Tektites

Tektites are small, pebble-like glassy objects of Earth material that have been melted by meteorite impact, splashed up into our atmosphere, and fallen to Earth again under gravity. They often acquire aerodynamic shapes when they partially melt on their return journey. Their name comes from the Greek word 'tektos', meaning 'molten'. The first written reference to tektites was about one thousand and fifty years ago, by Liu Sun in China, who gave them a name meaning 'Inkstone of the Thundergod'.

Where are they found?

Tektites have been found only in certain parts of the world, spread over large areas called strewn fields, mainly in low latitudes. The three major areas are south-east Asia (especially Thailand and the Philippines), Australasia; Caribbean-North America; and Ivory Coast, West Africa.

Other areas include the Czech Republic (Bohemia); Slovakia (Moravia); Aouelloul Crater, Mauritania, Africa; the Libyan Desert; Irgiz, C.I.S.; Dalat, South Vietnam; Laos; Kwantung province, China; and Malaysia.

Microtektites are tiny particles of tektite dust found in deep sea sediment in the Atlantic and Indian oceans. They have the same composition as tektites from the North American and Australasian strewn fields.

Over 600,000 tektites have been found in south-east Asia (heaviest 15 kg) and about 100,000 in Australasia (heaviest 0.4 kg). About 2,000 (heaviest 91 g) have been found in the Caribbean-North American strewn field; 55,000 (heaviest 0.5kg) from Bohemia and Moravia and 200 (heaviest 79 g) from the Ivory Coast, West Africa.

Australian tektites have been found right across southern Australia, mainly below 25 degrees latitude, particularly within an east-west belt extending over Northern Territory, Queensland, most of South Australia, Victoria, New South Wales, and Tasmania, and the southern parts of Western Australia.

How are tektites named?

Tektites are named geographically, i.e. Australites (Australia), Indochinites (Thailand & Cambodia), Thailandites (Thailand), Philippinites and Rizalites (Philippines), Javaites (Java), Billitonites (Billiton Island, Indonesia), Moldavites (Vltavins) from the Czech Republic and Slovakia, Bediasites (Texas, U.S.A.), Georgiaites (Georgia, U.S.A.), and Irgizites (Irgiz, C.I.S.).

Australian tektites

Australian tektites (Australites) have been known and used as artefacts and ritual objects by Aboriginal Australians for over 30,000 years. The commonly held view that the first Australian tektite was found by Sir Thomas Mitchell in 1836 in the Murray- Darling River region of New South Wales and given to Charles Darwin when he visited Sydney on the Beagle, needs reassessment, as it has some ambiguities and inconsistencies in timing and geographical details. A more likely scenario is that the specimen was found by Mitchell on the shores of Lake Boga in what is now Victoria, on his third expedition in 1836, and given to Charles Darwin a year later during Mitchell’s visit to England. Emus have been known to swallow Australites for gizzard stones which help grind up their food.

How old are tektites?

Tektites are geologically young, with a range of about 300,000 years to 35 million years. Many Australites are 610,000 to 750,000 years old. The North American tektites have been dated at 34.5 million years, and the Libyan Desert glass at 28 million years. The Bohemian and Moravian sites are dated at 14.7 million years, Aouelloul Crater at 3 million years and Ivory Coast tektites at about 1 million years.

Composition and properties

Tektites are made of opaque to translucent, green, brown, grey, yellow-grey or black glass. Moldavites are typically green, while Australites are usually black or dark brown.

Their chemical compositions are similar to both granite and impure sandstone (greywacke) or soils of these compositions, being high in silica (68-82%) with 10-14% alumina and lesser iron, magnesium, calcium, potassium and titanium. These components did not have time to combine and form crystals, but cooled quickly to form a glass.

Tektites do not contain any water. They can be mistaken for obsidian or pitchstone (black volcanic glasses), but these will emit some water on strong heating. With a hardness of 6-7 on Moh's scale, tektites will easily scratch window glass. They have a density range of 2.2 (Libyan glass) to 2.8 (Moldavites), but are usually 2.4 to 2.5 grams per cubic centimetre. This is a little lighter than quartz beach sand. Human built objects can be mistaken for tektites, particularly molten bottle glass, glass marbles, and black buttons. Many of these will not have the correctly-shaped rims, symmetrical structures or colours of real tektites, while others will be too heavy or too light.

