Tektites and Their Origin - Chapter One

DEFINITION AND HISTORICAL INTRODUCTION

DEFINITION

A tektite is a natural glass, usually black, but sometimes green, brown or gray, which occurs in lumps, usually a few centimeters in length, having no chemical relation to the local bed rock.  Tektites are broadly similar to some terrestrial volcanic glasses (obsidians); they can be distinguished by heating to the melting point with a blowpipe or a blowtorch.  Obsidians turn to a foamy glass, while tektites produce a few bubbles at most, because of their much lower content of water and other volatiles.  Under the microscope, obsidians are seen to have abundant microlites (microscopic crystals); tektites have essentially none.

Tektites are not found as isolated objects; they are found as members of large associations, called strewn fields (see Fig. 3), whose extent varies from 1 km or so to 10,000 km.

Associated with the macroscopic tektites in a strewn field there is a considerably larger mass (at least in many cases) of microtektites, ranging in size from 1 mm to the limit of detectability (currently about 40 μm).

The word tektite was coined by Suess (1900) from the Greek τήκτος meaning molten.

HISTORICAL INTRODUCTION

In Indochina, tektites were found associated with Neolithic pottery (Lacroix, 1931).  A large block of tektite glass was buried along with some quartz crystals in the sacred receptacle under an idol at Chom Khsan, some time in the Angkor Wat period of Khmer civilization (Lacroix, 1930).  A similar deposit was found at Kompong Speu (Lacroix, 1935b).

In modern times they have been used by European jewelers (Suess, 1900), Vietnamese (Lacroix, 1934a); Siamese (Lacroix, 1934a,b), natives of the Ivory Coast (Lacroix, 1934b), Texas farmers (Barnes, 1939) and Australian aborigines (Baker, 1959b, see Plate 2A, B).  The precise names used by the local people are of great importance to anyone wishing to collect tektites; a list is given by Baker (1959b).

The first written reference to tektites is by Liu Sun, about A.D. 950 (Lee Da-ming, 1963, translated by C.S. Peng, and abstracted by Barnes, 1969), who notes that they are called lei-gong-mo, and are collected in the fields after sudden rainstorms in Leichow.  Lee therefore proposes that all tektites from China be called lei-gong-mo.

The long history of human interest in tektites, including the prehistoric use of them, is important because in all this immense span of time, no one seems to have found an outcrop of tektite glass.  (By contrast, obsidian from the volcanic rock of the Lipari islands was an important item of Neolithic trade.)  Instead, tektites are always found as float, i.e. detached pieces.  Josef Mayer remarked, in the first known European description of tektites (1787, quoted in Suess, 1900) that no one has ever found the Mutterstein (mother lode).  Several of the most significant things that have been found out about tektites are negative statements, and this is one of them.

The Czechoslovakian tektites, which were eventually named moldavites (after the Moldau River, in Bohemia, and not after the territory of Moldavia, in Rumania) were the subject of about forty papers during the late 18th and 19th centuries, which are reviewed by Suess (1900).  The tektites were then most often called Bouteillensteine (bottle-stones); the question of natural versus artificial origin was debated.

The first scientific reference to the Australian tektites is by Charles Darwin, the evolutionist (1844).  Fig. 2 is a drawing by Darwin.  Darwin thought it was an obsidian, and attributed the flange to rapid rotation.  Nineteenth century Australian references to tektites, under the name of obsidian bombs, are collected by Walcott (1898).

The Libyan Desert glass seems to have been noted by French explorers; Spencer (1939) cites an 1850 memoir by Fresnel which refers to glass in this region.

Van Dijk (1879) described and photographed tektites from the island of Billiton in Indonesia.  They were discovered in the course of mining for tin.  Van Dijk's work is memorable for the fact that he drew attention to the similarity to Bouteillensteine, or moldavites.

In an important paper, Stelzner (1893a) studied some Australian tektites sent him by Victor Streich, who had gone with the Elder expedition.  In a private letter, Streich had apparently suggested that they were a kind of meteorite.  Stelzner noted that they were similar to moldavites.  He also saw that the curious flanged appearance of some Australian tektites, like ice-cream in a dish (see Fig. 16), could be explained if they had begun as spheres and had been partly melted.  The melt, moving backward as a result of air pressure, could produce the flange.  The pits, grooves and notches found on many tektites he regarded as the result of attack by aerodynamic forces.  He did not at first commit himself about their origin, but a little later (1893b) he decided that they could not be of extraterrestrial origin because glass is not found in meteorites.

