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Tektites are smooth, usually dark-colored stones consisting of glass-like material with a surface that appears pitted. The term comes from the Greek word teknos, meaning "melted." These stones come in a variety of shapes and sizes and from a number of known locations; radiometric and other dating methods place the dates of their formation at between 35 million and 750,000 years ago, depending on location. Often, they resemble types of volcanic glass such as obsidian, but they have distinguishing characteristics that point to a different origin. Over the years, a number of different theories have been put forward as to their formation, but it is now generally accepted that they result from the sudden heating and pressurization of soil and rock caused by meteorite impacts.
There are several locations on Earth where tektite stones are found; these are large areas over which the the objects are thinly scattered and are known as strewnfields. The largest of these locations covers most of Australasia and extends into Southeast Asia. The other major strewnfields are in North America — in Texas, Georgia and some other locations; the Czech Republic; and the Ivory Coast of Africa. Tektites also occur in a few smaller areas; for example, a yellow or greenish form known as “desert glass” is found in the Libyan desert. Many types of these stones take their names from the localities where they are found.
Tektites occur in a number of distinctive shapes as well as irregularly shaped masses, but can be divided into three main types. The Muong-Nong type — named after a town in Laos, southeast Asia — consists of irregular fragments, sometimes with a plate-like appearance. Splash-form types take a variety of shapes, including spheres, teardrops and dumbbells. The flanged button type has a circular shape with a ridge around it created by intense heating on one side.
Early investigators proposed various theories to explain tektites. Volcanic activity was one candidate; however, the structure and composition of these stones was found to differ from that of material ejected from volcanoes. Rock or soil heated by lightning seemed another possibility, but specimens of objects known to have been created in this way, called fulgurites, exist and are entirely different in appearance and structure; the lightning theory also fails to explain the non-uniform tektite distribution. Some theorists proposed that they were small meteorites or meteorite fragments; a variation on this theory being that they were ejected from the surface of the Moon by meteor impacts. Chemical analysis of meteorites and moon rocks, however, discounted this theory as well.
It seems that the meteorite theories were not too far from the truth, however. A large meteorite impact on land would heat the surface soil or rock almost instantaneously to very high temperatures, and subject it to huge pressure, before ejecting large amounts of molten material and propelling it high into the atmosphere. Blobs of molten material would then fall back to the surface, spread over a wide area. Analysis of the shapes, structure and composition of tektites strongly supports this theory.
The shapes of tektites are generally consistent with airborne molten material and the frequently observed pitted appearance suggests collisions with dust particles in the atmosphere. Spherical objects can be explained this way, with surface tension maintaining the shape. The dumbbell shape would result from a blob of molten material rotating in the air, with material being pulled toward either end — and a broken dumbbell would produce the common teardrop shape. The flanged button shape can be explained by molten material being pushed away from the downward direction of travel as the object fell back to the surface. The irregular, plate-like forms could originate in pools of molten rock near the impact site that would solidify into a form of tektite rock.
The chemical compositions of tektites are notable for their high silica (SiO2) content, which ranges from about 70% in Australasian specimens to about 98% in desert glass. The remainder is mainly made up of metal oxides. These materials all have very high melting and boiling points; the lack of volatile materials suggests that they were subjected to intense heat. The presence of a mineral called lechatelierite — formed from silica subjected to great heat and pressure — in many tektites also supports the meteorite impact theory. In several cases, impact craters have been found that may be associated with tektite locations.