THE SHAPES OF TEKTITES
INTRODUCTION
In this chapter we shall study the shapes of tektites, beginning with those whose form is best understood, and working back toward earlier and more primitive forms.
Definitions
Tektites occur in four general classes of forms:
Microtektites (see Plate 3), usually less than 1 mm in diameter, and found, so far, only in ocean-bottom cores, but clearly associated, both in composition and geographic location, with other tektites.
Muong Nong-type tektites (see Plate 4A, B), blocky in shape
and layered in structure.
A. View perpendicular to the layers.
B. View parallel to the layers.
Splash-form tektites (see Plates 5A, B and 6). These form the great majority of all known tektites. They look like congealed drops of some viscous liquid; they are shaped like spheres, drops, dumbbells, hamburgers, etc. They are typically decorated with corrosion markings of various kinds: cupules (hemispheric pits of sizes up to about 1 mm; see Plate 7A); gouges (elongated depressions, with sharp edges, U-shaped in cross section, and with a length which is several times the width; see Plate 7B); meandrine grooves (U-shaped in cross section, and in plan like worm tracks in old wood; see Plate 7C, D). Sometimes, especially in tektites from Anda (see Plate 7E) in the Philippines, the grooves show an astounding appearance of multiplicity, as if they had been dug out by the claw of an animal.
A. Disk
B. Two dumbbells.
A. Cast of Ivory Coast tektite, showing pits (cupules). Smithsonian.
B. Two bediasites (Texas tektites), B 78 (left) and B 58 (right).
Note grooves.
C. Billitonite. Smithsonian, USNM 3163. Note meandrine grooves. This was probably the anterior surface in flight.
D. Same tektite, reverse (probably posterior) surface. Note lack of meandrine grooves, presence of pits.
Plate 7 (continued).
E. Cast of Anda tektite. Note system of multiple grooves. Courtesy of E.C.T. Chao.
Flanged buttons (see Plate 8A), and related forms; these are found principally in Australia. The central part, or core, is typically lens-shaped. On one side there are often concentric rings, spaced a few millimeters apart, called ringwaves; these may also take the form of a double spiral. The side with the ringwaves is called the anterior side; it was almost certainly in front while the tektite was coming down through the atmosphere. The opposite, or posterior, side often has indications of corrosion, though usually not as strong as on the splash-form tektites. Around the edge there is, in some well-preserved specimens, a flange formed by glass dragged off the anterior surface presumably by the airstream.
The splash-form tektites and the flanged buttons, when sectioned, show a system of irregularly wandering striae (see Plate 8B), in the interior, of varying color, hardness and composition.
Plate 8. Australite structures.
A. Flanged australite. Courtesy of British Museum.
B. Striae in slice of flanged australite. The striae meet the posterior surface (the upward surface) at a sharp angle; but at the anterior surface, they turn and follow the surface, as a result of liquid flow. Courtesy of D.R. Chapman.
Many tektites have been subjected to a process of spallation, i.e. the breaking-off of a more or less flattened piece from the outside, apparently as a result of thermal shock. Occasionally, the spallation is incomplete: a portion of the spalled surface adheres, so that it is possible to be sure that this is how the remainder of the surface was lost (Chapman, 1964).
THE FLANGED BUTTONS AND RELATED FORMS
Observations
The best-understood shapes among the tektites are the forms of the flanged buttons (Plate 8A). Flanged tektites are almost unknown outside Australia; and the well-formed flanged buttons occur chiefly in southeastern Australia.
The understanding of these forms was developed in the papers of Stelzner (1893a, b), Fenner (1934, 1935, 1938, 1940a, 1949), Baker (1940, 1944, 1955b, 1958a, b, 1959a, b, 1960a, b, 1963c, 1967) and Baker and Forster (1943). The evolutionary sequence as finally worked out by Baker is indicated in Fig. 16. The tektite enters the atmosphere as a relatively smooth glass ball. Glass is first lost from the anterior surface; of this glass, the portion in the equatorial zone is lost, at least in part, by spalling. When half or more of the central diameter has been lost, it seems to become possible for glass to accumulate on the lee side of the tektite, to form the flanged button.
Experiment
Chapman et al. (1962) made some experimental studies in wind tunnels which showed that in fact an airstream is capable of producing the observed effects on tektite glass. Since the Mach 15 to 25 velocity of a tektite with respect to the air cannot be simulated in a wind tunnel, resort was had, as usual, to an arc jet, in which the air is strongly heated by an electric arc, and is then driven toward the model at about Mach 3. The total energy content of the stream (internal energy plus kinetic and mechanical energy) is the same as that in the actual case; and the fact that the velocity is different does not matter because behind the shock the air velocities are always subsonic in any case.
