The Type II modifications induced by IR femtosecond (fs) laser are used in many optical devices due to their excellent thermal stability at high temperatures (typically> 800 °C). The characteristic feature of Type II modifications is the formation of nanogratings, which can easily be detected through birefringence measurements. However, the measured birefringence is an aggregate value of multiple contributions that include form birefringence, stress-induced birefringence due to permanent volume changes, and point defects. In this work, we investigate the thermal erasure kinetics for each one of these contributions in silica glass. Firstly, we irradiate silica glass samples with a fs-laser using different conditions (polarization, energy). Secondly, we perform accelerated aging experiments to evaluate the stability of the laser-induced modifications, including defects, densification, stress field and porous nanogratings. Finally, the aforementioned contributions to the thermal stability of the nanogratings are identified and discussed using spectroscopic techniques (Raman and Rayleigh scattering, UV-Vis absorption) and electron microscopy. Moreover, porous nanogratings erasure kinetic is simulated using the Rayleigh-Plesset (R-P) equation. This work provides a valuable framework in the realization of silica glass-based optical devices operating at high temperatures (>>800 °C) by 1) evidencing the effect of annealing on each erasure mechanism and 2) providing information on the optical response (mainly the birefringence) upon annealing.