Figure F15. Characteristic structures of a'a flow. A. Photograph of an active a'a flow channel, where viscous tearing of the cooling lava is forming scoriaceous rubble on the flow surface. Levees are formed at the channel margins by sideward displacement of the flow top rubble. B. Sketch showing the main structural components of an a'a lava flow in a vertical section. Pattern of flow lines in moving lava is also shown. The hammer (lower center) is shown for scale. Modified from Lockwood and Lipman (1980).