HONOLULU (KHON2) — Most Hawaii people know the difference between pāhoehoe and ‘a‘ā. ‘A’ā is what you say as you walk across the clinkers that make up an ‘a’ā flow, or so the story goes.

According to USGS’s website, “‘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.”

The formation of an ‘a‘ā flow is seen in the video above. The USGS video was taken on the northeast flank of Mauna Loa.

Pāhoehoe, on the other hand is, “Basaltic lava that has a smooth, hummocky, or ropy surface. A pahoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust.”

These Hawaiian (‘ōlelo Hawaiʻi) words are now part of lava vocabulary around the world. You can explore more terms in the USGS’s Terms Glossary.

Mauna Loa means “long mountain” in ‘ōlelo Hawaiʻi and is, according to the National Park Service, “the quintessential shield volcano in its shape–signified by broad, rounded slopes.”

According to USGS, a shield volcano is “a broad shield-shaped volcano that is built up by successive, mostly effusive, eruptions of low-silica lava.”

So next time you cross a field of ‘a‘ā, you can wonder if the name really did originate from the sound you make when you cross the rough and, at times, sharp surface–“ah, ah, sore.”