  We consider the Sine-Gordon model coupled to 2D gravity. We find a
nonperturbative expression for the partition function as a function of the
cosmological constant, the SG mass and the SG coupling constant. At genus zero,
the partition function exhibits singularities which are interpreted as signals
of phase transitions. A semiclassical picture of one of these transitions is
proposed. According to this picture, a phase in which the Sine-Gordon field and
the geometry are frozen melts into another phase in which the fields and
geometry become dynamical.
