  We compute the luminosity of K_alpha emission lines produced by
astrophysically abundant elements in the soft X-ray spectra of the early
afterglow of gamma-ray bursts. We find that the detection of these lines can be
a diagnostic for the geometrical set-up of the reprocessing material. In
particular we can distinguish between a ``geometry dominated'' model, in which
the line emission is coming from an extended region and its duration arises
from light-travel-time effects and an ``engine dominated'' model, where the
line emitting gas is in a smaller region, irradiated for a longer period. These
lines therefore offer clues to the dynamics and time-scale of the explosion
leading to a gamma-ray burst.
