  We present multi-epoch infrared photometry and spectroscopy obtained with
warm Spitzer, Subaru and SOFIA to assess variability for the young ($\sim$20
Myr) and dusty debris systems around HD 172555 and HD 113766A. No variations
(within 0.5%) were found for the former at either 3.6 or 4.5 $\mu$m, while
significant non-periodic variations (peak-to-peak of $\sim$10-15% relative to
the primary star) were detected for the latter. Relative to the Spitzer IRS
spectra taken in 2004, multi-epoch mid-infrared spectra reveal no change in
either the shape of the prominent 10 $\mu$m solid-state features or the overall
flux levels (no more than 20%) for both systems, corroborating that the
population of sub-$\mu$m-sized grains that produce the pronounced solid-state
features is stable over a decadal timescale. We suggest that these
sub-$\mu$m-sized grains were initially generated in an optically thick clump of
debris of mm-sized vapor condensates resulting from a recent violent impact
between large asteroidal or planetary bodies. Because of the shielding from the
stellar photons provided by this clump, intense collisions led to an
over-production of fine grains that would otherwise be ejected from the system
by radiation pressure. As the clump is sheared by its orbital motion and
becomes optically thin, a population of very fine grains could remain in stable
orbits until Poynting-Robertson drag slowly spirals them into the star. We
further suggest that the 3-5 $\mu$m disk variation around HD 113766A is
consistent with a clump/arc of such fine grains on a modestly eccentric orbit
in its terrestrial zone.
