An Article In Meteorite Times Magazine
by Jim Tobin

 

Impactites: Glass? Maybe Just Barely

Its been a while since my article turned back toward Meteor Crater. Though I am never far from the crater. I am almost constantly writing something about to be used in a publication or website. My book is also done and if I can just create a back cover then it will be ready to print. I need to do that too or I will start editing and adding again. Then the whole project will go back into limbo. As it is I have had to make some decisions about what to include to maintain some reasonable size and affordability. In a different world I would have included a lot more of the old original papers. However, it is just not practical to try and print and sell a 1000 page book on Meteor Crater. So 200 pages will have to satisfy for the present time.

 

But, enough about that. It is the impactites found at the crater that I want to share this month. I have discussed them before but not to this monthís detail. The early years of investigation brought many prominent scientists to Meteor Crater. It is surprising that none would find the impactites. Decades would pass and finally Nininger would discover them. There had been much debate about the lack of evidence seen at the crater for great heat. Again, the billions of spheroids in the soil were misunderstood until Nininger revealed their origin as the vaporized asteroid. The rocks of the crater do show some heat effects, but the nice glass present at many impact sites is missing at Meteor Crater. There are no lovely black glass beads and drops as at Wabar. No elegantly spun ribbons and threads of glass as at Zhamanshin. Not even black glass masses as are found at Henbury. There just was not much glass at Meteor Crater. Or so it seemed until the impactites were discovered. And even then the impactites were a far cry from the finely vitrified glass of some other craters.

Meteor Crater impactites are bubbly crude masses of poorly melted rock. But, they are glass or contain glass. Most will be magnetically responsive. They contain small particles of nickel iron. The impactites are never very large at Meteor Crater. In fact I have impactites from other craters that are similar and they are often much larger than the ones from Meteor Crater. I took a complete sampling of the impactites from my collection and got an average weight of 3.07 grams. Of course since they are so bubbly they have a rather low specific gravity, they are mostly 15-20 millimeters in major dimension. Some specimens have been found that are larger than this running up to 40-50 millimeters. And they occur in smaller sizes down to microscopic. These I have seen in unsorted batches of spheroids.

Meteor Crater impactities have a slag-like appearance. Most are roughly ovoid in shape, less commonly are found ones with flow shapes. Rarely teardrop shapes. Most are rough on the exterior. Once in a while you may find one with a broken surface that exposes a large bubble. The texture of these exposed internal bubbles will be the smoothest of the surfaces. Even these will not be very shiny.

If the exterior of an individual is intact they will sometime show surface characteristics very similar to that of volcanic bombs or interestingly Reis bombs which are of course larger versions of impact melts.

Impactites have been referred to as splash droplets. This name offers us some good information about the product. They would appear to have been a high viscosity liquid that made a very short flight. Their superficial resemblance to volcanic cinders calls to mind pictures of bubbling lava and the splashing volcanic liquid. This raises the question of when did the nickel iron grains get incorporated into the melt. Were they falling down as rain onto the still liquid impactites already on the ground? Were there pools of melted material? Or was the mixing of the metal and the liquid rock something that happened during the formation and flight of the impactites? Or are the metal particles in the impactites even from the same rain of metal as the spheroids. Impactites donít look very much like condensation products. Which would by necessity have to form from fully melted material at extreme temperatures. The impactites look like poorly melted crushed rock, which got barely liquid and traveled only a short distance in flight. Their metal may be more like Niningerís slugglets; tiny surviving shards of metal that were mixed with the crushed rock. All this mix being poorly fused into the small impactite masses.

Because the craterís true floor hundreds of feet down was never reached to really study. Little of what might be a melt sheet has been seen. I am still wondering about what the melted material at the time of the impact was like. Though it is a little more glassy, the trinitite from the first atomic bomb test is also similar in appearance. The next picture is of a specimen of trinitite about the same size as the impactite show in the first picture of this article, 27mm in largest dimension.

Overlooked for several decades impactites are now evidence for the amount of thermal energy released at Meteor Crater. Can the differing states of vitrification seen in impact melts from craters be used as a thermometer. Water in the rocks at Meteor Crater had some kind of effect on the melting that occurred. Did its presence moderate the extent to which the temperature rose? Other craters often much smaller have much finer glass and show higher temperature effects. Is it just water being present? Or is it the kind of target rocks, the type of impactor, or other factors that determine the melting that occurs. Clearly from the size of the crater the impactor was traveling at cosmic speed the explosion is correct, but not the heat effects on the rocks. Several hundred feet of aquifer were punched through and immediate flooding is very likely to have happened at Meteor Crater.

Monturaqui Crater has impactites that are similar to those of Meteor Crater. Most from there also contain grains of metal. But, when cut there are clasts of several different rocks incorporated as angular bits. These impact breccias from Monturaqui like impactites from Meteor Crater are poorly melted. Melted just enough to hold the bits together. The clasts are mostly not melted from Monturaqui. Meteor Crater impactites are less brecciated and finer grained. A small amount of iron shale remains at the older Monturaqui crater. But, if there had been no remaining shale or meteorite fragments would the presence of the nickel-iron grains preserved in the impactites have been enough evidence to support an iron asteroid as the impactor. Other types of asteroids could have provided the iron as well. How do impactites and melt glasses fit as diagnostic indicators for crater parent bodies. Debate remains about what type body produced Libyan Desert Glass. Though there has been a big swing toward a chondrite body a cometary origin has not been ruled out. Like the metal particles in impactites the trapped material in LDG is analyzed for clues.

I love the Canyon Diablo specimens in my collection. But, in many ways the other materials from Meteor Crater are even more interesting.