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. Since the solid crust is brittle, it usually breaks to accommodate the "inflating" core of the flow. Such fractures generally extend along the length of a tumulus, and are frequently accompanied by smaller irregular cracks down the sides. Lava commonly squeezes out through these fractures, and sometimes drains from the tumulus to leave a hollow shell.