Flat-topped tumulus on Mauna Ulu lavas along Chain of Craters Road, Hawaii. Tumuli are just one end-member of a wide range of features formed by inflation of lava flows. A flat-topped tumulus is a half-way between a classic tumulus and a "lava-rise" or inflation plateau. This tumulus drained slightly but the crust did not deflate, leaving the empty cavity exposed when the road was made.
Tumuli are structures usually associated with slow-moving flows. When the top of the flow crusts over, the lava beneath can well upward and create an elliptical, domed structure. Oftentimes, the pressure creates an axial fracture along the length of the structure. The lava might burst through the crust, squeeze out slowly, or only push the crust so far before it freezes. Tumuli typically range from 2 to 10 meters high. While tumuli are most often associated with pahoehoe lavas, they have been documented in ‘a’a lava as well. Tumuli are one of several related features formed by inflation of lava flows.
Due to their relatively small size, tumuli are difficult to spot aerially. There is often debate over images of suggested Martian tumuli.