The Edinburgh Geologist
by Bill Baird
Until recently, the stone spheres of Central America were regarded as the mysterious vestiges of an unknown culture. Ranging in diameter from less than 1 inch to over 11 feet and almost perfectly spherical, their mode of construction was an enigma. The precision with which these spheres had been shaped was incredible. A sphere from Costa Rica, 7.03 feet in diameter with an estimated weight of 16 tons, for example, was measured as being within one quarter of an inch of a perfect sphere.
Apart from those found in Costa Rica, precisely rounded stone spheres were also known from Honduras, Belize and Mexico. It was not until 1967 however, when Matthew Stirling and his colleagues saw hundreds of spheres littering the ground at Agua Blanca, near Guadalajara, in Mexico, did they suspect that the origin of the spheres was a geological rather than archaeological problem. Stirling's report to the National Geographic Committee for Research and Exploration resulted in a joint National Geographic - Smithsonian Institution - United States Geological Survey expedition to the Agua Blanca area in 1968. The members of the expedition concluded that the spheres were indeed of geological origin and had probably formed by the nucleation, at high temperature, of glassy material around individual, widely spaced glass shards, within the matrix of an ashfall tuff. The tuff was formed during an episode of Tertiary volcanism. Hot gases were released as the glass solidified, permeating the rock in all directions and remelting the surrounding material to form the spheres. The process of sphere growth continued until either the rock had cooled sufficiently or the spheres coalesced.
Spheroidal structures are not uncommon in rocks, especially those of volcanic origin, but most are the intermediate products of weathering processes and not normally perfectly round. Many geologists will have seen examples of onion skin weathering in the dolerite exposures of Scotland's Midland Valley and structures of this type within igneous rocks appear in the literature, from as far afield as the Karroo, of South Africa and Klondyke, Arizona. What is uncommon about the stones of Central America is the near perfection of their spherical form. Perhaps the master stonemasons of that unknown culture first credited with making them, simply honed to perfection the spheres they liked best?
Augustithus, S S, 1982. Atlas of the spheroidal textures and their genetic significance. Theophrastus Publications S.A., Athens, Greece. 15
Roos, R de, 1965. Costa Rica, free of the volcano's veil, National Geographic, Volume 128, No. 1, p. 125-152.
Simons, F S, 1962. Devitrification dykes and giant spherulites from Klondyke, Arizona, The American Mineralogist, Volume 47, p. 871-885.
Stirling, M W, 1969. Solving the mystery of Mexico's Great Stone Spheres, National Geographic, Volume 136, No. 2, p. 295-300.
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