An Article In Meteorite-Times Magazine
by Jim Tobin, Editor
Meteor Crater's Impact Materials
It seems only appropriate that I write in this first issue on the topic I love best of all: Barringer Crater. I wonder sometimes as I drive away from the crater if I will ever get tired of visiting. Yet soon after I get home I begin to figure out how to fit a side trip over to the crater into my next vacation. I am running out of locations in North Central Arizona to use as excuses for a visit. I got out some of the specimens in my collection and photographed them recently. The Internet is so visual, I thought I would share those shots with descriptions for this issue.
From practically the first day that Barringer arrived he put great emphasis on the pumiceous type metamorphic rocks he called Variety B. This material has been called many other names, but since pieces often float perhaps pumiceous is a good term to use. The material is laced with glass and is a froth of vesicles. Descriptions often talk of filaments of glass. Here is a picture of these threads of glass that often bridge cavities in the rock.
At various locations around the crater and out on the plain small glassy bits of rock of a very different type can be found. H. H. Nininger called these objects impactites and the name has stuck. In fact it has been applied to similar material from other craters ever since. The impactites from Meteor Crater have some properties that distinguish them for their cousins found elsewhere. They have very tiny blebs of metal compared to those of Monturaqui for instance. The number of clasts of native rock is less as well, with the major portion of their make up being the bubbly glass. As can be seen in the picture above there is only one clast and the small dots of metal are almost invisible. The white clast of native rock is approximately three millimeters in longest dimension. The next picture is of Monturaqui impactite photographed at the same scale. There are numerous pieces of the native rock and the metal bits are much larger and very visible. Even though the metal is very scare in the Barringer Crater impactites the magnetic attraction is quite similar. Both impactites are only slightly affected by an ordinary magnet. I am sure that my rare earth magnet would make a strong response but that seemed of less value to me as a test.
Unlike the metallic spherules that were known but not recognized for their true significance, the impactites were not even discovered until very late in the investigation of the crater. At any location around the rim and extending out onto the surrounding plain it is possible to collect with a magnet the tiny metallic spheres. They are fascinating objects. The best way I have found to separate them is to take a sample of soil (which can be purchased from the Meteor Crater Gift Store) and extract all the magnetic portion. Then with the point of a very weakly magnetized needle pluck out the perfect little balls of nickel iron. They are more magnetically attracted than the microscopic bits of iron shale that also abound in the soil. The largest of the spheroids seen in the accompanying photo is approximately .75 millimeters in diameter. The smallest ones are truly tiny, but as you can see perfectly round.
To finish out this discussion of impact materials from Meteor Crater I offer these pictures of shocked quartz grains. The pulverized sandstone is now rock flour of several types and found in several different layers. The allogenic breccia has features that distinguish it from the fallout layer or the lakebed deposits. Likewise they are different from each other also. What is easiest to see in the microscope is the condition of the grains themselves. Some of the grains are round and nearly whole. Though they will often have a flat edge or sliver missing. Grains from other deposits will have several missing edges and be very angular. The worst of the crushed material will have almost no large pieces of relic grains being predominately pulverized dust. The percentage of damaged grains and the percentage of tiny rock flour particles in the various layers is readily seen in the scope. I have included photographs from three of the most commonly discussed layers. I will leave the viewing and deciding to you.
What is obvious from all these differing materials is the tremendous level of energy that was expended in forming the four thousand foot wide chasm. We know from Dr. Nininger's work that a large amount of the mass of the asteroid was converted into vapor which cooled forming the tiny nickel iron spheroids. Sometimes it is the tiniest evidence that yields the greatest amount of information. Till next month.