  Context: The origin of GRBs' prompt emission is highly debated. Proposed
scenarios involve dissipation processes above or below the photosphere of an
ultra-relativistic outflow. Aims: We search for observational features that
would favour one scenario over the others by constraining the dissipation
radius, the outflow magnetization or by indicating the presence of shocks.
Bursts showing peculiarities can emphasize the role of a specific physical
ingredient, which becomes more apparent under certain circumstances. Methods:
We study GRB 070110, which exhibited several remarkable features during its
early afterglow: a very flat plateau terminated by an extremely steep drop and
immediately followed by a bump. We model the plateau as photospheric emission
from a long lasting outflow of moderate Lorentz factor ($\Gamma\sim 20$) which
lags behind an ultra-relativistic ($\Gamma> 100$) ejecta responsible for the
prompt emission. We compute the dissipation of energy in the forward and
reverse shocks resulting from this ejecta's deceleration by the external
medium. Results: Photospheric emission from the long-lasting outflow can
account for the plateau properties (luminosity and spectrum) assuming some
dissipation takes place in the flow. The geometrical timescale at the
photospheric radius is so short that the observed decline at the end of the
plateau likely corresponds to the shut-down of the central engine. The
following bump results from dissipated power in the reverse shock, which
develops when the slower material catches up with the initially fast component,
after it had been decelerated. Conclusions: Our interpretation suggests that
the prompt phase resulted from dissipation above the photosphere while the
plateau had a photospheric origin. If the bump is produced by the reverse
shock, it implies an upper limit ($\sigma \lesssim 0.1$) on the magnetization
of the slower material.
