Numerical calculations in two dimensions show that solitary wave disturbances can propagate along thermal plumes in homogeneous, viscous fluid with a thermally activated rheology similar to the mantle. Comparison of the numerical results with analytic results from an idealized two-fluid model indicates that the two-fluid model indicates that the two-fluid model can be applied to solitary waves in the thermally activated plume. A two-fluid model of solitary waves on cylindrical conduits is accordingly applied to mantle plumes to estimate the propagation speeds, time duration, and pulse lengths of solitary waves in mantle plumes as a function of background mantle viscosity, plume flux, plume density deficit, plume viscosity, and the volume of material transported by the solitary wave. It is inferred that mantle plumes may be episodic via solitary waves and that these disturbances might account for observed 0(10 m.y.) variations in the durations and spacings of episodes of enhanced hotspot volcanic activity. Solitary waves in mantle plumes could be generated by interactions among plumes and between plumes and the large-scale, time-dependent mantle circulation. ¿ American Geophysical Union 1989