Various structures observed in Earth’s mantle are considered to have a high density and therefore affect the mantle flow and the plate motion. We analyze the onset time of surface motion for 2D basally heated models with and without dense material. We find that the first motion is delayed for more, deeper or denser chemical heterogeneities.</br> Due to a temperature-dependent viscosity, the temperature difference between the cold surface and a hot mantle leads to a highly viscous surface region. In contrast, in a cold mantle, the temperature difference is too small to form rigid plates. The heat from the core first needs to be transported upwards by plumes. Therefore, it takes longer for highly viscous plates to form.</br> In the period after the Earth’s mantle was (partially) molten, the mantle was hotter than today. Under these conditions, plate motion can occur within approximately the first 0.5 Gyr if no chemical structures are present or if they are initially located at the surface. For a system that additionally has deep chemical heterogeneities, the onset of plate motion will be delayed by at least one order of magnitude. The motion will have been less stable than today, probably showing short subduction events.