Plate 9. Three australites, diameters 16-26 mm, and three artificial models made from tektite glass ablated in the NASA-Ames arc jet. Courtesy of D.R. Chapman.
Chapman's remarkable australite-like models (see Plate 9, upper line) were not produced starting from spherical bodies; instead, his models in these cases started from lens-shaped pieces of tektite glass. Hawkins (1963), working with AVCO windtunnels, attempted to produce the flanges experimentally by starting with spheres; he was not successful. In his experiments the melt flow went around to the back of the sphere; the results seem to have resembled some javanites described by Von Koenigswald (1963b) in which an outer layer, apparently of melt glass, forms festoons on the posterior surface. Among the australites these objects are called crinkly tops by Fenner. It is known from Fenner's studies (1938) that flanges are not found attached to tektites until the main body of the tektites has been reduced to a lenticular form; in addition, Hawkins gives aerodynamic reasons for expecting that in his wind tunnel the flange glass will be lost, while in Chapman's somewhat faster airstream, it will stay on.
Calculations
It is possible to calculate the ablation phenomenon, using a theory developed for heat shields of artificial satellites and missiles. The plan of the calculations is to account in detail for the heat dissipated as the tektite moved through the air. A mass m of air encountered by the tektite moving at a velocity V as seen by the tektite, contains an energy ½ mV², plus relatively unimportant quantities or energy which it contains as a result of the initial temperature. Of this energy, a calculable fraction is dissipated at the shock; this is radiated away. Back of the shock is a layer of air a millimeter or so deep called the gas cap. In contrast to the ambient atmosphere, which is cold, thin, and moving at hypersonic speed past the tektite, the gas cap consists of hot, dense air, which moves relatively sluggishly, at subsonic speed, over the tektite surface. Most of the energy of the airstream has been converted at the shock from kinetic energy of motion to thermal energy. As the gas cap flows over the tektite surface, it carries away with it most of the energy of the airstream, which then disappears into the wake.
The gas cap is at a temperature on the order of 7000° K. Between it and the tektite surface there is a very thin layer of air, the boundary layer, whose base is at the temperature of the tektite surface, and whose top is at the temperature of the gas cap. It is through this layer that most of the heat comes which the tektite must cope with. This heat is denoted qaero. A small amount of qaero is radiated back into the air; another small amount is absorbed in heating the body of the tektite. But most of the heat goes into melting and vaporizing a thin layer of glass a tenth of a millimeter thick or less. The critical question concerns the balance between melting and vaporization.
If the layer is thick, then the forces of skin drag will pull it away from the stagnation point; it accumulates in the lee of the tektite edge, as the flange.
If the layer is thin, then viscous forces, which depend on velocity gradients, are able to hold the liquid in place until the heat supplied is enough to vaporize it. In the first case, about 500-600 kcal/kg are consumed; in the second case, around 3000 kcal/kg are used up.
A first attempt at a calculation of the flow was made by O'Keefe (1960); this was immediately superseded by the work of Chapman (1960). Adams and Huffaker (1962, 1964) and Chapman and Larson (1963) produced detailed mathematical programs which gave the calculated ablation as a function of entry angle and entry velocity. Of these calculations, those of Chapman and Larson indicated that most of the ablation in tektites always occurs as the result of melt flow; in their calculations, this always predominates over vaporization by a large factor. For 1-cm tektites entering at 11 km/sec the ablation ranges from 8 to 15 mm, depending on the entry angle, and being largest for the shallowest angles of entry. The calculations of Adams and Huffaker (1964), on the other hand, were based on a vapor pressure for tektites which was much greater than that accepted by Chapman and Larson. Adams and Huffaker found large ablation and melt flow only for angles which nearly graze the upper atmosphere (skipping trajectories, in particular, which actually climb out of the atmosphere and then fall back); moreover, they found that vaporization can remove as much as 60% of the tektite. Their use of a high vapor pressure was attacked by Centolanzi and Chapman (1966) who demonstrated that the vapor pressures of Walter and Carron (1964) – which were used by Adams and Huffaker – referred to the most volatile constituents of the tektite only (for example, water); and that the vapor pressure corresponding to the majority of the matter of the tektite was not far from that of pure silica, as Chapman and Larson had assumed. The accuracy of the calculations of Chapman and Larson was confirmed by O'Keefe et al. (1973) using the theories of Adams and of Warmbrod (1966; see Fig. 17).
The vitally important fact, on which all these calculations agree, is that the amount of ablation observed on australites corresponds to velocities coming from space (i.e. on the order of 11 km/sec). It is much too great for the kind of velocities to be expected for tektites which are following ballistic trajectories from one point to another on the earth (see Table II); these range up to about 6 km/sec. As we shall see, it is possible to understand how tektites moving at 11 km/sec might suffer much less ablation than that of Fig. 17; what is not comprehensible is how tektites moving at 6 km/sec or less could suffer as much ablation.
