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Updated: May 4, 2025
Long-buried glassy lavas devitrify, or pass to a stony condition, under the unceasing action of underground waters; but their flow lines and perlitic and spherulitic structures remain to tell of their original state. Ancient tuffs are known by the fragmental character of their volcanic material, even though they have been altered to firm rock.
Spherulites, which also mark the first changes of glassy lavas toward a stony condition, are little balls within the rock, varying from microscopic size to several inches in diameter, and made up of radiating fibers. Perlitic structure, common among glassy lavas, consists of microscopic curving and interlacing cracks, due to contraction.
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