During sudden outbursts of a mass of rock and gas a surface is formed separating the undisturbed body of the rock from the mass of crushed rock and gas ejected into the working. This surface is being displaced towards the inside of the undisturbed rock. It is assumed that the rock-and-gas medium are represented as a multiphase medium made up of a skeleton of a solid body, forming a collection of pores, and gas deposited in the pores both in adsorbed and in free forms. From the solution of the nonlinear equations for the conservation of mass, momentum and energy, which govern such a medium, there follows the possibility of the generation of shock waves during which the “catastrophic gradient” of stresses, velocity and density takes place. Within the region of the shock wave there may occur the destruction of the rock-and-gas medium, as well as phase transition of gas deposited there. Within the region of such a transition, the medium is characterized by a property, described mathematically in the paper, this property being a necessary condition of the generation of a rarefaction shock wave. Such a wave initiates the phenomenon of a sudden outburst, in which event the thermal energy of the wave is drawn from the medium within which the wave is propagating. This causes a temperature drop of the medium, an effect observed in the outburst of a mass of coal and gas.