Tektites are typically rounded black silicate glass objects, and range from less than a millimeter wide to chunks weighing several pounds.  The parent material is sedimentary rock from the Earth's surface.  They formed under conditions of high pressure and sudden intense heating, most likely from meteorite impacts.  Areas where tektites of similar composition are found are called strewn fields.  Each strewn field is thought to have been formed by a single event.

The Australasian tektite strewn field is the largest in the world, yet no source crater has ever been found.This area is just where one would expect to find debris from a giant impact north of Madagascar and heading west to east.

Microtektites that appear to be part of the Australasian strewn field have now been found about 3000 km south of eastern Australia, in the Victoria Land Transantarctic Mountains of Antarctica, making the largest tektite strewn field even bigger than the area shown above.
 Folco, L., P. Rochette, N. Perchiazzi, M. D'Orazio, M.A. Laurenzi, M. Tiepolo. April 2008. Microtektites from Victoria Land Transantarctic Mountains. Geology, Vol. 36, No. 4, pp. 291-294.

Large tektites from forward jetting would fall to Earth faster than microtektites.  In the Australasian strewn field, large tektites are found on land that was down range of the impact in the original protocontinent.

(Muong Nong tektites from Indochina; scale in centimeters)

Map of the Australasian strewn field.¹  Dashed line is the commonly used boundary.  Actual findings of tektites/microtektites are marked by symbols: small "x"s are tektites on land; open circles and black dots are microtektites on the ocean floor, with the greatest density at the black dots; asterisks are both tektites and microtektites; the 3 diamonds are unmelted ejecta but no microtektites; large "X"s are ocean floor drilling sites where none have been found; the plus sign and shaded oval mark a proposed impact site, although it has not been found.

"The largest strewn fields originate from an approximately 30 degree impact (more oblique impacts eject a lot of high velocity melt, but it moves through the low, dense atmosphere, while less oblique impacts eject melt into steep trajectories)."  "Ejecta with the highest velocity (and hence - the most distal [i.e. farthest]) originate from a very thin surface layer."  If the meteorite is small, "all ejected material is quickly decelerated by the atmosphere; molten droplets cool quickly, and descend at the distances of a few crater radii."
Artemieva, N. March 2008. Tektites: model versus reality. 39th Lunar and Planetary Science Conference, LPI Contribution No. 1391, p. 1651.

Map of an ash layer overlapping or just above the microtektite layer.¹  The black dots are ocean floor drilling sites where this combination has been found.  The shaded area is the presumed area covered by the ash, estimated to be 3.7 x 10⁷ km².  The "X" at Toba is a volcanic caldera proposed as an ash source by some.

Glass produced in atomic bomb tests shares most of the characteristics of tektites, including shape, color, transparency, inclusions of quartz and other minerals, iron ratios, and very low water content.  Even the percentages of types of shapes are similar.²

1.  Glass, Billy P., Christian Koeberl. 2006. Australasian microtektites and associated impact ejecta in the South China Sea and the Middle Pleistocene supereruption of Toba. Meteoritics and Planetary Science, Vol. 41, No. 2, pp. 305-326.

2.  Rocca, M.C.L. 2005. Australasian tektites and atomic bomb glass: close similarity in their shape percentages. Meteoritics & Planetary Science, Vol. 40, Supplement, Proceedings of the 68th Annual Meeting of the Meteoritical Society, September 12-16, 2005, Gatlinburg, Tennessee. Abstract, p. 5001.