Stelzner's ideas were taken a step further by Verbeek (1897a,b), who knew Stelzner.  Verbeek joined the tektites of Billiton (the billitonites) to those of Australia (australites) and the moldavites, and suggested that all three were the ejecta of lunar volcanoes.  He had studied the 1883 eruption of Krakatoa, and believed, probably incorrectly, that the eruption had sent out some rocks with velocities as great as 2.37 km/sec which is the escape velocity from the moon's gravitational field.  Despite this weakness, it now appears possible that Verbeek's idea may be essentially right, after all.

Verbeek's hypothesis was warmly received in Australia, probably chiefly on the basis of the report in  Nature (Anonymous, 1897); very similar ideas were published by Twelvetrees and Petterd (1897) and Walcott (1898).  In Europe, similar ideas were published by Krause (1898), Suess (1898) and Rzehak (1898).

Shortly afterward, Suess (1900), the second of the dynasty of Austrian geologists, brought out a fundamental memoir on the origin of moldavites.  In this paper, the three types of glass are first brought together under the common name tektites, which Suess coined.  Suess was thoroughly convinced that tektites are extraterrestrial in origin.  He attributed most of the sculpturing of tektite surfaces to aerodynamic attack.  With advice from Ernst Mach, then teaching at Vienna, Suess built a sort of wind tunnel, and simulated the aerodynamic ablation of glass, using rosin models, and a hot airstream from his brother's cement factory.  Suess's main arguments remain valid today (see Chapter 3).  His memoir was widely read; many authorities thereafter spoke of tektites as glass meteorites.

Artificial versus natural origin

During the period from 1900 to the outbreak of World War I, the question of artificial versus natural origin of tektites was debated.  The moldavites, by an unfortunate coincidence, occur in a region which was a center of early European glass-making. Suess considered that they could not be artificial because the temperature required to melt moldavites is so high that from experiments run by the great authorities Ernst Abbé and Otto Schott, at the Zeiss works in Jena, it seemed clear that no furnaces earlier than the Siemens regenerative furnace could get high enough temperatures. Johnsen (1906) pointed out that artificial glasses can be distinguished from natural glasses, in general, by their low contents of Al₂O₃, and their high contents of alkalis; these differences account for the lower viscosity of artificial glasses.  By this test, tektites are natural glasses.

A flurry of excitement was produced when Weinschenk (1908) brought forward two glass balls from Kuttenberg, which he claimed were moldavites.  A crust on the outside he attributed to fusion in the atmosphere, like the crust seen on meteorites; this, he said, was proof of the correctness of Suess's ideas.  Weinschenk's paper was attacked by Rzehak (1912a, b), who showed that the glass was typical of some early artificial glasses, although a little low in silica.  He also showed that the crust was a porous material of low index of refraction (1.495 versus 1.521 for the glass itself), and very resistant to melting.  Highly silicic residues with these properties are found to be typical of weathered glass.

The meteoriticist Berwerth (1917) pointed out that tektites have a remarkably close chemical resemblance to sedimentary rocks, especially sandstones; this is a crucial point which has often been rediscovered.  Berwerth went on to suggest that tektites were the product of an ancient human culture, extending all over the earth.  This part of his argument did not convince people, particularly because Suess could already show (Suess, 1916) that tektites had existed as far back as the Aurignacian, which he knew to be earlier than -20,000 years.  Berwerth's paper was the last serious suggestion that tektites are artificial; the conclusion that they are not artificial is another of the important negative statements about the tektite problem.

Tektites as terrestrial volcanic rocks?

A more difficult question was the distinction between tektites and terrestrial volcanic rocks.  The early Australian investigators (W.B. Clarke, 1855; Stephens, 1898; Twelvetrees and Petterd, 1897, 1898) regarded tektites as obsidian bombs.  Walcott (1898) and Suess (1900) pointed to the enormous distances between the tektite strewn fields and the nearest volcanoes.  The possibility of distribution by the natives could not, however, be satisfactorily eliminated.

