Before southern Australia was occupied by the white man, the native
tribesmen of that region treasured certain small rounded pieces of black
glass as medicine stones, rainmaking stones, and message stones. The
Wadikali tribe referred to these objects as _mindjimindjilpara_, a word
meaning “eyes that look at you like a man staring hard.” The early
European settlers of the area called the same black glassy masses
“blackfellows’ buttons.” Both phrases applied to objects that modern
scientists call “australites,” which are now one of the best known types
of _tektites_ (Greek: _tēktos_, molten).
These Australian tektites and the tektites from many other countries
around the world are a problem to meteoriticists. The question is, are
they really meteorites? Many investigators believe that the answer is
yes, and they are inclined to add to the three main divisions of true
meteorites listed in the preceding chapter, a fourth: the tektites.
These mysterious glassy objects occur in such widely separated
localities as Czechoslovakia, the Philippine Islands, Borneo, the Ivory
Coast of Africa, Australia, Indo-China, Texas, Malaya, and Java. In
these and still other areas, they have been found by the thousands in
surface deposits of sand, clay, and gravel.
[Illustration: (left) “Flanged buttons” from Australia. (right)
Several sizes of “dumbbells” from Australia. See p. 136.]
Tektites have never been seen to fall. In spite of this fact, as we
noted above, a number of scientists believe that, like the meteorites,
the tektites really did come from outer space but, that they fell to
earth before man was here to see them come down—or at least before he
had acquired the means and skill to make lasting records of such an
occurrence.
Tektites are usually quite small, weighing between 1 and 100 grams,
although a few of much larger size have been found. One large specimen
from the Philippines weighed about ½ pound. Two giant tektites, one
weighing ¾ pound and the other over 1 pound, are in the collection of
the British Museum. In composition, tektites are an impure silica-glass
containing low percentages of the oxides of such elements as iron,
magnesium, calcium, and titanium.
If tektite fragments are held under a lamp and observed by reflected
light, their thicker parts generally appear to be jet-black. If,
however, these same specimens are held up _between_ the observer and the
light, then their thin razor-sharp edges are seen to be bottle-green,
yellow-green, brownish, or even colorless.
In shape, many tektites are roundish or oval. Others are shaped like
dumbbells, ladles, canoes, and teardrops. So they are known by those
descriptive terms. One particularly interesting example is the unusual
“flanged button” of Australia. Tektites of this type look like miniature
South American gold-pans, the _bateas_, heaped high with pay dirt.
Australian gold-field workers regarded these tektites as magical, and
used them as good-luck charms. Superstitious American gold-seekers
brought them into the United States all the way from Australia!
[Illustration: (above) Rounded tektite from Texas. (below) Deeply
grooved bediasite from Texas.]
Some tektites (for example, many of the “bediasites” from Texas) are
deeply grooved and channeled, and have a very jagged and irregular
appearance. Even the smoother tektite surfaces are characterized by flow
lines, flow ridges, and bubble pits.
Many weathered pebbles and fragments of obsidian somewhat resemble the
tektites superficially. There is a very simple test by which you can
distinguish true tektites from obsidian. If you hold a thin splinter of
tektite glass in a blowpipe flame, the glass melts quietly but only with
the greatest difficulty. On the contrary, when you test in the same
flame the terrestrial glass, obsidian, it froths up much more easily,
into a bubbly, whitish mass.
Although the question of where the tektites came from is still not
entirely settled, most scientists agree that all tektites did have a
_common_ origin. For example, tektites from widely scattered localities
on the earth’s surface show not only similar queer shapes and surface
markings (technically known as “sculpturing”), but also have very much
the same chemical composition and, in particular, the same content of
radioactive elements.
Because the tektites chemically resemble certain terrestrial rocks,
scientists at first believed that some kind of earth process must have
created them. One suggestion was that lightning had fused dust particles
suspended in the air to form them; another, that they had come from
volcanoes; still another, that the tektites were simply inclusions that
had weathered out of terrestrial rocks. A few scientists once took
seriously the possibility that tektites were refuse from primitive glass
factories!
[Illustration: Tektite vs. obsidian, after blowpipe test.]
While such theories have not yet been completely discarded, most
scientists now feel that the tektites had their origin somewhere outside
the earth. There are several reasons for this belief. First, the shape
of such unusually symmetrical forms as are found, for example, among the
australites, indicates that these small bodies at one time were members
of a swarm of freely-spinning liquid masses. Again, flow features
observed on the surfaces of certain tektites (and the fusion crust
definitely identified on one specimen) show that these bodies at some
time must have traveled through the earth’s atmosphere at high velocity.
If, then, the tektites were not produced by earth processes, where did
they come from? According to primitive legends, they were “rocks” or
“pebbles” from the moon. Indeed, one of the earliest scientific theories
as to their origin (proposed by the Dutch authority Verbeek in 1897)
likewise attributes them to debris jetted out from the moon. Another
holds that tektites are fragments of the outermost glassy layers of some
so-called “meteorite-planet,” or planets.[8] Still another idea is that
tektites are what is left of a comet when it passes so close to the
blazing-hot sun that the “ices” which make up most of the cometary
nucleus (head) are all distilled away.
These theories of the origin of the tektites are based primarily on
their observed shapes, surface features, and compositions. The senior
author of this book has suggested still another possible theory based on
the very unusual nature of the observed distribution of the tektites on
the face of the earth.
