VHP Photo Glossary: AA
Glowing `a`a flow front advancing over pahoehoe on the coastal plain
of Kilauea Volcano, Hawai`i.
`A`a (pronounced "ah-ah") is a Hawaiian term for lava flows that
have a rough rubbly surface composed of broken lava blocks called
clinkers. The incredibly spiny surface of a solidified `a`a flow makes
walking very difficult and slow. The clinkery surface actually covers a
massive dense core, which is the most active part of the flow. As
pasty lava in the core travels downslope, the clinkers are carried
along at the surface. At the leading edge of an `a`a flow, however,
these cooled fragments tumble down the steep front and are buried by
the advancing flow. This produces a layer of lava fragments both at
the bottom and top of an `a`a flow.
Related photo glossary terms: