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Meteor Crater Bomblets

Harvey Harlow Nininger did about the best examination of the impact materials found at Meteor Crater in Arizona as anyone ever has. Some of the things that he reported on in depth had been seen and described poorly by others over several decades. The tiny bits of iron in the ground had been collected with magnets from the first investigation by Gilbert in 1891. But it was Nininger that proved finally that most of the mass of the asteroid was vaporized and that thousands of tons of little metallic droplets were still present in the soil. Nininger created methods for separating the small bits of iron shale and meteorite fragments from the iron droplets that he named spheroids.

Iron Spheroids are tiny metallic particles that have the appearance of being melted. As tiny as they are Nininger actually divided them into large and small sized groups. These pictured here are ones that he would have called small. They are approximately .88 milligrams each in weight.

Nininger also recovered masses of terrestrial rock fragments held together with fused material. It might seem OK to use the words glass or glassy for this material but mucht of this melted material would be better described as slag. It formed from rock material derived from the Moencopi and Kaibab layers. The Kaibab is limestone and dolomite with some silica. And unlike the fine quality glass found at other craters where the target material is pure silica, the limestone with just a little silica produced a poorer fused slag product. The Coconino Sandstone which could have produced beautiful true glass was likely full of water which became super heated steam. This prevented the pulverized particles of quartz sandstone from fusing. Impactites named such by H. B. Stenzel are found at many craters. Generally they are masses of small broken bits of the target rocks held together by melted target material. Sometimes the melt is shiny high quality glass other times as at Meteor Crater it is less fused dull lusterless material that holds the small clasts of target rock together. The impactites from Meteor Crater are often similar in appearance to bubbly volcanic rock. However the presence of particles of nickel iron proves without doubt that they are asteroid impact in origin. The ones at Meteor Crater are the size of small pebbles. They were not completely missed by all the investigators exploring Meteor Crater before Nininger. G. P. Merrill makes mention of a “silico-ferruginous slag” that gave a strong reaction for nickel. This would be different from the small amounts of lechatelierite Barringer hit with his drills on the crater floor and found pieces of on the crater rim. Barringer’s finds did not have any nickel and were well fused glass. This would indicate that Merrill had found the slaggy impactite which was later really brought to the attention of the world by Nininger.

This is a group of impactites found at Meteor Crater. Externally they have a resemblance to small volcanic bombs. But closer examination shows they are different.
It would break my heart to cut any of my Meteor Crater impactites so I cut several Monturaqui crater impactites years ago. They are very similar to the ones found at Meteor Crater. They have the same kind of irregular iron bits inside them.

Nininger collected impactites from several areas around the perimeter of the crater and from its slopes. These poorly fused masses are often breccias of the several rock types pierced by the asteroid during the crater’s formation. They were found almost without exception to have some bits of nickel iron. Even the ones that seemed to have none did have a little metal if finely examined. This makes the impactite masses responsive to a magnet. It was I think a surprise for Nininger when he later discovered the very small melted objects which he named bomblets. The bomblets were his original discovery. They had never been described by earlier investigators. He found that there was a horizon of them buried under the surface in the soil of the crater slopes. He believed they were slag created by the upper layers of target rocks. He felt they had splashed to where they landed while the debris of the lower Coconino rock layer was still falling. The impactites were covered by this later ejecta and missed being discovered until they were dug up. He found the bomblets in large numbers. Like the various magnetic particles in the soil the separation of the bomblets from other material proved a challenge. But he overcame the difficulties and was able to concentrate the bomblets.

Like their larger cousins the impactites they are fused slag material. Essentially small droplets of melted rock. Where the larger impactites are generally responsive to a magnet with nearly all containing at least one bit of meteorite iron. The bomblets on the other hand seem to be much less responsive. Only around 40-50% are attracted by a magnet. The comparison of microtektites to the bomblets found at Meteor Crater is to this author not unreasonable. They both form in cosmic impact events and are both made of fused material derived from the target rocks. Though the distribution of the bomblets is restricted to an area very near Meteor Crater and the distribution of some of the microtektites is world wide. Still the two objects have some characteristics in common.

Both microtektites and Meteor Crater bomblets are tiny impact products. The bomblets have a wide size range within any group of collected specimens. The range I sampled is about .027 inches or .685 mm for the smallest to about .080 inches or 2.032 mm for the largest dimension of the bigger ones. As for weight that is of course all over a range too. But I took a sample of 100 and without sorting them weighed them on one of my most sensitive scales and the result was an average of 1.72 milligrams per bomblet.

The bomblets have both aerodynamic shapes like those of splashform tektites as well as fragmented shard-like shapes as if they were broken from larger pieces. Yet there is something about the overall look of the shard-like ones that tells the eye that it is complete within itself even though its shape is in no way smooth or contoured. They are bubbly vesicular objects with a coating of solid “glass”. Internally some are microscopic breccias with clasts of more than one color of rock. Others are just froathy droplets with a smooth surface. Egg shapes and round ended rod shapes are fairly common. Teardrops and flatter patty shapes are not uncommon. Specimens that are thready blobs and twisted rope-like pieces are seen in every batch examined under the microscope. These have a very similar appearance to certain Irghizites though very much smaller. Scarcer but still not rare are bomblets with a pimply surface of tinnier adhering dots of glass. Unlike Irghizites, Wabar Pearls, or even Henbury Glass where this pimpling is commonly seen on pure black glass, the bomblets are white, tan, brown and gray but never black. And they are not as shiny as the black impact glass products. They have a dull grainy surface for the most part. But even the ones that are smooth are dull. In this they resemble the much devitrified microtektites from deposits in Haiti.

Some of the bomblets have adhering iron spheroids. These are easy to see microscopically as dark brown spheres attached to the lighter colored bomblet. These are also easy to pick up with a magnetic needle. Most of the others will not be affected by the magnetic needle very much at all. As mentioned earlier bomblets were originally found by Nininger in a buried horizon in the southeastern part of Meteor Crater’s slope. He later found them in other locations and out to a distance from the crater. Nininger does not mention if iron spheroids were found in the same horizon as a separate material. And the bomblets are not mentioned as being a significant material in the soil sample he collected for the iron spheroids. Though he does mention in passing that the bomblets would not have been collected by his magnetic separation of the iron products. This made him think they could have been missed in those samples. However finding iron spheroids attached to bomblets would seem to indicate that they were forming or settling out of the impact cloud at the same period of time. This is in contrast to the metal in the larger impactites which matches the metallurgy of the meteorites. The composition of the spheroids is highly variable and often has much larger quantities of nickel and cobalt than the meteorites. It was thought by Nininger that this meant the metal bits which were usually irregular in shape in the impactites was the result of material stripped directly from the asteroid and immediately fused into the crushed rock of the impactites. The bomblets with round melted spheroids might indicate a later time of formation and that they formed after the major portion of the asteroid had been vaporized and condensed again.

The concentration of the bomblets in the soil drops off strongly after at a distance of just a half mile from the crater. Erosion has removed them from some areas and in other areas they are found still in great numbers.

From this point on I will share images of bomblets. These are not all the possible shapes I am sure. But it is hoped these will give the reader an introduction into these tiny impact products that were overlooked for so long. What else is Meteor Crater hiding for future investigators to find? It has been amazingly stubborn sometimes about yielding up its secrets.

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