Tektite

The widely accepted explanation is that tektites form during large hypervelocity impacts on Earth. The impact melts surface rocks and soils, ejecting molten material into the atmosphere; droplets cool into glass before or during their return to the ground. Unlike ordinary volcanic glass, tektites are impact-related and are commonly depleted in water. Their compositions vary by group but are generally rich in silica, and their rapid cooling prevents the melt from crystallizing normally.

Tektites range from microscopic particles called microtektites to larger glass bodies. Their shapes include spheres, rods, teardrops, dumbbells and flattened forms. Some forms record rotation or reshaping while molten; many surfaces also show evidence of atmospheric ablation. Related specimens are grouped into strewn fields, extensive deposits whose members share broadly similar ages and compositions. Major fields include the Australasian, Central European, Ivory Coast and North American fields.

Many names refer to the region where a variety is found. Moldavites, for example, are green tektites associated with the Central European field and the Ries impact structure in Germany. Ivory Coast tektites are linked to Ghana's Bosumtwi crater, while North American tektites are associated with the Chesapeake Bay impact structure. The source crater for the immense Australasian strewn field has not been identified with certainty. Proposed associations should therefore be distinguished from established links.

Tektites can resemble obsidian and other glassy rocks, but they are not volcanic products. Impact glass formed at or near a crater is also related to impacts but is not necessarily a tektite: the term is generally used for ejected glass that travelled through the atmosphere and fell back over a wide area. Fulgurites, by contrast, form when lightning melts material at the strike site.