The mechanisms of laser-induced modification of transparent materials are complex combinations of different processes that depend on the material itself and a range of processing parameters. As such, the mechanisms are still subject to ongoing study. We use a custom-built adaptive third-harmonic generation (THG) microscope to study those mechanisms. New femtosecond-laser-written phenomena are revealed through this method of imaging. This Letter, together with previous reports by Miyamoto Opt. Express 24, 25718 (2016) [CrossRef]  and Fernandez J. Phys. D 48, 155101 (2015) [CrossRef]  suggest that the distribution of the generated plasma during writing is responsible for the newly revealed phenomena.