fireball hot enough to cause severe flash burns at a range of up to 10 km (7 miles). A shock wave moving out at 2,000 km/h (1,200 mph) leveled everything within a radius of 14-22 km (8.5-13.5 miles), dissipating to hurricane-force winds that persisted to a radius of 40 km (25 miles).
Despite this destruction, the Barringer impact did not throw up enough dust to seriously affect the Earth's climate. The area was likely recolonized by the local flora and fauna within a century. This did not much affect the crater itself; its preservation was aided by the local climate's shift to its present-day arid conditions.
The meteorite itself was mostly vaporized. Relatively large chunks of nickel-iron fragments, ranging from gravel size to blocks weighing up to 640 kg (1,400 lb), have been recovered from the debris field surrounding the crater. Several thousand tons of tiny nickel-iron droplets, the size of sand grains, fell in and around the crater after condensing from the cloud of metallic vapour produced by the impact. Very little of the meteorite remained within the pit that it had excavated.
[edit] Discovery and investigation
Although the local Native American peoples would have known about the crater – the Ancient Pueblo Peoples lived relatively nearby at Wupatki – it was not until the 20th century that its origins were explained scientifically. The crater had come to the attention of scientists following its discovery by European settlers in the 19th century. Dubbed the Canyon Diablo crater, it had initially been ascribed to the actions of a volcano. This was not an unreasonable assumption, as the San Francisco volcanic field lies only about 40 miles to the west.
In 1903 a mining engineer and businessman named Daniel Moreau Barringer suggested that the crater had been produced by the impact of a large iron-metallic meteorite. Barringer's company, the Standard Iron Company, purchased the crater and conducted research on its origins between 1903 and 1905. It was concluded that the crater had indeed been caused by a violent impact. Barringer and his partner, the mathematician and physicist Benjamin C. Tilghman, documented the evidence for the impact theory in papers presented to the U.S. Geological Survey in 1906 and published in the Proceedings of the Academy of Natural Sciences in Philadelphia.
Barringer's arguments met with skepticism, as there was a general reluctance at the time to consider the role of meteorites in terrestrial geology. He persisted nonetheless and sought to bolster his theory by uncovering the remains of the meteorite. At the time of first discovery by Europeans, the surrounding plains were covered with about 30 tons of large oxidized iron chunks from the meteorite. This led Barringer to believe that the bulk of the impactor could still be found under the crater floor. As impact physics were poorly understood at the time, Barringer was unaware that the meteorite had in fact vaporized on impact. He spent 27 years trying to mine the crater and find metallic iron, drilling to a depth of 419 m (1,376 ft), but no significant deposit was ever found.
Although many geologists remained skeptical of the crater's meteoritic origins until as late as the 1950s, it gained increasing acceptance as planetary science gained in maturity. Professor Herman Leroy Fairchild, an early promoter of the idea of meteorite impact cratering, argued Barringer's case in an article in Science in 1930. [1] [2] It was not until 1960 that later research by Eugene M. Shoemaker would confirm Barringer's hypothesis. The key discovery was the presence in the crater of the minerals coesite and stishovite, rare dense forms of silica found only where quartz-bearing rocks have been severely shocked by a large meteorite impact. They cannot be created by volcanic action; the only known mechanism of creating them is through an impact event.
Shoemaker's discovery caused a sensation in the geological world, as it was the first definitive proof of an extraterrestrial impact on the Earth's surface. Since then, numerous impact craters have been identified around the world.
[edit] Meteor Crater today
Meteor Crater is today a popular tourist attraction, easily reached via meteor crater road off I-40. There is an entrance fee to see the crater. Despite its importance as a geological site, it is not protected as a national monument. The crater is still owned by the Barringer family.
A visitor center operated by the Barringer Crater Company stands on the north rim of the crater. The crater continues to be a focus for scientific research; during the 1960s, NASA astronauts trained there for missions to the Moon.
[edit] References
[edit] See also
[edit] External links
- Barringer Crater official site
- Alternate official site
- Meteor Crater Rim Tour and Floor Tour
- CNN: New computer analysis shows that the meteor may have been traveling more slowly than previously thought
- Computer generated movie based on satellite imagery and topography data
- Aerial Exploration of the Barringer Structure
- Herman Leroy Fairchild: An Early Promoter and Defender of Meteorite Impact Cratering - includes details of early investigations into Meteor Crater
- Metallic Meteorite Spheroids from Meteor Crater From Meteorites Australia - Meteorites.com.au
[edit] View the Crater