Tektite shapes

The streamlined shapes of many tektites suggest a rapid movement through the atmosphere under low gravity conditions, with melting early in their formation. About 30 distinctly different shapes have been described. They start as a molten blob projected through our atmosphere, then 'freeze' into a shape as they solidify.

The shape depends on whether the initial molten blob was rotating, and the speed of rotation. The spherical and button types (common in Australites) had no or very little rotation with a fairly steady flight path direction. Their rear surfaces were more protected from heat, but the front surface, facing the flight direction, melted and was pushed towards the rear, piling up to form ridges and a rim or flange.

These 'flanged button' shapes have been copied artificially at the Corning Glass Company in America, where molten glass spheres were placed in the jetstream of a wind tunnel. The tear-drop shapes formed from stretching and breaking apart of rapidly rotating, elongated dumb-bells. Flat disks, ovals and boomerang shapes have all been found.

What happens after tektites fall back to earth?

After falling back to Earth as showers of glass, many tektites lay exposed to weathering and erosion at the surface or shallow depth. Many thousands of Australites have been washed into shallow lakes or depressions and are now found on claypans, and shallow drainage depressions.

Effects of high temperatures, wind, rainfall and chemical attack have produced various types of surface etching or corrosion - pits, furrows, grooves or 'saw-cuts'. Many Australites lose their rims under these conditions and become rimless cores. There is a strange corroded, layered type, the Muong Nong from Laos, south-east Asia, which forms masses up to 24 kg and may have melted only at the Earth's surface rather than being projected up into its atmosphere and melting again on re-entry.

There is no record of anyone having ever witnessed the fall of a tektite, and their formation may depend on the very infrequent arrival of large meteorites or asteroids which would cause great devastation if they came today.

Origins of tektites - impact craters

There have been many theories for tektite origins, with a lunar origin (rock splashed out from the moon by meteorite impact, or erupted from volcanoes) being dismissed only recently. The most favoured theory suggests an origin from Earth, where rock was hurled up into our atmosphere by the impact of a large meteorite or asteroid, then partially re-melted as it fell back through the atmosphere to the ground.

In this theory's favour is the chemical similarity between some Earth rocks and tektites, and the composition of gas bubbles in Moldavites. The gas in these bubbles was under very low pressure, equivalent to 32 km altitude in our atmosphere.

A number of impact craters on Earth have been suggested as sources of tektites. The Ries Crater in southern Germany has shock effects extending down to over 2 km, and is linked to Moldavites, while the Lake Bosumtwi Crater, Ghana is linked to Ivory Coast tektites. Possible sources of the Australasian and south-east Asian tektites are unproven although several have been suggested, including a large depression containing Lake Tonle Sap in Western Cambodia. The North American or Chesapeake tektite event has a crater centre in Cape Charles, Virginia near the southern tip of the Delmarva Peninsula.

Impactites

Glassy, molten rocks can also be produced directly by the high heat and pressure of a meteorite impact or just its shock wave, without projecting the molten material up into our atmosphere. Molten glass and rocks fractured by high pressure have been found near impact craters. The Ries Crater in southern Germany has molten rocks containing high pressure forms of silica, while other sites have 'shocked' quartz showing strain and fracturing, pulverised rock flour and shock breccia (rock fragmented and compacted by shock). Sometimes a yellow glass made from fused quartz sand is found in desert areas, 'Libyan glass' from the Libyan desert being a well-known example. Such examples are probably a result of an impact, but there may not be direct evidence of associated craters.

In the Northern Territory of Australia, small rounded pieces of frothy, molten rock have been found at the Henbury craters, and rocks fractured into a regular 'shatter cone' pattern by intense, sudden pressure have been found at Gosses Bluff impact crater. Mt Darwin crater near Queenstown, Tasmania has molten glass (' Darwin glass') very much like tektites, but it has probably not travelled through our atmosphere.