Microscopic investigations of UV-induced formation of laser damage on LiB(3)O(5) optical surfaces during long-term sum-frequency generation (SFG) uncovers a significant growth of a SiO(2)-amorphous layer spatially limited to the illuminated area. The layer gives rise to a catastrophic break-down of the LiB(3)O(5)-output surface upon long-term laser operation even at intensities far below the laser-induced damage threshold. The interaction of UV laser light, LiB(3)O(5) surface and foreign atoms in the ambient atmosphere is discussed in the frame of a two-step process for surface-damage formation.