SPLASH-FORM TEKTITES: THE CORROSION PROBLEM
Thus a difficulty has arisen in understanding tektite ablation. The aerodynamic calculations indicate that nearly all ablation is by melt flow, and ablation is always a matter of the order of a centimeter, and hence sufficient to change the overall shape of a tektite of typical size in a significant way. In fact, however, splash-form tektites almost never show evidence of melt flow; the only exceptions are some javanites (Von Koenigswald, 1963b; see Plate 10). Moreover, the shapes of a large class of tektites are those of liquid drops: spheres, dumbbells, etc., without the changes of overall shape which is observed in australites and is predicted by the theories of Chapman and his co-workers. Chapman explains the absence of evidence of melt flow by saying that the outer surface, with the marks of aerodynamic ablation on it, has been removed by spallation, followed by etching by ground chemicals. There is no doubt that spallation does account for important features, particularly in the sequence from australites to Philippine tektites (Plate 11). The meandrine grooves seen on the lower (anterior) side of these tektites are probably an indication that thermal shock has taken place; hence spallation is plausible.
In general, however, spallation does not account for the absence of visible melt flow, hence the question of the amount of corrosion by ground chemicals is a critical point in understanding the aerodynamic effects on tektites. Many authors consider that attack by ground chemicals may have destroyed the flanges and the ringwaves, and may have produced the system of pits and grooves. The abundant evidence on this point is widely scattered through the literature; it will be summarized in the next section.
Plate 10. Flange formation on a javanite. Note the thickness of the melt layer, compared with that on the australite (Plate 8B). Courtesy of G.H.R. von Koenigswald.
Causes of tektite corrosion
Let us adopt the term corrosion (Suess, 1900, p. 256) for the process by which many tektites acquired their characteristic sculpture of grooves, pits, notches, and gouges. In using this term we do not intend to imply anything about the origin of the corrosion, whether aerodynamic or chemical or from any other cause.
Corrosion on australites. A typical australite button has three different kinds of surface: the anterior surface, the rear surface of the flange, and the true posterior surface (seen bulging upward within the ring of the flange). It will be shown that chemical attack has been negligible in shaping each of these surfaces.
Clearly there has been little attack on the anterior surfaces of the flanged buttons. Corrosion is normally not shown at all; occasionally there are a few small hemispherical pits. The maximum amount of loss by ground chemicals can be estimated from the curvature of some striae which are observable in the glass. The striae are due to variations in the composition of the glass; they form a complex system of thin-folded structures inside the tektite, like a crumpled wad of paper. As the striae approach the anterior surface, they turn aside and run parallel to it (see Plate 8B); this is an obvious result of the flow of the glass, under the influence of aerodynamic drag, away from the center of the anterior surface and around toward the sides and back. By fitting theoretical curves to the observed striae, Chapman et al. (1962, p. 14) have estimated that only about 0.12 mm was removed after the tektites stopped ablating.
Like the anterior surfaces, the posterior surfaces of the flanges show very little evidence of corrosion. Study of the flanges has not been made in the same way as on the anterior surface, but it is reasonably clear from photographs of sections of flange glass that there has been little chemical loss.
On the posterior surface of the main body of the australite – which is usually referred to as the posterior surface – there is often corrosion, particularly in the form of small hemispherical pits, and it often happens that the striae are seen standing out in relief. McColl (1966) notes that this resembles the corrosion seen in other tektites. It is important to decide whether this corrosion is due to ground chemicals or was already present when the tektite fell.
The difference between the anterior and the posterior surfaces of the australites cannot be somehow a result of their position in the ground (e.g. the lower surface being differently attacked from the upper surface) because the flange glass shows no corrosion in many cases when the posterior surface of the main body is corroded.
It is reasonably certain that at least some of the corrosion on the posterior surface is produced before the tektite strikes the ground, because it has been found under the flange glass. Dunn (1912) illustrates this point by photographs of flanged australites which have been sliced perpendicular to the axis of symmetry, right through the region where the flange joins the core. Baker (1944) noted pits under the flanges, sometimes infilled with flange glass. Barnes (1962b) also noted hemispheric pits, which he attributed to bubbles, filled with flange glass. Baker (1963a) concluded that the sculpture of the posterior surface is pre-atmospheric.
On the other hand, it is clear from other thin sections made in the same way that the posterior surface of the australite is sometimes much rougher outside the flange than under it. This roughness is on a scale of a few tenths of a millimeter; and it follows the striae. Clearly, this attack occurred after the tektite reached the earth. Clearly, also, it is entirely different from the sculpture of the splash-form tektites, since it follows the striae, while the typical sculpture of splash-form tektites does not.
