Definition of aa lava Basaltic lava flows with blocky broken surfaces.The term is of Hawaiian origin, its name originating from thesound that a person typically makes when attempting to walkacross the lava flow in bare feet. Aa lava flows are typically10–33 feet (3–10 m) thick and move slowly downhill out ofthe volcanic vent or fissure, moving a few meters per hour.The rough, broken, blocky surface forms as the outer layer ofthe moving flow cools, and the interior of the flow remainshot and fluid and continues to move downhill. The movementof the interior of the flow breaks apart the cool, rigid surface,causing it to become a jumbled mass of blocks with angularsteps between adjacent blocks. The flow front is typically verysteep and may advance into new areas by dropping a continuoussupply of recently formed hot, angular blocks in front ofthe mass of broken blocks. These aa lava fronts are ratherthe flow, with the internal parts of the flow slowly overridingnoisy places, with steam and gas bubbles rising through thehot magma and a continuous clinking of cooled lava blocksrolling 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 aretherefore hazardous to property and may bulldoze buildings,forests, or anything in their path, and then cause them toburst into flames as the hot magma comes into contact withcombustible material. Since these flows move so slowly, theyare not considered hazardous to humans.See also PAHOEHOE LAVA; VOLCANO.
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