A’a (pronounced “ah-ah”) is a type of lava flow with a rough, jagged, clinkery surface made up of broken lava blocks called clinkers. The term comes from Hawaiian, and it describes one of the two main forms that flowing basalt lava takes, the other being the smooth, ropy-surfaced pahoehoe. If you’ve ever seen photos of a volcanic landscape covered in what looks like a field of sharp, dark rubble, you were likely looking at an a’a flow.
What A’a Looks and Feels Like
The defining feature of a’a is its surface: a chaotic pile of loose, angular, often razor-sharp rock fragments. These fragments, called clinkers, form as the outer crust of the flowing lava breaks apart during movement. Walking across an a’a flow, even one that cooled centuries ago, is notoriously difficult. The surface is uneven, unstable, and abrasive enough to shred boots and skin.
Beneath that rubbly exterior, though, the interior tells a different story. The core of an a’a flow is dense, solid, and nearly free of gas bubbles (less than 5% vesicularity in many measured flows). This creates a dramatic contrast: a crumbly, low-density shell surrounding a massive, homogeneous interior. The brecciated (broken-up) top crust can reach about half a meter thick, while a thinner vesicular layer sits at the base, typically 0.2 to 0.5 meters.
The crystals inside a’a flows tend to be very fine-grained. This happens because a’a cools faster than pahoehoe, sometimes at rates above 20 to 30 degrees Celsius per hour. That rapid cooling drives high rates of crystal formation, producing many tiny crystals rather than fewer large ones.
How A’a Forms
A’a isn’t a fundamentally different kind of lava from pahoehoe. Both can erupt from the same volcano, even during the same eruption. What determines whether lava becomes a’a or pahoehoe comes down to two interacting factors: viscosity and strain rate. Viscosity is how resistant the lava is to flowing. Strain rate is how fast the lava is being deformed, stretched, or sheared as it moves.
When lava is fluid and moves gently, it can stretch and deform in a smooth, ductile way, forming the glassy, rope-like surface of pahoehoe. But when the lava becomes more viscous (from cooling and crystallization, for example) or gets subjected to higher strain rates (by tumbling over a steeper slope or accelerating through a narrow channel), it can no longer stretch smoothly. Instead, the surface tears and fractures, breaking into the rough clinker fragments that define a’a. Scientists describe this threshold as “viscous rupture,” the point where the lava’s surface breaks rather than bends.
A complex web of variables feeds into this transition: the lava’s temperature, how many crystals have already formed, the dissolved gas content, the bubble content, the slope of the ground, the eruption rate, and the chemical composition of the lava itself. Both increasing the cooling rate and increasing the shear rate lower the crystal content needed to trigger that rupture. It’s a non-linear process, meaning small changes in conditions can tip the balance quickly.
The Pahoehoe-to-A’a Transition
One of the most interesting things about a’a is that pahoehoe flows frequently transform into a’a as they travel, but a’a almost never reverts back to pahoehoe. This is a one-way street. As a pahoehoe flow moves downhill, it cools, crystallizes, and loses dissolved gas. All of these changes increase viscosity. If the flow also encounters a steeper slope or gets funneled into a narrower path, the strain rate climbs. At some point, the combination crosses the threshold, and the smooth surface begins tearing apart into clinkers.
There are also intermediate forms. “Slab pahoehoe” occurs when relatively fluid lava experiences very high strain rates, producing broken slabs of smooth-surfaced lava rather than true clinkers. “Spiny pahoehoe” forms under low strain rates but in lava that has become too crystalline and viscous to produce a smooth glassy surface. These transitional textures show that the boundary between the two lava types is a spectrum, not a hard line.
How Fast A’a Flows Move
A’a flows are generally not fast enough to outrun, but they’re relentless. On gentle slopes, the leading edge of a basalt lava flow typically advances at less than 1 kilometer per hour, roughly one foot per second. On steep slopes, flow fronts can reach up to 10 km/h (6 mph). When lava is confined within a channel or tube on a steep slope, the main body of the flow can hit speeds above 30 km/h (19 mph), though the advancing front is usually slower than the flow behind it.
Most individual a’a flows are relatively modest in size: typically 1 to 10 meters thick, tens to hundreds of meters wide, and hundreds of meters to tens of kilometers long. Flows an order of magnitude larger are not uncommon, though, particularly during sustained eruptions.
Why the Name Matters
Hawaiian volcanology gave the world much of its lava terminology because Hawaii’s shield volcanoes produce such clear, accessible examples of different flow types. “A’a” entered the geological vocabulary directly from Hawaiian, and it’s now used by geologists worldwide regardless of where the lava is found. You’ll see a’a flows on every continent with active or recent volcanism, from Iceland to the Galápagos to Mount Etna. The term applies to the surface texture and formation process, not to any specific chemical composition or geographic origin.