Tektites (Etymology: Greek tektos, "molten") are pieces of glass formed when a large meteorite or asteroid strikes the Earth. Tektites and associated impact melted rock are found in only a few regions on Earth (called tektite strewn fields) and are, in most cases, associated with young impact craters on or near land. Tektite glass looks similar to obsidian glass but can be differentiated by color and chemical composition.
Tektites come in two forms. The more common form, "splash-form tektites" have rounded, aerodynamic shapes such as spheres, tear-drops, dumbbells, and disks when they are well-preserved. The second variety, "layered" or "Muong-Nong-type" tektites, are found in abundance only in southeast Asia. They have blocky, fragmental shapes and commonly display compositional layering and variations in bubble content. Some larger pieces have a surface tektite reminiscent of lava or "breadcrust" lava bombs.
Tektites are similar to shale in composition and have SiO2 contents that range from 68-82%. They have very low water content 0.005% on average. Relative to splash-form tektites, layered tektites are more compositionally heterogeneous, contain more water, and were heated to lower temperatures. These differences, and the fact that layered tektites are found over a more restricted area in southeast Asia, suggest to scientists that splash-form tektites are formed closer to "ground zero" of the impact and layered tektites are formed further from "ground zero."
There are far fewer tektite localities on Earth than there are impact craters. This is because tektites, being made entirely of glass, dissolve slowly with time. We estimate that after 40 million years only one millionth of the original deposit may remain. Therefore, tektites are preserved in abundance only from large, young, impact events.
Some scientists have proposed that tektites are material from the Moon. The geochemical evidence from the Moon and from tektites themselves clearly shows that this is unlikely. Furthermore, the clear association of tektites with at least three young craters on Earth provides strong evidence that tektites are a product of terrestrial impact.