The lack of attack by ground chemicals on Australian tektites would of itself suggest that attack by ground chemicals is very slow, except for the unfortunate fact that, as mentioned in Chapter 2, the date of arrival on earth of the Australian tektites, which include almost all of the flanged tektites, is controversial. In that chapter, however, strong arguments were put forward indicating that the australites belong to the Australasian strewn field. It follows that the absence of terrestrial etching on australites (more exactly, the very low level of etching) is evidence that the rate of attack has been small. This evidence is significant not only for the very arid areas of Australia, but even for the more humid areas; Baker (1960b) goes so far as to say that the best-preserved australites are from humid areas.
Corrosion on splash-form tektites. Von Koenigswald (1963a) points out that flight markings, melt flow and flanges also occur on some javanites (Plate 10) (though no flanged buttons); in this case the tektites are found associated with mid-Pleistocene fauna (Von Koenigswald, 1958): Homo erectus and a primitive elephant-like animal Stegodon. The presence of the melt flow suggests that the Australian tektites are only an extreme case of general trends across the Australasian strewn field, and hence should have the same age as the others; these are dated by by K-Ar and fission track, and also by standard geological methods, at -700,000 years.
The evidence for melt flow consists of rolled-up flanges, like those on australites, flow ridges, like the concentric flow ridges of australites, and in many cases an anterior surface whose curvature is markedly less than that of the tektite as a whole. The ablated surfaces are free of corrosion; yet in general the javanites belong to the family of splash-form tektites and are corroded (Von Koenigswald, 1964).
Among the javanites, Von Koenigswald (1961b) has drawn attention to a particular hollow specimen which appears to have been plastic when a pit was inflicted on it. The tektite was afterward broken open by natural causes, and it is possible to see the inside of the tektite; there is a lump on the inside just back of the pit on the outside, as if the pit had resulted from some inward-acting force while the tektite was still plastic.
Beyond Java and Indonesia, in the South China Sea, between the Philippines and Indochina, four tektites were brought up by a dredge haul from the bottom of the sea (Saurin and Milliès-Lacroix, 1961) whose exterior sculpture differed little from that of land tektites. These were examined by Barnes (1971a) who found attached nannofossils which could be dated at -1.0 to -1.3 million years. Since the sculpturing must have been complete before the nannofossils were attached, Barnes concluded that the rate of attack must have been much more rapid in the earlier part of the life of these tektites on earth. The evidence is obviously more easily explained as a result of sculpturing before arrival at the earth.
If in fact the sea tektites were etched by seawater, then it is difficult to understand the existence of the microtektites. There are many of these in the size range around 40 μm. Since the total amount of corrosion is typically on the order of 1 or 2 mm, it is clear that if this corrosion had persisted for a time only 2% greater, all of the smaller microtektites would have disappeared completely. It is also difficult to understand why tektites whose initial radius was, let us say, 1.02 mm should have been so much more abundant than those whose initial radius was 1.05 mm; yet if etching had removed 1 mm, the first set would have become the present 40-μm spheres, and the second the present 100-μm spheres, which are much less abundant. It is much more likely that the etching has been essentially zero in the sea, and therefore that the sea tektites were already corroded when they arrived. This conclusion is strengthened by the recent discovery (Glass et al., 1972a) of microtektites in the Caribbean, which have survived for 35 million years under the sea, or about 50 times longer than the Australasian microtektites.
A large number of tektites have been found on the Indonesian island of Billiton, in the course of tin-mining operations. These were examined by Easton (1921) who pointed out that while many of them have the forms of complete droplets (spheres, pears, etc.); there are also many forms which appear to have broken. He found that the typical tektite sculpture never appears on the broken surfaces. This could be understood only if the break occurred after the period of sculpturing. Yet the broken surface was not truly fresh, as it would be if the miners themselves had broken it.
The absence or scarcity of sculpture on broken surfaces was also noted by Suess (1900, pp. 257-258) on moldavites; by Van der Veen (1923) on billitonites; by Lacroix (1929, 1932) on indochinites; by Von Koenigswald (1961b) on javanites (as well as billitonites); by Barnes (1939, p. 503) on North American tektites; by Rost (1969) and Žebera (1968) again for moldavites. Rost notes that when two fragments of the same tektite are found separately and are reunited, it can be seen that the common surface is only slightly etched. Kaspar (1938) notes that when moldavites are broken, exposing the interior of an ancient bubble, the bubble surface is never corroded. Lacroix (1930) notes the same for indochinites (Plate 2E, F); it is shown on a thailandite (see Plate 12A, B, C) and on a lei-gong-mo (Plate 12D, E). It is incredible that in every case the tektite could have been broken only a short time before it was found.
