We have investigated the development of axisymmetric mantle plumes in a compressible mantle with temperature-dependent viscosity, using two different values for the thermal activation energy. Thermodynamic parameters were based on realistic Earth values in the shallow mantle. We assumed a compressible mantle with a phase transition and a two layer viscosity structure (mantle 30 times more viscous beneath the phase change). A dramatic transition in plume dynamics occurred as the thermal activation energy was increased from 250 to 500 kJ mol-1. A mega-plume was formed from the merging of small-scale instabilities swept into the plume's axis, rapidly flushing the entire contents of the hot thermal boundary layer into the upper mantle. Hot plume material with a temperature anomaly of 250 K reached the lithosphere for our assumed temperature drop of 500 K across the base of the mantle. Viscous heating over four orders of magnitude greater than chondritic radiogenic sources was observed during the initial plume-lithosphere-interaction. ¿ 1998 American Geophysical Union