Merrill (1911) set the tone for many U.S. discussions of the tektite problem by saying that the markings on tektites do not differ essentially from those on obsidian (see also Buddhue, 1941).

Dunn (1911, 1912), on the other hand, noted the existence of large, nearly spherical internal bubbles in some tektites.  He reasoned, correctly, that these tektites must have been formed, and must have hardened in some kind of fluid, since on a solid surface, the hollow sphere would promptly collapse.  The fluid could not have been water, since the liquid lava would have exploded if plunged in water; it was therefore, Dunn argued, some kind of gas.

He then elaborated a unique idea of large, thin bubbles of obsidian, produced by lightning strokes in a dusty atmosphere filled with hot gases.  He suggested that these bubbles might be formed during a volcanic eruption, and might be thin enough to float through the air for thousands of miles.  Eventually, he believed, the bubbles broke up; the thin walls were completely destroyed.  Nothing was left except the congealed drop at the bottom of the bubble (like the drop often seen at the bottom of a soap bubble); this, he thought, was the origin of the flanged australites.  His photographs of sections of flanged australites are among the best ever made (Dunn, 1912).

Dunn's theory was criticized by Summers (1909, 1913), who pointed out that tektite compositions are unlike those of terrestrial volcanic rocks; in particular, tektites occupy very thinly populated classes in the usual Cross, Iddings, Pirsson and Washington (CIPW) classification of igneous rocks.  Summers also notes that the dust would have to be unbelievably thick.  Ideas like those of Dunn, but starting from ordinary windblown dust were later put forward by F. Chapman (1933) and Vogt (1935).  Vogt drew attention to the remarkable similarity between tektites and loess (windblown deposits of dust).  Nininger (1952) remarked that if this mechanism worked anywhere, it should have worked on the Great Plains, where dust storms and lightning often occur together; but in his searches, despite the examination of some 50,000 allegedly meteoritic specimens brought in by the general public, no tektites were found.

Suess (1914) restated the chemical differences between tektites and obsidians:  tektites have more FeO + MgO, and less Na₂O + K₂O than obsidians of the same silica content; also K₂O predominates over Na₂O.  These chemical distinctions were reiterated by Muller (1915), Suess (1916), Dittler (1933) and Loewinson-Lessing (1935).  They have led to another generally accepted statement:  tektites are not the products of terrestrial volcanism.

New discoveries of tektites, to 1918

In the meantime, tektites were discovered in new places.  Scrivenor (1909, 1916) found them in Malaya, Java, Borneo and the Natuna archipelago (see Fig. 7).  Hills (1915) reported a new kind of tektite, with SiO₂ up to 85%, found by Conder in Tasmania (discovery described in Conder, 1934; see Fig. 11).  Suess (1914) wanted to call this queenstownite, but the name was already in use, and the name Darwin glass is therefore given, after Mt. Darwin, in Tasmania.  The status of Darwin glass as a kind of tektite was debated for the next 60 years.

The interwar period

Theories of the 1920s.  World War I interrupted tektite research; when it recommenced, three remarkable new theories opened the discussion.

Goldschmidt (1921, 1924) proposed to explain tektites as the result of a collision between a meteorite and a "cosmolith" or natural earth satellite.  The idea was not later referred to.

Easton (1921) suggested that tektites are produced by the drying-out of siliceous gels, resulting from the action of humic acid.  Although the theory has not survived, his detailed discussion of the decorations (pits, grooves, navels, etc.) on billitonites is valuable.  Like Suess, he noted the relation of sculpture to overall form.  Basically, he objected to Suess's idea of extraterrestrial origin because, he noted, a granitic shell would not be likely to exist on a planet; again, he asked, why no tektites in historic times, while there are plenty of meteorites?

Easton's paper was rebutted effectively by Van der Veen (1923) in another useful paper.  Van der Veen found that X-ray diffraction patterns of gels indicate that they are really crystalline, despite the amorphous appearance; X-ray patterns for tektites indicate only amorphous structure.  Van der Veen also pointed to the wide interval from softening to melting (800 - 1200°C) as evidence of a glassy structure.  He suggested that the major sculpturing of tektites may result from chemical enlargement of fine cracks, and that these in turn may result from thermal shock, an idea later confirmed by Centolanzi and Chapman (1967).  These results seem to apply principally to the worm-track markings which are prominent in billitonites.  Van der Veen noted some very delicate ornaments on billitonites (mushroom-shaped projections a few millimeters in size; see Plate 2C, D) which could not, he felt, be expected to survive the fall to the ground, and must therefore be due to attack by ground chemicals.