A. Thailandite. Smithsonian, NMNH 2349. Interior surface, showing little corrosion.
B. Same as A, exterior corroded surface.
C. Same as A, edge, showing bald spot.
D. Lei-gong-mo (Chinese tektite) from the island of Tung Hai (Tan Hai). View showing walls of broken bubbles. Note lack of corrosion on bubble walls and broken edge. Smithsonian.
E. Same tektite as shown in D. Corroded exterior.
The evidence on the moldavites is particularly interesting; some of these have obviously been worn by stream erosion. Žebera (1968) notes that even when the stream action can be dated, and took place millions of years ago (Pliocene), there is no sculpture formed on the worn surfaces. Baker (1937) noted that in a collection of 83 tektite fragments only two could be put together; he concluded that fragmentation took place in flight.
Nininger and Huss (1967) found two indochinites (see Plate 13A, B) which appeared to have suffered incomplete breaks while still in a plastic condition. The surfaces of these tektites are covered with the usual decorations, except where the plastic break exposed new surfaces. There is a clear implication that these tektites broke while still in a plastic condition, but after the completion of the sculpturing. Since the tektites could scarcely have been made plastic after they reached the ground, this appears to be evidence that these tektites at least were already corroded when they struck the ground.
Suess (1900) made a fundamental contribution to this problem in a monograph on the Czechoslovakian moldavites. He found that the markings on the moldavites are arranged in patterns which depend on the overall shape of the tektite. This result is understandable if the patterns are produced by vortices and shock waves in hot gases, since these must satisfy differential equations of fluid flow which involve the shape of the specimen. The result does not make sense if the patterns were produced by the blind action of underground chemicals or plant roots.
For example, Suess found that if the tektite has the general shape of an oblate spheroid (like a Gouda cheese) then a system of gouges is often found, which radiate from the center of one face (see Plate 14A, B). On very flattened spheroids (watch-shaped bodies) there is often a set of gouges which go across the rim in a direction perpendicular to the equator of the body. On convex surfaces, the gouges tend to run in the direction of greatest curvature, while on concave surfaces the gouges run in the direction of least curvature. A hemispherical pit is often found to be the center of a star-like configuration of radial gouges. When a surface contains mostly hemispherical pits, the gouges are not seen (Plate 7A). When a surface has pits on one part, and gouges on another, the regime of pits appears to precede the regime of the gouges.
Somewhat similar rules were found to hold for billitonites by Easton (1921); navel-like depressions occur on the most strongly curved surfaces, for example.
Suess also found that the sculpture of moldavites is not usually related to variations in chemical composition. The surfaces of tektites generally show some faint swirling marks (see Plate 14C, D) which look like what you would see if you cut through a crumpled and folded stack of pancakes. These lines are called the streaky structure. The streaks are related to the chemical composition of the tektite; streaks with more silica tend to stand out very slightly. They correspond to the striae (Plate 8B); they are the lines where the striae come to the surface. The important point is now that the streaky structure may run at any angle to the gouges of the main tektite structure. In a detailed piece by piece description of some 43 specimens, Suess (1900) makes this point again and again.
If, on the other hand, a tektite is put into a weak solution of hydrofluoric acid, so that it is slowly etched away, then Suess found that this artificial attack follows the lines of the streaky structure. One would therefore expect that if the sculpture is due to ground acid etching, it would also follow the lines of the streaky structure; but the major sculpture does not.
When tektites have been found chipped by primitive man, it is always found that the chipped surfaces are uncorroded. For moldavites found at Willendorf (Plate 1) this is noted by Suess (1914). Beyer (1934b, p. 106) reports the same for late Paleolithic artifacts made from Philippine tektites. Baker (1962) finds a similar result for Australian chipped tektites (Plate 2A, B).
When glass is attacked by ground chemicals, a residue of highly silicic material is left behind (Rzehak, 1912b; Brill, 1961). This residue has been observed on obsidians (Wright, 1915); it is in fact used for purposes of dating. Baker (1961) found that etching with citric acid, a normal soil constituent, produced a white crust. It is not produced by hydrofluoric acid. But no such residue has ever been noted on a tektite when found.
In favor of the origin of tektite sculpturing by ground chemical activity is the fact noted by Berwerth (1910) to the effect that tektite sculpture is not really like the sculpture of meteorites; and that on meteorites the effect of atmospheric ablation tends to round the bodies, rather than to roughen them. Linck (1928) used similar arguments. Diaconis and Johnson (1964) attempted to produce the typical tektite sculpture by using artificially heated tektites, since this can tend to make the boundary layer turbulent. Once again, it turned out that the effect of aerodynamic ablation, when entry into the earth's atmosphere is simulated, is to smooth the specimens rather than to produce the characteristic tektite sculpture. Only when air of much higher density was used did the sculpture appear (Golden and Blackledge, 1968). This result can be understood if the sculpture is not atmospheric but is due to envelopment in some gas such as that which launched the tektite. Linck (1928) suggested that the tektites had been launched from a lunar volcano, and that the sculpture was the mark of these volcanic gases. For the moment, the point to see is that this objection, namely that the earth's atmosphere will not do the trick, does not necessarily imply that the sculpturing is produced by ground chemicals.
