Laser irradiation (λ = 248 nm, E = 5 eV) of thermal, amorphous SiO2 is found to produce thermally reversible, linear compaction reaching a maximum of 16 % (accumulated, incident dose of ∼ 2000 J/cm2 followed by irreversible compaction and photoetching. Assuming the volume fluctuation to be ΔV/V = 3(Δ1/1)/(1+2σ) this corresponds to a maximum volume fluctuation of −36 %. An approximately linear refractive index versus ΔV/V relationship found in the linear compaction regime agrees with the predictions of the Lorentz-Lorenz law. Laser compaction results are found to be consistent with those obtained using hydrostatic pressure. That is argued by comparison with crystalline SiO2 polymporph, that compaction occurs via collapse of a high order ring structure into one having predominately 2 or 3 membered rings. Irreversible compaction/etching occurs when the lattice can no longer support a reduction in ring dimension and dangling bond defects are created. The presence of a large number of defects is sufficient for mechanism of superficial desorption and photoetching to become important.