Suess (1922) pointed out that Easton's idea would lead to a relation between soil type and tektite chemical composition, which is not observed; he also said that silica gels could not become as dry as tektites. Easton's idea is no longer considered.

Michel (1922, 1939) noted that Wahl had suggested that in the hypothetical parent body from which meteorites come, the light metals might not be completely oxidized.  He suggested that tektites might result from the burning of these light metals in the earth's atmosphere.  This idea was supported by Suess (1933) and Lacroix (1934c).  It was criticized by Watson (1935; see also Fenner, 1938), who calculated that the rate of penetration of heat into the interior of the mass would not permit the burning of the tektite during the relatively short period of flight in the earth's atmosphere.  Watson pointed out that the Widmanstätten figures are observational evidence that heat does not penetrate deeply even in iron meteorites, which have a high thermal conductivity.

Hardcastle (1926) suggested that tektites are formed by the superficial heating of stony meteorites.  He thought that aerodynamic forces might sweep off the liquid layer, and form it into drops.  This idea has not been accepted in its original form, since stony meteorites are very different from tektites in chemical composition; but the more plausible idea that tektites might be ablation droplets from a larger body of tektite composition was formulated (independently of Hardcastle) by Hanuš (1928).  It was taken up again in the 1960s and will be further discussed in Chapter 8.

T.W.E. David et al. (1927) and De Boer (1929) independently pointed out that all tektite occurrences known up to that time fell along a great circle.  They suggested that tektites had come from a shoal of bodies in orbit around the earth.  The idea was generally given up when Lacroix (1934b) found tektites in the Ivory Coast, 45° of latitude off the proposed great circle.  La Paz (1938), however, then postulated two great circles, and when tektites were found in Texas by Barnes (1939) and Stenzel, he suggested (La Paz, 1944) three great circles.

An interesting extension of Verbeek's (1897a, b) theory of tektite origin from lunar volcanoes was put forward by Linck (1928).  Linck noted that the aerodynamic sculpturing of meteorites is not in general much like tektite sculpture; it tends to soften the features of the surface, while moldavite sculpture, and to a less extent, tektite sculpture in general, tends to be very sharp.  He suggested that the sculpture may have been produced by the gases which expelled the tektites from lunar volcanoes.  This idea remains defensible to the present.

New discoveries of tektites, 1920-1940.  In this interwar period, the boundaries of the tektite strewn fields were rapidly extended.  The tektites reported during the 1920s from South America have been shown to be volcanic, and are not discussed here.  But in 1926, H. Otley Beyer found some tektites at Novaliches, on the island of Luzon, in the Philippines.  They were recognized as tektites by H. Oberbeck, and an account was distributed in typewritten form (Beyer, 1928).  Beyer subsequently collected a very large number of tektites – on the order of one million – from all over the Philippines (see Fig. 9).

Soon after Beyer's discovery, Lacroix (1929) announced the discovery of tektites in Cambodia.  Later (1930) he reported that tektites were to be found all over Indochina, from 21° to 7° N latitude, and from 103° to 121° E longitude.  His research on the indochinites was summarized in an important memoir (Lacroix, 1932) which tended strongly to bridge the traditional gap between the billitonites and the australites.  The memoir mentions tektites in China at Kwang-chow-wan.

In 1934, Lacroix reported the discovery of a completely new group of tektites in the Ivory Coast (Lacroix, 1934b, 1935a).  Unlike all other groups, the Ivory Coast tektites have more Na₂O (by weight percent) than K₂O.

Lacroix next found the tektites of the type now called Muong Nong type.  These are found in the same areas as the indochinites, but they are larger and are blocky in overall shape.  They show a layered structure, like a sedimentary rock, but chemically they are nearly identical with the rounded tektites (called splash-form tektites by Barnes) from the same area.  It was blocks of this Muong Nong-type material that were treasured by the Khmers at Chom Khsan and Kompong Speu.

