Many aspects of the history of natural glasses are related to the physicochemical properties of silicate melts. Glasses are thermodynamically unstable; with time and under a variety of conditions, they become crystalline. When little water is present, devitrification may be described in much the same terms as those which apply to the crystallization of silicate melts. The writer uses simple rate theory and the available experimental data to set an approximate lower limit to the time required for thermal reconstruction of natural glasses as a function of temperature. The time required for a glass at 300° C to become a felsite is estimated to be at least a million years, at 400° C, at least a few thousand years. The secular disappearance of strain birefringence in volcanic glasses at low temperatures indicates an intermediate stage in the process of devitrification.
Devitrification along fractures in most volcanic glasses, however, is dependent upon water. In a number of samples, the width of the layer hydrothermally reconstructed is moderate to vanishingly small. The geologic ages of these glasses indicate that the rate of devitrification at low temperature (around 20° C) is no more than about 4–5 μ in 100 million years. Hydrothermal reconstruction is very sensitive to temperature; the depth of devitrification generally varies substantially among macroscopically unaltered samples from a single flow. In the least devitrified specimens, the depth of devitrification seems to be related to the age of the flow.
Presumably, the diffusion of water controls the rate of hydrothermal reconstruction. The diffusion constant at 20° C is estimated to be 10−23 cm2/sec, and pressure-bomb experiments indicate an activation energy of 30 kcal/mole. Time-temperature relationships are estimated for hydrothermal reconstruction. In contrast to thermal reconstruction, devitrification in the presence of water requires only a short time (within experimental range) at temperatures no more than about 300° C.