An ‘a‘ā lava flow on the northeast flank of Mauna Loa. This lava flow is several meters thick and moving to the northeast. 'A'ā flows have a rough rubbly surface composed of broken lava blocks called clinkers. The clinkery surface 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‘ā 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‘ā flow. Via USGS