  EX Lup is the prototype of a class of pre-main sequence eruptive stars
defined by their repetitive outbursts lasting several months. In 2008
January-September EX Lup underwent its historically largest outburst,
brightening by about 4 magnitudes in visual light. In previous studies we
discovered on-going silicate crystal formation in the inner disk during the
outburst, but also noticed that the measured crystallinity fraction started
decreasing after the source returned to the quiescent phase. Here we present
new observations of the 10 $\mu$m silicate feature, obtained with the MIDI and
VISIR instruments at Paranal Observatory. The observations demonstrate that
within five years practically all crystalline forsterite disappeared from the
surface of the inner disk. We reconstruct this process by presenting a series
of parametric axisymmetric radiative transfer models of an expanding dust cloud
that transports the crystals from the terrestrial zone to outer disk regions
where comets are supposed to form. Possibly the early Sun also experienced
similar flare-ups, and the forming planetesimals might have incorporated
crystalline silicate material produced by such outbursts. Finally, we discuss
how far the location of the dust cloud could be constrained by future JWST
observations.
