A pressure ridge (sometimes referred to as a tumulus or tumuli in plural form) is created in an active lava flow. Formation occurs when the outer edges and surfaces of the lava flow begin to harden. If the advancing lava underneath becomes restricted it may push up on the hardened crust, tilting it outward. Inflation also takes place and is a process where the plastic layer of lava underneath expands as it cools and small crystals form. The end result is a raised mound of hardened lava rock, usually a relatively narrow but long ridge. Tension cracks form on the surface of pressure ridges and run along the axis of elongated ridges, and at both edges of broader ridges, sometimes referred to as pressure plateaus. Sometimes, along the edges of a pressure ridge, the tension crack can be large enough to create a liftup cave. Other caves can form inside pressure ridges in which conditions arose for the ridge to form and the lava inside subsequently drained leaving an inflationary cave.