  We calculate the thermal and dynamical evolution of the surface layers of an
accreting neutron star during the rise of a superburst. For the first few hours
following unstable 12C ignition, the nuclear energy release is transported by
convection. However, as the base temperature rises, the heating time becomes
shorter than the eddy turnover time and convection becomes inefficient. This
results in a hydrodynamic nuclear runaway, in which the heating time becomes
shorter than the local dynamical time. Such hydrodynamic burning can drive
shock waves into the surrounding layers and may be the trigger for the normal
X-ray burst found to immediately precede the onset of the superburst in both
cases where the Rossi X-Ray Timing Explorer was observing.
