  The local and global dynamics of a sheared granular medium are studied in a
model experiment as a function of several macroscopic parameters. We observe
that by changing the shear rate or the loading stiffness, the system crackles,
with intermittent slip avalanches, or exhibits periodic motion. By analyzing
the global force, induced while shearing, we capture the transition from the
crackling to the periodic regime and associated scaling laws. We deduce a novel
dynamic phase diagram as a function of the shear rate and the system's
stiffness. Using photo-elasticimetry, we also capture the grain-scale stress
evolution, and investigate the microscopic behavior in the different regimes.
