Pristine silica glass fiber is well-known to become mechanically weaker when heat-treated in air but the cause of such weakening is not presently known. The time dependence of mechanical degradation of various silica glass fibers containing varying impurity contents were studied in the range from 500 degrees C to 1000 degrees C. Two possible sources of strength degradation were considered: surface crystallization and water diffusion. Surface crystallization kinetics of silica glass fibers were investigated in a wide temperature range, including nanoscale surface nucleation at low temperatures via scanning electron microscopy. From the comparison of the strength degradation, surface crystallization, and water diffusion data in literature, it was concluded that surface crystallization may be responsible for the mechanical weakening observed in silica glass fiber surface during heat-treatment at temperatures above similar to 800 degrees C, whereas water diffusion into the glass surface may be responsible for the strength degradation at lower temperatures.