aa lava

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.
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