To explain this theory, we first recall that the planet on which we live
is more nearly a true sphere than are such familiar “spherical” objects
as baseballs or basketballs. Consequently, any plane through the center
of the earth cuts its surface in a curve that to all intents and
purposes is what geometers refer to as a _great circle_.
[Illustration: Every plane passing through the center of a sphere
intersects the surface in a great circle. In this figure, only the
front half of the great circle cut out by the plane is shown.]
Now the significant fact is that all the tektite deposits known at
present are located on or very near to three great circles on the
earth’s surface. Mathematics shows that if some earth process had
created the tektites at random over the surface of the earth, then the
odds would be very strongly against the existence of this peculiar
“great-circle distribution.” But such distribution along great circles
would be _expected_ if the tektites had resulted from what might be
likened to “chain-falls” upon the earth of objects like nearby
satellites moving in orbits encircling our globe.
This notion brings up the interesting possibility that at some time in
the remote past, the earth may have been the proud possessor of a set of
equatorial rings. These rings would have been similar to those at
present circling in the plane of Saturn’s equator. (Jupiter, too, may
once have had its own set of equatorial rings.) The rings of Saturn are
known to be made up of countless very small meteorites. In the same way,
the “earth rings” of prehistory could have consisted of swarms of tiny
nearby meteoritic satellites—the tektites—moving about the earth in the
plane of its then-existing equator.
Eventually, the innermost of these small natural satellites collapsed
onto the earth’s surface, falling along the old equator. At least twice
thereafter, this process was repeated, the points of impact of the later
tektite falls again lining up along whatever great circle of the earth
happened to be the equator at the time of fall.
As the geologists and other investigators have shown, major shifts have
occurred in the position of the earth’s equator during past geologic
ages. This fact is well-substantiated by discoveries of fossil shells
and plants on the cold Antarctic continent and of glacial deposits in
hot South Africa. Therefore, we could hardly expect the tektite
deposits, which are believed to have fallen at widely separated
intervals of time, to have all occurred along a _single_ great circle on
the earth’s surface.
As you can see, the so-called “tektite-puzzle” is a complex one. As if
this were not bad enough, Mother Nature has added to the confusion by
creating in addition to the tektites another type of silica-glass not
only found along the very same three great circles sprinkled with true
tektites, but also having other features in common with the tektite
glasses.
At Mount Darwin in Tasmania and at Wabar in the Rub’ al Khali desert of
Arabia, large and small fragments of this curious silica-glass have been
collected. At Wabar the masses of silica-glass were found in and about
the rims of a series of meteorite craters formed in nearly pure sand, as
we pointed out in Chapter 4. These meteorite craters are known to have
resulted from the high-speed impact of iron meteorites upon the sand
dunes of the Wabar site. Since the silica-glasses of Wabar have been
found to contain countless spherules of nickel-iron of the same
composition as the iron meteorites discovered about the Wabar meteorite
craters, it seems quite certain that both the sand of the earth target
and the nickel-iron of the falling meteorites were vaporized by the
intense heat generated at impact. Consequently, it is natural that these
Wabar masses of congealed silica-glass and nickel-iron be called
_impactites_. They are silica-glasses, created chiefly from
_terrestrial_ materials by the impact of large crater-forming
meteorites. This same name is now applied to all silica-glasses believed
to have the same origin as those at Wabar.
As regards size if not composition, the crater-forming meteorites
responsible for the Wabar and other impactites may have been big
brothers of the small-fry responsible for the showers of true tektites.
Or these big ones may have moved about the earth in orbits distinct from
those followed by the tektite swarms but lying in the same plane as one
of these swarms.
In addition to the curious puzzle of the tektites, meteoriticists have
also run up against the problem of “fossil” meteorites or, more exactly,
the problem of the _lack_ of “fossil” meteorites. As we have already
mentioned, no positively identified meteorite has ever been found in
other than the most recent rock layers. With all the mining—particularly
coal mining—that has gone on throughout the world in historic times,
this fact does seem astonishing.
A number of explanations can be suggested for this absence of ancient
meteorites. In the geologic past, meteorite falls may not have occurred
as often as they do today. For example, the primeval atmosphere of the
earth may have been so much denser than at present that even quite large
meteorites were totally vaporized as they passed through it and
therefore never reached the ground. Again, even if the rate of infall of
meteorites was the same in the remote past as now, still various
weathering processes active ever since the earliest meteorites fell may
have so changed them in appearance and composition that they are no
longer recognizable for what they are.
Several unusual lumps of rock from England and a mass of iron from
Austria, all found at some depth by coal miners, have been tentatively
put forward as “fossil” meteorites. But studies of these masses have so
far produced no conclusive results. Still, we should not ignore the
possibility that someday meteorites may be found and identified in rocks
of considerable age.
[Illustration: L. J. SPENCER PHOTO
COURTESY OF AMERICAN MUSEUM OF NATURAL HISTORY
Mysterious glass objects found in the Libyan Desert. (right) Cut
and polished specimens.]
Does it seem as if we have posed more problems than we have solved in
this chapter? It is very true that we have done just that. In speaking
briefly about the tektites, the impactites, and the absence of “fossil”
meteorites, we have by no means tried to present the last word on the
troublesome but highly interesting problems connected with these
objects—problems that admittedly may take scientists years or even
decades of further research to solve. Perhaps you will find here the
kind of unusual and thought-provoking problems that make the study of
meteorites a rather special challenge. If so, you may wish to take an
active part someday in unraveling these puzzles.