Van der Veen (1923) removed all of the existing sculpture from some tektites, then heated them and quenched them in a water jet. He found that a set of cracks developed on the outer surface. If the tektite is then attacked by HF, the attack follows the pattern of the cracks, and the resulting pattern of grooves is very much like the meandrine grooves seen especially on some billitonites (Plate 7C, D). Like the natural grooves, the grooves so produced were found to have a U-shaped cross section. These results were confirmed by D.R. Chapman and F.J. Centolanzi in some unpublished work which they kindly communicated to me (1973; see also Chapman et al., 1967b). In this case, the resemblance to tektite sculpture is really convincing. It seems possible that some kind of attack really can occur along such cracks. A possibility that does not seem to have been excluded is that during the formation of the crack itself, powerful local stresses produced a mesh of small cracks around the main crack. Later the chips fell out, or were dissolved out by some very short-range action. A similar explanation might apply to the deepening of the crack between the flange and the core, which can be seen on some australites; the flange glass is at a different temperature from the core glass when they are welded together. Glass (1974) has found that when two microtektites are welded together, a groove is found along th line of contact. In this case, however, the groove is found to be V-shaped in cross section.
Baker (1963c) considers that the sculpturing must be due to ground chemicals because he feels that the Australian flanged buttons are very young. The buttons are clearly not corroded on the anterior surfaces, and he feels that these two facts are connected. The argument evidently falls to the ground if australites have the same age as the other Australasian tektites. Curiously, Baker regards the sculpture of the posterior surface of flanged australites as preterrestrial. He notes (1963b) the mixture of corroded and uncorroded tektites side by side in Western Australia at Nurrabiel. He finds (1961) that acid attack is closely related to the striae (which is not usually true of the corrosion).
Summary of arguments. Summing up, the arguments in favor of the terrestrial origin of the tektite sculpture are:
(1) The absence of sculpture on australites, especially the anterior surfaces, combined with 14C evidence for the low ages of the australites.
(2) The demonstrated relation of the meandrine grooves to cracking, particularly thermal spalling.
(3) The failure of experiments to produce sculpture by aerodynamic processes which simulate atmospheric entry.
Against the terrestrial origin of this sculpture are:
(1) Evidence that the australite flanged buttons have been on earth as long as other Australasian tektites; this evidence comes from fission tracks, K-Ar dating, paleomagnetics (for the microtektites), and the close chemical ties of australite groups to other Australasian tektite families. If the high age is accepted, then the lack of pits and gouges on the anterior surfaces of australite flanged buttons and the evidence for low (0.12 mm) loss of surface glass becomes evidence pointing against the terrestrial origin of the tektite sculpture.
(2) Sculpturing, especially pits,on the posterior surface of australites, especially that under the flange glass, which is qualitatively similar to the sculpture on the splash-form tektites.
(3) Pits and other sculpture on some surfaces of javanites, with australite-like ringwaves on other parts of the same tektites. Flanges on tektites directly associated with Pleistocene fossils.
(4) Existence of microtektites, many less than 40 µm in diameter, for periods up to 35 million years in seawater. Some Australasian tektites with sculpture are found in waters which also have microtektites 700,000 years old.
(5) Absence of sculpture on broken surfaces, on the interior of broken bubbles, and on surfaces formed by plastic breaks.
(6) Correlation of pits and gouges with overall shape; lack of correlation with compositional variations.
(7) Lack of the siliceous crust usually formed by glass decomposition.
The arguments against the terrestrial origin of the sculpture appear overwhelming. The arguments for terrestrial origin can be met if:
(1) The australites have the same age as other Australasian tektites.
(2) The meandrine grooves are due to thermal cracks enlarged by some mechanism other than chemical attack. Note that the Bikol (Coco Grove) tektites (Plate 15A, B; Beyer, 1938, part 2, p. 143) were dredged from the sea bottom, yet have enlarged meandrine grooves.
(3) The sculpture is produced, not during entry into the earth's atmosphere, but (except for the meandrine grooves) in some earlier phase.
It is therefore concluded that most tektite corrosion cannot be due to action by ground chemicals; that it was already present on the tektites when they reached the earth's surface.
