The surfaces of pahoehoe flows on flat or gentle slopes often exhibit elliptical, domed structures called tumuli. A tumulus is created when the upward pressure of slow-moving molten lava within a flow swells or pushes the overlying crust upward and breakes it. Lava commonly squeezes out through fractures, and sometimes drains from the tumulus to leave a hollow shell.
Tumulus below South East Crater, Etna November 1999. Photo: J. Alean