Clayton and Spencer (1934) reported on a transparent yellow-green glass, called Libyan Desert glass.  The relation of this to the tektite problem is disputed; but in this book the Libyan Desert glass is regarded as a kind of tektite.

The first tektites identified in North America were brought to light during the Depression by a Works Projects Administration (WPA) project in April, 1936, and identified by H.B. Stenzel.  They were described by Barnes (1939), who notes that they had been locally known for 50 years previously.  He also mentions (p. 548) the identification of the first Georgia tektites by Oscar Monnig in a letter to E.P. Henderson.

Tektites were found in Java by Von Koenigswald (1935) in the Trinil formation, which also yielded a skull of Homo erectus and some mid-Pleistocene mammals.

Theoretical and laboratory work of the 1930s.  In 1932, the explorer H. St. J. B. Philby (father of Kim Philby, the Soviet agent),having embraced the Moslem faith, set out to explore the Rub'al Khali (the desert of southeast Arabia).  He was searching for a circle of rock in the sand which the Arabs considered to be the burned-out ruins of the Biblical treasure-city of Ophir, which the Arabs pronounced Wabar.  He reached it, saw that it was some kind of crater, and brought back rock from the area.  He secured the help of L.J. Spencer in studying it, and Spencer (1933a) wrote an appendix to Philby's book, The Empty Quarter.  Spencer perceived that  Wabar was a meteorite impact crater; he was especially interested in some silica glass formed there from the desert sand by the heat and shock of the impact.

Spencer was led by this event to put forward an idea which he supported to the end of a long and influential career,  namely that tektites are the product of meteorite impacts like that at Wabar.  He noted a resemblance of Wabar glass to Darwin glass (Spencer, 1933b) and even to indochinites, as shown in the papers of Lacroix.  He was immediately attacked by Scrivenor (1933) who pointed to the absence of partially fused rock or sand in tektites, and by Fenner (1933) who said that the slaggy masses from Wabar resembled tektites neither in form nor in composition.  Spencer replied, referring to some glassy bombs from Wabar, which resemble tektites both in form and in composition.  He and Hey (1933) described a similar glass from the meteorite impact crater at Henbury, in Australia.  Suess (1933) abandoned his earlier opinion that Darwin glass is a tektite, but adhered to the extraterrestrial origin of other tektites. The controversy between Fenner and Spencer continued for the next 25 years.

Fenner wrote a series of papers (1935, 1937, 1938, 1940a, 1949, 1955) on the distribution and morphology of australites (Australian tektites, not including Darwin glass) in which he developed the idea that the flanges which are observed on australites result from the flow of melted glass, and that the underlying shapes before ablation are those of a rotating liquid mass under surface tension.  Except for the point about rotation, the ideas which he developed on the forms of australites have been generally accepted.

In Czechoslovakia, Janoschek (1934, 1937) studied the stratigraphic relations of the moldavites; he attributed the moldavites to the Helvetian, which in Central Europe is a subdivison of the middle Miocene; specifically, Janoschek connected the moldavites with layers containing the fossil Oncophora.  Since the Australasian tektites were already known or believed to be of Pleistocene age, this meant that the great-circle idea was wrong. 

Martin (1934a) and Koomans (1938) separated tektites from amerikanites (tektite-like obsidian bombs found in the Philippines, and originating from terrestrial volcanoes) on the basis of composition, especially water content.  Martin felt that tektites form a clear petrological sequence:  billitonite – australite – moldavite – Darwin glass.

Preuss (1935) in his dissertation gave a very detailed analysis of some tektites by spectrographic analysis.  He noted some broad regional trends in the Australasian strewn field:  an inner region in southern Indochina and Billiton, and an outer region including north Indochina and Australia.  These are distinguished by a difference in the abundance of nickel and chromium, which are enriched in the central zone.  In this book the tektites of the central zone are called, following Chapman and Scheiber (1969), the high-magnesium clan.

Preuss further found that tektites are chemically much like terrestrial sedimentary rocks, in particular a Norwegian loam.  Tektites tend to differ, however, from the most nearly comparable terrestrial materials through excess of silica, deficiency of Na₂O, and deficiency of a number of elements on the right-hand side of the periodic table (Cu, Ge, Sn, Pb) which are volatile at temperatures around 1000° C.  Preuss suggested that the differences are due to an episode of strong heating.  Preuss's work has been confirmed to a remarkable degree by later studies (S.R. Taylor, 1966, Chapman, 1971).  Heide (1936b) concluded from Preuss's work that Spencer must be right.  He later noted (1938b) that tektites from Thailand and the Philippines belong to the outer, nickel-poor zone of the Australasian strewn field.

