A small mound or dome-like uplift, up to 20 m or more in diameter, on the crust of a lava flow. Upwarping of the flow crust is caused by the hydrostatic overpressure of lava within the flow interior, or by excess pressure developed due to the difference in rate of flow between the cooler crust and the more fluid lava below it. Unlike a lava blister, a tumulus is a solid structure.