VHP Photo Glossary: Tulumus
Photograph by J.D. Griggs on 23 September 1984. Tumulus about 30 m in diameter on the Hilina Pali road
on the south flank of Kilauea Volcano, Hawai`i.
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. 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.