Tekties and strewnfields

By: Alan Morgan

Tektites are small pieces of glass ranging in size from sand grain-sized particles to isolated fragments weighing as much 1 kg (2 lb). Usually smoothly rounded in contour, they take the symmetrical forms of buttons, dumbbells, teardrops, disks, winged bodies, and rods (see figure). The glass, which may appear very dark, is coloured green, brown, or amber. The shapes of tektites show clearly that they solidified from droplets free to adjust their outlines to surface tension. Moreover, they may show markings such as grooves and flanges, that suggest the effects of attrition by air resistance during their fall.

Most opinion seems to be in agreement that tektites solidified from liquid drops falling through the atmosphere. Hypotheses about the origins of tektites range from extra-terrestrial sources (likely lunar) to terrestrial origins. The lunar origin has been ruled out because of the dissimilar analyses of tektites and lunar rock samples. However, the terrestrial origin hypothesis is widely accepted. Melting occurred through the impact of large meteorites, producing craters from which surface material was thrown upward into the atmosphere in a molten state. The tektites were formed as these melted particles fell back through the atmosphere.

All tektite glasses are high in silica, which ranges from 70 per cent Si0₂ (Australasian tektites) to as high as 97.6 per cent Si0₂ (Libyan tektites). Aluminum oxide is the second ranking constituent, forming about 13 per cent of the Australasian tektites. Iron, magnesium, and calcium are important lesser constituents. Tektite glass is unique in having a very high melting point (higher than that of Pyrex glass) and a low coefficient of expansion. Tektite glass with its high silica content is not the same as any glass that might be produced by melting of a stony meteorite.

Areas with concentrations of tektites on the Earth's surface are known as strewnfields. There appear to be five large strewnfields. The biggest of these is the Australasian field, which includes southern Australia, the Philippine Islands, Indonesia, and parts of Indochina and China. There is no obvious crater attached to this strewnfield and a few authors have suggested that there might be many local, and much smaller, impacts that were responsible for this field. Extremely minute tektites, known as microtektites, have been found in deep-sea sediments from a broad area of the Indian Ocean, and also constitute a part of the single Australasian strewnfield. Recent studies are pointing toward a likely impact site or sites in Indochina, associated with layered tektites of the 'Muong Nong' type.

A second strewnfield is that of the Ivory Coast. The possible impact site is in Ghana. Microtektites have also been recovered from deep-sea sediments off the Ivory Coast. A third locality encompasses the Czech Republic, Slovakia and a few finds in Austria. Here tektites are concentrated in small areas of Moravia and Bohemia. The name 'Moldavites' has been applied to these tektites. The possible impact site might be the Ries Crater in Germany. A fourth strewnfield is in the southern United States, where tektites have been found at two localities, one in Texas and one in Georgia. The possible origin of these tektites is from an impact structure detected recently beneath Chesapeake Bay. A fifth strewnfield is in the Libyan Desert.

A sixth area of tektite glass is seen at Wabar in the 'Empty Quarter' of Saudi Arabia. This glass area is from quite small local impacts, certainly less than 7,000 years ago, perhaps to as late as 450 years ago and possibly within historic time (either mid or late 1800s). However, there is only one impact time and likely only one correct age. The uncertainty lies in the isotopic ages from the glasses and records of a fireball landing in this area.

Tektite glass and microtektites from the presumed Cretaceous/Tertiary Period (K/T) boundary impact at Chicxulub Crater (beneath northernmost Yucatan) have been recently found in Haiti and in Montana. These glass areas have both been dated (Ar39/Ar40) at 64.6 my ± 0.1 my. The K/T tektite and microtektite glasses are the oldest found so far.

Radiometric ages have been determined for the strewnfields, as follows:

Here is a reference (there are over 500 available on the web) that illustrates some aspects of tektites:
http://woodshole.er.usgs.gov/epubs/bolide/ (Chesapeake Bay descriptions)