  We present new results from the Interface Region Imaging Spectrograph showing
the dynamic evolution of chromospheric evaporation and condensation in a flare
ribbon, with the highest temporal and spatial resolution to date. IRIS observed
the entire impulsive phase of the X-class flare SOL2014-09-10T17:45 using a 9.4
second cadence `sit-and-stare' mode. As the ribbon brightened successively at
new positions along the slit, a unique impulsive phase evolution was observed
for many tens of individual pixels in both coronal and chromospheric lines.
Each activation of a new footpoint displays the same initial coronal up-flows
of up to ~300 km/s, and chromospheric downflows up to 40 km/s. Although the
coronal flows can be delayed by over 1 minute with respect to those in the
chromosphere, the temporal evolution of flows is strikingly similar between all
pixels, and consistent with predictions from hydrodynamic flare models. Given
the large sample of independent footpoints, we conclude that each flaring pixel
can be considered a prototypical, `elementary' flare kernel.