ABLATION OF SPLASH-FORM TEKTITES
It follows that we cannot explain the absence of the marks of aerodynamic ablation on tektites by appealing to corrosion by ground chemicals. Unless these marks have been removed by breakage (e.g. spalling), the tektite must be carrying the marks of aerodynamic ablation. The role of spalling in removing the aerothermal stress shell, as Chapman (1964) calls the portion of the tektite stressed by aerodynamic heating, cannot be denied. It is clear, however, that every unbroken tektite form must carry either spall marks or a place where the effects of ablation are visible.
In the case of the flanged buttons, the results of aerodynamic ablation are clear; but what about the much commoner splash-form tektites? Should we follow Suess in regarding the sculpturing itself as the result of downward passage through the earth's atmosphere?
Probably not, for the following reasons:
First, as mentioned above, this kind of sculpture, though it may result from gas flow, seems to require turbulent flow of a relatively dense gas. It does not seem to be possible to explain the attack by dense gases of the kind required here under conditions which simulate tektite entry; at least, all efforts to do so have failed so far.
Second, as pointed out by Berwerth (1910) and others, the sculpture observed on meteorites is not really like that on tektites; by comparison, meteorite sculpture seems to smooth the surface, at least if we are thinking on a scale of millimeters.
Third, some of the sculpture on the posterior surface of australites seems to predate the formation of the flanges, as noted above. Conceivably it could represent an earlier stage of aerodynamic ablation; but this is improbable because higher atmospheric density favors turbulent flow over laminar flow. The flanges are the result of laminar flow; if there is to be turbulent flow in descending flight, it should come after the laminar flow; but a small amount of corrosion is observed under the flanges.
Fourth, the sculpture on the Nininger and Huss (1967) specimens seems to have been put on while the tektite was still plastic. Watson (1935) pointed out the difficulty of heating a mass the size of a tektite all the way through during a meteoric passage through the atmosphere. It would require a nearly grazing approach; and would also, probably, mean that the tektites would have to form by the sweeping-off of a liquid layer (O'Keefe, 1963). This idea has had to be given up (O'Keefe, 1969a) because it cannot be made to fit the microtektite data. Hence the Nininger specimens point to sculpturing at the source.
Similarly, Bouška (1972) found pairs of moldavites in which one had plunged into the other while the specimens were still plastic. The pattern of corrosion is different on the two pieces as if already established when they were joined.
If the Nininger and Huss specimens are examined, it will be seen that in addition to the plastic breaks, they have considerable areas which are bare of all corrosion (Plate 13B). These areas may be called bald spots. Is it possible that these bald spots represent the results of aerodynamic ablation?
If we examine the splash-form tektites with this question in mind, we will note that a very large number of them do have a sort of bald spot, where the sculpture seems to have worn away (Plates 12C, 14D and 15C). In museum specimens, this is usually the place where the curator puts the label on. It does not, of course, always appear on broken tektites. On unbroken tektites, the spot is often more easily detected by the sense of touch than by sight; but it is almost always there. In the few cases when it cannot be found, the reason may be that the tektite tumbled in flight, so that the ablation was evenly distributed over the whole object.
D.R. Chapman (personal communication, 1973) argues that these bald spots result from spallation; and there can be no question about the fact that this is sometimes true. On the other hand, there also appear to be cases when the bald spot includes some pits in it, as if these had been too deep to be scrubbed off. When these surviving pits are numerous, it becomes evident that the spalled fragment, if any, would have been lacy with holes. It does not seem mechanically plausible that such an object would break off in one piece.
The main point is, however, that ablation on the splash-form tektites must have been much less than that on the australites. This is not a matter of minor details, but of overall form. Where there are spheres among the indochinites, there are lenses among the australites; where there are drops or dumbbells or rods, the australites have the equivalent form but flattened. The flattening is clearly not a matter of deformation while in a plastic condition; it is rather a matter of the loss of some material. There are some tektites outside Australia whose overall form simulates that of the australites (King, 1964a; Chao et al., 1964b; Von Koenigswald, 1967; Soukenik, 1971b); but they are rare. The general rule is that the overall shapes of the splash-form tektites resemble the shapes which the australites must have had before they were ablated.
We seem to be driven to suppose that the splash-form tektites suffered some kind of ablation (connected with the bald spots) but that this ablation was quantitatively much less than that of the australites.
Furthermore, it appears that the ablation of the splash-form tektites occurred without melt flow, or with only very minor melt flow, as in the case of some rare javanites.
Is it possible to imagine circumstances such that in a single event some of the infalling objects are deeply ablated, losing up to a centimeter in depth, at least some of it by melt flow, while in the same fall (but not in the same area) other objects lose only a millimeter or two in depth, with very little of the loss being due to melt flow? Chen (1974) considers that the differences may result from roughness in the preatmospheric shape; he investigated this point in detail.
MUONG NONG TEKTITES; MICROTEKTITES
The Muong Nong-type tektites (Plate 4A, B) appear to be chunks broken out of an extensive layered mass; their forms do not appear to have any further significance. The Libyan Desert glass (Plate 16A, B) belongs to this category (Barnes, 1963b). The Darwin glass (Plate 17A) and the Aouelloul glass (Plate 17B) are also closely related to the Muong Nong category (Barnes, 1963b); they show clear evidence of a layered structure, but it is often contorted.
The overall forms of the microtektites resemble those of the splash-form tektites. They are presumably governed by the same considerations.
BODY SHAPES OF TEKTITES
Underlying the australite flanges, and the pits and gouges of the splash-form tektites are the general body shapes. Most australites and many splash-form tektites are spherical. Others are oblate spheroids, rod-shaped bodies, dumbbells, tear drops, or canoes (see Plates 5A, B and 6).
The spheres can obviously be thought of as large drops. For the shapes of the other bodies, Fenner (1934) suggested that rotation had played a major role. This idea has been widely accepted (e.g. Baker, 1959b); but according to an important paper by Tobak and Peterson (1964) it is wrong. They remark that under surface tension, a figure of equilibrium must be rotationally symmetrical around the actual axis of rotation; this rules out both the dumbbell and the (prolate) ellipsoid, because both bodies could rotate stably only around their short axes. Tobak and Peterson deduce that the shapes of splash-form bodies resulted from the breakup of a jet, which was turning slowly or not at all.
CONCLUSIONS
The most important conclusion is that the flanged australites appear to have entered the atmosphere, as Chapman and his co-workers have claimed, at velocities near 11 km/sec, and at angles to the horizon on the order of 30°.
Australites seem to have entered the atmosphere as smooth bodies, usually spherical (since the surfaces under the flanges are smooth).
Splash-form tektites seem to have entered the atmosphere as rough bodies; the roughness, judging from the bent tektites of Nininger, was impressed while the inside of the tektite was still hot and plastic.
It may be that the very different pattern of ablation found on most splash-form tektites (some bald spots but no liquid flow) is connected with differences in initial surface sculpture.
It does not seem likely that tektite sculpture was produced by ground chemicals.
Plate 13. Casts of tektites of having plastic breaks. Courtesy of H.H. Nininger and G. Huss. A. View of plastic breaks. Note that the corrosion of the exterior surface must have occurred while the tektite was still hot and plastic.
Plate 13 (continued).
B. Bald spot on the larger tektite shown in A. Possibly the result of aerodynamic ablation.
Plate 14. Tektite sculpture.
A. Moldavite with radial gouges. Smithsonian, NMNH 3172.
B. Reverse side of the moldavite shown in A.
Plate 14 (continued).
C. Indochinite, Viet Nam. Smithsonian, NMNH 2141.
D. Reverse of C, showing bald spot and streaky structure.
Plate 15. Tektite sculpture.
A. Philippinite. Coco Grove (Bikol type). Smithsonian, NMHN 2039-3 [note - that looks like a typo in original? and perhaps should be NMNH 2039-3 - LH]. Note deep meandrine grooves.
Plate 15 (continued).
B. As for A. Smithsonian, NMNH 2039-11.
Plate 15 (continued).
C. Lei-gong-mo from Kwang-chow Wan, Smithsonian No. 4. Note the bald spot and the streaky structure.
Plate 16. Libyan Desert glass. Smithsonian, NMNH 5739.
A. Wind-facetted surface.
B. Reverse of A. Surface partly wind-facetted, and partly corroded.
Plate 17. Darwin and Aouelloul glass.
A through E. Darwin glass. Smithsonian, NMNH 5642.
F. Aouelloul glass, NMNH 5617, showing layered structure.
Plate 18. Tektite internal structure.
A. Moldavite, about 3 cm in diameter, from the Smithsonian collection. Immersed in light machine oil, and viewed between crossed polaroids to show internal strain pattern. Pattern is explicable as due to cooling as a unit..
B. Spinous voids in a Muong Nong tektite from Phaeng Dang. Loaned by V. Barnes.
Plate 19. Tektite internal structure.
A. Fine structure in a Muong Nong tektite, revealed by etching with HF. Photo by L.S. Walter.
B. Quartz grain in thin section between crossed polaroids. In Muong Nong tektite from Nong Sapong. Loaned by V. Barnes.
C. Lechatelierite in a moldavite from Lhenice. USNM 2057-2. From Chao, 1963b. (Tektites, Plate IId;
© 1963, The University of Chicago).
Plate 20. Impact glass from the Ries crater, showing inhomogeneity. Courtesy of E.C.T. Chao.
Plate 21. Lunar volcanic craters in Alphonsus. NASA photograph 118M from Orbiter V.
