IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
BARRINGER IMPACT CRATER
a Three different dating techniques, taking advantage of the natural decay of naturally radioactive elements, have been used to determine the age of the impact crater. All three techniques are in agreement:
General Area: This best known of all impact craters is 60 km ESE of Flagstaff, AZ on a flat plain south of the Little Colorado River. Although the crater is 1.19 km wide, it is very conspicuous because its bright rim contrasts with the darker plain of sedimentary rocks.
D. Barringer assembled evidence to support an impact origin for the Barringer Crater. He then presented the following arguments for the impact origin of the crater to the Academy of Natural Sciences in Philadelphia in 1906, and again in 1909:
- The presence of millions of tons of finely pulverized silica, which could only have been created by enormous pressure.
- The large quantities of meteoritic iron, in the form of globular "shale balls", scattered around the rim and surrounding plain.
- The random mixture of meteoritic material and ejected rocks.
- The fact that the different types of rocks in the rim and on the surrounding plain appeared to have been deposited in the opposite order from their order in the underlying rock beds.
- The absence of any naturally occurring volcanic rock in the vicinity of the crater.
These conclusions were championed by geologist George P. Merrill. Merrill analyzed a new type of rock discovered by Barringer at the crater, which Barringer called "Variety B". He concluded that it was a type of quartz glass which could only be produced by intense heat, similar to the heat generated by a lightning strike on sand. Merrill also pointed to the undisturbed rock beds below the crater, which proved that the force which created the crater did not come from below (Merrill 1908).
It was in 1920 that the structure was finally recognized to be an impact crater, the first feature on Earth to be so recognized.
-the 260 million year old Coconino sandstone (Permian Age),
-the yellowish Toroweap formation, a Permian sandstone,
-the cream coloured 250 million year old Kaibab formation sandstone,
-the Kaibab 10 million year unconformity (the contact between older rocks and younger sedimentary rocks in which at least some erosion has removed some of the older rocks before deposition of the younger), and
-the 240 million year old reddish brown Moenkopi sandstone top layer of the crater wall (over the unconformity).
The rim of the 1.19 kilometre diameter Barringer Crater is still well defined, even after approximately 49 thousand years of erosion. It has been estimated that the first two stages of the cratering process (time from initial contact of the impactor until the end of the excavation stage) here at Barringer took approximately 6 seconds! Almost 63 million cubic metres were evacuated from this area in that time to form the crater. The height of the rim over the surrounding plain is 36 - 61 metres. Investigations around this rim confirmed an "overturned rim sequence".
An overturned rim sequence is also present at the rim of the Barringer Crater and is now recognized as one of the hallmarks of an impact crater.
Some of the Barringer Crater ejecta curtain can been seen here in visible light as illustrated in this image. I took this image of the crater from about 1000' above the ground. The "lighter" coloured sand is the remnants of the ejecta curtain. The pattern of the ejecta implies that the direction of the impactor was from the upper left of this image. If I was flying here at the time of impact, 49 thousand years ago, I would not know what hit me!!
We did a complete orbit of the crater in order to document it from each direction. Note the “square” shape of the crater, technically called a jointed crater. This shape is determined by the faults in the geology of country rock at the impact site.
In the "east view" image, if you look at the crater floor at the 5 o’clock position you will see a small white dot. Apparently that is the remains of a small aircraft. The pilot flew into the crater but with the tight turning radius required to stay away from the crater wall, he could not climb to exit the crater. It is a lesson in density altitude VS climb rate in steep turns. The altitude of the crater rim is over 5500’. The pilot and passenger survived albeit in a slightly “bent up” condition.
An excellent documentation of an exploration trip to the bottom of the Barringer Crater can be viewed at: www.meteorite.com
Barringer, D. M., 1906, Coon Mountain and its crater: Proceedings of the Academy of Natural Sciences of Philadelphia, v. 57, p. 861-886.
Buchwald, V.F. (1975) Handbook of iron meteorites. University of California Press, Berkeley, v. 3, pp. 937-942.
Kring David A., Guidebook to the Geology of Barringer Meteorite Crater, Arizona (a k a Meteor Crater), The 70th Annual Meeting of the Meteoritical Society, August 2007
Merrill, G.P. (1908) The Meteor Crater of Canyon Diablo, Arizona; its history, origin, and associated meteoric irons. Smithsonian Miscellaneous Collections, v. L, no. 1789, pp. 461-498 (with muliple plates).
TAGLE, R. and HECHT, L., Geochemical identification of projectiles in impact rocks. Meteoritics & Planetary Science Volume 41, 26 JAN 2010.