  A two-dimensional system of photoelastic disks subject to vertical tapping
against gravity was experimentally monitored from ordered to disordered
configurations by varying bidispersity. The packing fraction $\phi$,
coordination number $Z$, and an appropriately defined force chain orientational
order parameter $S$, all exhibit similar sharp transition with small increase
in disorder. A measurable change in $S$, but not $\phi~\&~Z$, was detected
under tapping. We find disorder-induced metastability does not show
configurational relaxation, but can be detected via force chain reorientations.
