Chemistry and petrography of tektites

Ordinary tektites are typically composed of about 70 percent silica (SiO₂) and are much like granites in composition, though deficient in sodium monoxide (Na₂O) and potassium oxide (K₂O) and enriched in magnesium oxide (MgO) and ferrous oxide (FeO; see Click Here to see full-size tableTable 2). These trends are similar to those found in rocks intermediate between the shales and sandstones. Tektites are thus generally considered to have been derived from sedimentary rather than igneous rocks. Refined analyses have shown that water is present only to the extent of about 100 parts per million (ppm) in tektites—far below the value for terrestrial igneous or sedimentary rocks. Much information about tektite history is obtained by radioisotopic dating; the ages cited above for the tektite strewn-fields were found by potassium-argon dating.

The problem of tektite origin

The origin of tektites was a subject of intense debate throughout much of the 20th century owing to their unusual shape and their chemical composition, which differs from glassy rocks of igneous terrestrial origin. It was assumed that tektites formed from the molten ejecta of high-energy impacts, but the question remained whether such impacts had taken place on Earth or on the Moon or elsewhere in space. Radioisotope measurements indicated that tektites could not have originated from beyond the Earth-Moon system but did not rule out a possible lunar origin. Under this theory, tektites were lunar rocks melted by the impact of meteorites on the Moon’s surface. These tektites were thrown up into space, and some of them eventually landed on Earth. The lunar theory was most strongly supported by the distinctive lens-and-flange shape of australites, which suggested that they came from outside Earth’s atmosphere. The shape of these tektites could only be explained by two heating events; the first, which created the glass, and a second episode of heating apparently due to air-friction melting as the australite entered Earth’s atmosphere at a relatively high velocity. The lens-and-flange shape apparently results from a glassy sphere melting on its forward side (the area of greatest friction); as the object slows down, some melt flows toward the lee and curls to form a flange. (The same could happen, of course, to tektites thrown outside Earth’s atmosphere which then underwent aerodynamic heating in the course of their reentry.)

The lunar-origin theory was advocated by a number of scientists, but one important argument against it was the localized distribution of tektites on Earth. If tektites came from the Moon, their distribution over Earth’s surface should be completely uniform, which is certainly not the case. More importantly, the analysis of lunar rocks obtained by the Apollo space missions in the late 1960s and early ’70s showed that they were not suitable parent rocks for tektites, thus fatally weakening the lunar-origin theory.

In support of tektites’ terrestrial origin, it was long pointed out that the composition of tektites strongly resembles that of some rocks that are widespread in Earth’s crust. It was only with the emerging recognition of the links between impact craters with some strewn-field areas on Earth, however, that the theory of terrestrial origin gained general support. The correlations between tektite strewn-fields and terrestrial impact craters are numerous and impressive. Ivory Coast tektites, for example, occur 300–1,600 km (180–960 miles) from the Bosumtwi Crater in Ghana; and the formation of both the tektites and the crater has been dated at 1.3 million years ago. Similarities have been traced between the crater glass of Bosumtwi and the Ivory Coast tektites in chemical composition, radioisotopic ratios, and other aspects. The same similarities can be traced for several other impact craters and their associated tektite groups. The cumulative effect of this data is impressive, and a generic connection between craters and tektite fields is now claimed by virtually all authorities.