During this period there was an active school of Czech students of tektites.  Their work is inaccessible because of the language difficulty; it is summarized by Kaspar (1938).  He concludes that the microsculpturing of tektites is due to chemical corrosion, but the macrosculpturing is not.  He notes that many tektites appear to be fragments of larger bodies which contained bubbles (Plate 2E, F).  The bubble cavities are now broken open in most cases.  The inner walls of the bubble cavities are found to be unsculptured, while the outer walls are heavily sculptured.  Evidently the process of corrosion, whatever it was, stopped before the bubbles were broken open.  Most tektites were protected against breakage by burial in the ground during most of the time between fall and recovery.  Hence the breakage probably occurred during the fall to earth or shortly thereafter, prior to burial.  Hence, Kaspar argued, the macrosculpture is not due to ground chemical attack.  The paper of Oswald (1942) has been translated; it presents his strong opinion that the markings on moldavites cannot be attributed to chemical attack after fall to the earth.

La Paz (1938) suggested that the very broad distribution of tektites, compared to the very restricted areas of individual meteorite falls, might indicate that they had moved in a low satellite orbit around the earth, like the great meteor procession of February 9, 1913 (the Cyrillids).  The idea was supported by Fenner (1938) and later by O'Keefe (1958, 1963).

Volarovich and Leontieva (1939) measured the viscosity of tektite glass as a function of temperature; this work has been much used for physical theories.

Barnes (1939) suggested that tektites are fulgurites (glass produced by lightning striking the earth).  This suggestion was criticized by Fenner (1949).

From World War II to Sputnik I

World War II halted most work on tektites, although Baker's morphological studies (1937, 1939, 1940, 1944, 1946, 1955a, b, 1957, 1958a, b) continued through this time.  An important memoir (Baker, 1944) confirmed Dunn's observation that some pits occur under the flanges of australites, and that these pits are infilled with glass from the flanges.  These pits were therefore certainly not produced by ground chemicals.  The same memoir notes the presence of bands of glass in australites with index of refraction up to 1.535.

In his Halley lecture, Paneth (1940) remarked that Michel's burning-light-metals idea will not work; the volume of oxygen demanded (at the relevant atmospheric heights) is too great.

Nininger (1940, 1943a, b, 1952) suggested that tektites originate by meteorite impact on the moon.  His chief argument was that impacts must launch portions of the  lunar surface at escape velocity; some of the debris must reach the earth.  He argued that tektites are not found outside an equatorial belt some 90° in width.  He considered (incorrectly, as it turned out) that this would result from the position of the moon's orbit.  His hypothesis was supported by Kuiper (1953, 1954) who explained the composition of tektites as the result of a fractionation process:  material driven off the earth at high temperatures condensed on the cool moon.  Nininger's hypothesis is one of the important ideas of the present-day study of tektites.

Khan suggested (1947) that tektites are produced by the fall of anti-matter.

Cross (1948) brought forward some tektite-like glass bodies from Valverde County, Oklahoma; these were rejected as tektites by La Paz (1948) because they foam before the blowpipe, and contain crystals.  They are presumably obsidians.

Fenner (1949) found that the anterior portion of australites has a tendency to spall or crack off; this point was later developed by Chapman (1964) as evidence of ablative heating.

Raman, who got the Nobel prize in physics, found (1950a) that Libyan Desert glass shows a kind of weak optical anisotropy, different from strain birefringence, and like that observed in plate glass.  It seems to indicate flow during a semi-molten state.

The first study of oxygen isotope ratios in tektites was made by Baertschi (1950); he found that Java tektites show a deviation of +9.5 parts per thousand in the ratio of  ¹⁸O to ¹⁶O, compared to Hawaiian seawater; moldavites and Darwin glass were also measured; and Silverman found similar values (1951).

Hammond (1950) calculated the rate of cooling of tektite glass from the strains left in the glass; he found that they cooled at a rate of about 50° C per minute, or more, especially between 700 and 600° C.

