"Rock of the Month #156, posted for June 2014" ---

A "soccerball" tektite from the Philippines

A superb, near-spherical tektite of black glass, the surface patterned by polygonal grooves up to 5 mm deep, whence the informal "soccerball" name. The measured diameter of the spheroid varies from 43 to 48 mm. The sample weighs 138.05 grams. A related form is shown in Koeberl (1988, p.262). The specific gravity is measured by two labs at 2.430. Australite S.G. varies from 2.36 to 2.52, depending on silica content and the occurrence of bubbles (McNamara and Bevan, 1991). The set of smaller examples shown in Figure 2 was measured at 2.44-2.48, that is, relatively dense glass.

The sample has a bulk magnetic susceptibility of 0.043x10^-3 SI units (estimated logχ = 1.56 [10^-9 m³/kg]), higher than a pure sandstone or quartzite but about 20% that of a typical metasediment.

Tektites are formed by the generation and kinetic distribution of glassy material in large terrestrial impact events. Because glass is a metastable state in nature, such tektite strewnfields are short-lived phenomena. Just four are known on Earth: by far the largest covers much of southeast Asia and Australia (see map in McCall, 2006, p.476). The crudely mushroom-shaped field extends east from Vietnam to the Philippines, WSW across the Indian Ocean to the east coast of southern Africa, and southwards across most of Indonesia and Australia to the Transantarctic Mountains. The other strewnfields are much more localised, in the southeastern United States and Caribbean (e.g., Cuba and Barbados), Ivory Coast (west Africa) and central Europe (the moldavite tektites). The australites are found from the east coast of Australia to the west, finds somewhat biased to the south coastal belt, possibly a function of population distribution. The largest such tektite known in Australia weighs 437 grams and was recovered near Notting in Western Australia. A possible impact site in northwest Laos, near the eastern border of northern Thailand, has been suggested (see maps in McCall, 2001, p.74; McCall, 2006, p.488; Howard, 2011, pp.486-487). Other sources have been suggested, in Cambodia and further afield (McCall, 2001). The heaviest, largest, sometimes layered tektites (Muong Nong tektites) are found near the suspected source region (e.g., Howard, 2011) and may weigh in excess of 10 kg. Typical tektites are far smaller, maximum dimensions no more than 1-10 cm, mass from a few grams to <250 grams.

A recent study, including precise dating of 37 lava flows, offers four lines of evidence that would place the "missing" source crater beneath the extensive and long-lived Bolaven volcanic field of southern Laos (Sieh et al., 2019). These are:

1) Tektite geochemistry implies presence of young, weathered basalts near source.
2) Mapping and Ar-Ar age dating affirm presence of pre- and post-impact lavas on site.
3) A gravity anomaly suggests a buried, 17x13-km crater, and
4) An outcropping sandstone and mudstone breccia, 10-20 km from the crater, may be part of the proximal ejecta blanket.

Tektites are silica-rich glasses (65-85 wt.% SiO₂) with rare earth element patterns consistent with upper crustal sediments (Koeberl, 1988). East and south from Indochina, the Australasian tektites are generally smaller, and exhibit a range of rounded, tubular, button and shard-like morphologies. The common tektite shapes are termed splashform, and show considerable variety (see, e.g., McCall, 2001; Howard, 2011). The diversity in tektite form is partly attributable to variable speeds of rotation of glassy blobs formed in impact (McNamara and Bevan, 1991). The illustrated example, with grooved "breadcrust" texture, demonstrates fracturing formed in heating during atmospheric re-entry, some way from the likely impact point in Indochina, some 780,000 years ago (Whymark, 2009). The Philippine tektites from the Bikol region have been variably etched by groundwater, resulting in a pock-marked surface (Whymark, 2009).

The indochinite / australite tektites from the Philippines are now generally termed philippinites, an earlier term being rizalites. The australites are the youngest of the four strewnfields, at about 0.8 Ma, cf. the Ivory Coast (1 Ma), central Europe (15 Ma) and southeast USA (35 Ma: see Koeberl, 1988).

The origin of tektites was long in dispute, with some arguing for a lunar origin in impacts on our Moon (see O'Keefe, 1978; McCall, 2006). The argument was settled, in many minds at least, by the isotopic arguments of Shaw and Wasserburg (1982), who favoured terrestrial sediments as the principal source material of the siliceous tektite glasses.

Henry Otley Beyer (1883-1966) was a prominent American-born anthropologist. He was born in Iowa and graduated in chemistry from Denver University. He arrived in the Philippines as a teacher in 1905, and spent most of his life, both career and retirement, in the Philippines. Much of his collection of books, papers and maps are at the National Library of Australia in Canberra. He published on the peoples of the Philippines, in various themes of archaeology and anthropology, as well as writing on tektites: he was the first to document Philippine tektites, which he first encountered in 1926, and named rizalites in a 1928 paper. For more details on Beyer's great interest in tektites, and his tektite catalogue, see the essay by Aubrey Whymark on his website (www.tektites.co.uk).

Figure 2. Three small, discoidal philippinites, weights 23.2, 50.0 and 63.8 grams. The specific gravity of each sample ranges from 2.44 to 2.48, consistent with these impact glasses. Samples from Blaine Reed, January 2020 (smallest sample: John Pocock). Note the pitted (finely vesicular) grooved surfaces of each specimen. Margins of these glasses are translucent.

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