  We present ultrafast pump-probe reflectivity and Doppler spectrometry of a
silicon target at relativistic laser intensity. We observe an unexpected rise
in reflectivity to a peak approximately $\sim$9 ps after the main pulse
interaction with the target. This occurs after the reflectivity has fallen off
from the initially high "plasma-mirror" phase. Simultaneously measured
time-dependent Doppler shift data show an increase in blue shift at the same
time. Numerical simulations show that the aforementioned trends in the
experimental measurements correspond to a strong shock wave propagating back
towards the laser. The relativistic laser-plasma interaction indirectly heats
the cool-dense ($n_{e}\geq10^{23} cm^{-3}$ and $T_{e} \sim 10 eV$) target
material adjacent to the corona, by hot electron induced return current
heating, raising its temperature to around 150eV and causing it to explode
violently. The increase in reflectivity is caused by the transient steepening
of the plasma density gradient at the probe critical surface due to this
explosive behaviour.