Barnes (1951, 1961) reported some light olive-brown tektites from the Muldoon area in Texas.  He abandoned his earlier ideas of a terrestrial origin of tektites, and suggested origin from sedimentary rocks on a destroyed planet.  A similar idea was put forward by Cassidy (1956) and Stair (1956).

A crater at Aouelloul discovered by Galouédec was examined by Monod and Pourquié (1951).  They found a silica glass near it which obviously resembled Darwin glass.  It was studied at the British Museum by Campbell-Smith (1951) and by Campbell-Smith and Hey (1952a,b) who analyzed it and found it like Darwin glass in composition.  The crater appears to be an impact crater but the glass did not, at least at first sight, match the local sandstone in chemical composition.

H.E. Suess, son of F.E. Suess, measured the gas composition in tektite bubbles.  He found a pressure of less than 10^-3 atmosphere.  The composition was principally CO₂ and CO, with some H₂ and H₂O, but very little N₂.  For the age of the Australasian tektites, Suess et al. (1951) found upper limits of a few tens of millions of years by potassium-argon methods.  Gerling and Yaschenko (1952) obtained similar results.

Kuroda and Sandell (1954) confirmed Preuss (1935) in finding that molybdenum in tektites is low (0.5 ppm) compared with the figures for obsidian (about 1.0).

Urey (1955, 1957, 1958a,b) argued that the distribution of tektites over the earth could not have resulted from the fall of a shoal of bodies in interplanetary space, because the shoal would have been enlarged to the size of a meteor shower (enveloping the whole earth and more) by differential gravitational action of the sun.  He thought that the distribution probably resulted, as Spencer had suggested, from a meteorite impact on the earth.  Later (1957) he modified this to a cometary impact, since comets are probably less compact than meteorites, and might thus explain the lack of a crater correlated with, e.g., the Australasian strewn field.  Urey felt that it would be surprising if the moon could produce material whose composition is so much like that of a terrestrial clay.

Hubbard et al. (1956) applied the principles of glass technology to the tektite problem.  They expected lumps of glass to break up by thermal shock during entry into the atmosphere, forming fragments of centimeter size.  They noted that between 550 and 650° C the expansion coefficient of tektites other than australites decreases (because of the approach to the transition temperature); they said that this means that the glass was quenched.  They remarked that the striae which can be seen inside a tektite do not in general turn, as they come to the surface, but end abruptly, as if the tektite had been broken from a larger mass.  (Turning is observed on the front surface of australites; see Chapter 3).  They measured the spectral transmission characteristics, as did also Houziaux (1956).

Stair (1956) made the fundamental point that the production of a homogeneous glass, such as tektite glass, cannot be done suddenly; time is required for the component oxides to mix.  A collision (i.e. a meteorite or comet impact) does not give enough time.

Von Koenigswald (1957) began a series of investigations of Java tektites, and Wilford (1957) reported tektites from Brunei, in the northern part of Borneo.

The years 1958 and 1959 opened a new period in the history of the tektite problem.  Ehmann and Kohman (1958a, b) reported (incorrectly, as it turned out) the detection of the radioactive isotope ²⁶Al in some tektites.  This implied a long stay in space, and hence an extraterrestrial origin.  The first international tektite conference was held in Washington.  Baker's comprehensive monograph (1959b) with its strong plea for the origin of australite sculpture by aerodynamic forces, was published.  Nature published a series of short papers in favor  of a lunar origin by Gold (1958), Varsavsky (1958a), and me (O'Keefe, 1958), and against it by Urey (1958a), Kopal (1958) and Barnes (1958a).  The launching of artificial satellites by the U.S.S.R. and the U.S.A. increased interest in the tektite problem.

The period of the past 16 years is too close to be seen in historical perspective.  It is therefore discussed, subject by subject, in Chapters 2-10.

CONCLUSIONS

The study of tektites up to 1958 led to the following conclusions, which underlie modern work:

(1)  Tektites always occur as detached pieces, connected neither physically nor, with some doubtful exceptions, chemically with their surroundings.

(2)  Tektites are not artificial glasses.

(3)  Although tektites resemble terrestrial volcanic glasses in their physical properties, they are not the products of terrestrial volcanoes; they are, in some chemical respects, more like sedimentary rocks.