Basaltic lava flows with blocky broken surfaces. The term is of Hawaiian origin, its name originating from the sound that a person typically makes when attempting to walk across the lava flow in bare feet. Aa lava flows are typically 10–33 feet (3–10 m) thick and move slowly downhill out of the volcanic vent or fissure, moving a few meters per hour. The rough, broken, blocky surface forms as the outer layer of the moving flow cools, and the interior of the flow remains hot and fluid and continues to move downhill. The movement of the interior of the flow breaks apart the cool, rigid surface, causing it to become a jumbled mass of blocks with angular steps between adjacent blocks. The flow front is typically very steep and may advance into new areas by dropping a continuous supply of recently formed hot, angular blocks in front of the flow, with the internal parts of the flow slowly overriding the mass of broken blocks. These aa lava fronts are rather noisy places, with steam and gas bubbles rising through the hot magma and a continuous clinking of cooled lava blocks rolling down the lava front. Gaps that open in the lava front, top, and sides may temporarily expose the molten lava within, showing the high temperatures inside the flow. Aa flows are therefore hazardous to property and may bulldoze buildings, forests, or anything in their path, and then cause them to burst into flames as the hot magma comes into contact with combustible material. Since these flows move so slowly, they are not considered hazardous to humans. See also PAHOEHOE LAVA; VOLCANO.
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